What is asbestos and why is it important ...

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What is asbestos and why is it important? Challenges of defining and characterizing asbestos a

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Brian R. Strohmeier , J. Craig Huntington , Kristin L. Bunker , a

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Matthew S. Sanchez , Kimberly Allison & Richard J. Lee a

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RJ Lee Group, Inc., Monroeville, PA, USA

Available online: 21 Apr 2010

To cite this article: Brian R. Strohmeier, J. Craig Huntington, Kristin L. Bunker, Matthew S. Sanchez, Kimberly Allison & Richard J. Lee (2010): What is asbestos and why is it important? Challenges of defining and characterizing asbestos, International Geology Review, 52:7-8, 801-872 To link to this article: http://dx.doi.org/10.1080/00206811003679836

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International Geology Review Vol. 52, Nos. 7–8, July–August 2010, 801–872

What is asbestos and why is it important? Challenges of defining and characterizing asbestos

1938-2839 Geology Review, 0020-6814 TIGR International Review Vol. 1, No. 1, Feb 2010: pp. 0–0

Brian R. Strohmeier*, J. Craig Huntington, Kristin L. Bunker, Matthew S. Sanchez, Kimberly Allison and Richard J. Lee

International B.R. Strohmeier Geology et al. Review

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RJ Lee Group, Inc., Monroeville, PA, USA (Accepted 18 January 2010) Asbestos is a term used to describe a group of six fibrous silicate minerals whose unique set of properties has led to widespread use in a variety of commercial products. Asbestos is also commonly associated with potential disease, increasing government regulation, and the upward spiralling costs associated with asbestos abatement and litigation. Yet what exactly is asbestos? The term is in common use and has often been incorrectly applied to many elongated or fibre-shaped mineral particles. However, it has become important to be more precise: which elongated or fibre-shaped mineral particles should be defined as asbestos and which analytical methods should be used to make an accurate identification? This review article is intended to highlight differences among the various mineral particles identified as asbestos and to address controversies that have arisen from the use of the term by a wide range of interested parties. Historical information and summaries of the latest research trends are presented for various academic and professional communities, including geologists, medical doctors and health researchers, regulatory professionals, and legal professionals, in order for them to better understand asbestos-related issues as they consider potential solutions to specific questions. Keywords: asbestos; fibre; cleavage fragment; amphibole; chrysotile; regulations;

health

Introduction The naturally occurring minerals that have collectively become known as ‘asbestos’ have been used for thousands of years owing to their unique fibrous characteristics of flexibility, high tensile strength, large surface area to mass ratio, electrical resistance, and resistance to heat and chemical degradation. The industrial/commercial world that mines and processes those materials uses the term ‘asbestos’ to refer to a group of six naturally occurring fibrous silicate minerals mined for the distinctive properties listed above. Asbestos was incorporated into thousands of industrial and commercial products beginning in the middle of the nineteenth century, and asbestos-bearing products became ubiquitous in modern society. The term ‘asbestos’ took on a different connotation in the twentieth century, when it became evident that airborne asbestos inhalation could cause pulmonary diseases, including asbestosis, lung cancer, and mesothelioma. Scientists, particularly those involved in the identification of asbestos, use a mineralogical definition that allows distinction between the several ‘asbestos’ minerals. They base the distinctions between *Corresponding author. Email: [email protected] ISSN 0020-6814 print/ISSN 1938-2839 online © 2010 Taylor & Francis DOI: 10.1080/00206811003679836 http://www.informaworld.com


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these unique minerals with fibrous morphologies on their crystal structures and compositions, the usual and acceptable way of characterizing any mineral particle. With the advent of a range of sampling techniques and analyses by high-resolution methods, it is now possible to identify samples of single particles or aggregates, which are less than a few microns in size, wherever they occur, e.g. airborne particles in response to health concerns related to dust inhalation. This article is in two parts: Part I presents the physical and chemical scientific details that are the basis for the mineralogical definition of asbestos and the analytical methods used to distinguish asbestos from other minerals and man-made fibres. ‘Asbestos’ has been studied for over 100 years; however, the term ‘asbestos’ has been misapplied in some published literature. The intent of this article is to address those discrepancies. Because airborne particles are extremely small, typically less than 0.5 μm in diameter, accurate identification, especially in dusts, is not a trivial matter. Therefore, the sophisticated analytical methods used for identification are presented in Part I. Part II discusses the interaction of asbestos and man, including a brief history of its health effects and current regulatory issues so that the controversies that have arisen can be highlighted. In spite of extensive published research and other literature on asbestos and its health effects, there remain unresolved scientific, medical, and regulatory issues, such as the relative health effects among the several asbestos minerals, asbestos fibres, and elongated fragments of the same minerals. These health aspects and other regulatory issues are discussed in Part II. This article also contains a table of abbreviations and a glossary of terms useful for understanding discussions regarding asbestos and other amphibole minerals.

Part I: Asbestos mineralogy and analytical techniques Asbestos, related asbestiform minerals, and definitions A mineral is a homogeneous, naturally occurring, inorganic, crystalline element or compound with a characteristic or ideal chemical composition, a known three-dimensional crystal structure, and a distinct mineral species name (Campbell et al. 1977; Skinner et al. 1988). Minerals with similar or essentially the same (or comparable) structures and related compositions are known as members of ‘mineral groups’, and each of these groups has a name. There are three silicate mineral groups that commonly exhibit fibrous morphology: the serpentine, amphibole, and zeolite mineral groups. Several minerals of the first two mineral groups have species that have been mined or are currently mined for commercial use. Due to the highly elongated morphology or fibrous ‘habit’ of these silicate minerals, they are specifically labelled ‘asbestiform’ (Skinner et al. 1988). All of the asbestos minerals are naturally formed – they are not man-made. Six asbestiform minerals are currently regulated as asbestos by the US Federal government (US Code of Federal Regulations 2003) – chrysotile, from the serpentine mineral group, and five minerals from the amphibole group: crocidolite (riebeckite asbestos), amosite (cummingtonite-grunerite asbestos), anthophyllite asbestos, tremolite asbestos, and actinolite asbestos. See Table 1 for further details. The regulated minerals in the amphibole group must be designated as asbestos because the same minerals also occur in non-asbestiform morphologies. There are many amphibole mineral species in the group, and most never occur with an asbestiform habit. The term ‘asbestiform’ describes the unusual crystallization morphology that these minerals display when formed as aggregates of thin, hair-like fibres. Figure 1 shows hand-sized specimens of asbestiform and non-asbestiform pairs of the six regulated asbestos minerals. The asbestiform mineral habit illustrated on the left of

International Geology Review Table 1.

The six regulated asbestos minerals.

Regulatory name Chrysotile Tremolite asbestos Actinolite asbestos Anthophyllite asbestos Crocidolite Amosite


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Mineral name

Mineral group

Ideal chemical formula

Chrysotile Tremolite Actinolite Anthophyllite Riebeckite Cummingtonite-grunerite

Serpentine Amphibole Amphibole Amphibole Amphibole Amphibole

Mg3Si2O5(OH)4 Ca2Mg5Si8O22(OH)2 Ca2(Mg,Fe2+)5Si8O22(OH)2 Mg7Si8O22(OH)2 Na2 Fe32+ , Fe3+ 2 Si8O22(OH)2 (Mg,Fe2+)7Si8O22(OH)2

Asbestiform

Rock

Asbestiform

Rock

Chrysotile

Antigorite

Anthophyllite asbestos

Anthophyllite

Crocidolite

Riebeckite

Tremolite asbestos

Tremolite

Amosite

Cummingtonite-grunerite

Actinolite asbestos

Actinolite

Figure 1. Hand specimens of the six asbestos minerals and their non-asbestiform counterparts. (Note the ability of asbestiform particles to be easily separated into smaller particles relative to the rock mass.)

each pair is contrasted with the massive non-asbestiform mineral habit on the right. Although the corresponding pairs shown in Figure 1 are of the same mineral species, the non-asbestiform minerals are not asbestos; the physical expression or morphology is key. The asbestiform morphology is a special type of fibrosity in which the fibres exhibit fine fibre thickness, flexibility, separability, and general parallel arrangement of fibres en masse. These asbestiform minerals are usually found as mineral aggregates concentrated in veins or slip fractures in certain rocks, which makes them easily seen and mined if in high enough concentration (Skinner et al. 1988). In addition to the six regulated asbestos

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minerals, 388 minerals (including 92 silicate and aluminosilicate species) are known to occur, at least occasionally, in fibrous form (Skinner et al. 1988). Only a few of these fibrous minerals occur with an asbestiform habit. The term ‘fibrous’ as distinct from asbestiform merely describes the habit of many minerals to be observed as long, thin particles; they are inorganic, but there are many naturally occurring organic fibres such as the common protein, collagen, which is of biologic origin.


Asbestiform versus non-asbestiform particle characteristics Figure 2 illustrates the significant differences between asbestiform and non-asbestiform amphibole tremolite using a higher magnification technique – polarized light microscopy (PLM). The original habit of the mineral tremolite is blocky or prismatic (Figure 2c); after crushing (Figure 2d), the mineral does not exhibit the long, curved, very thin fibres of asbestiform tremolite (Figure 2a) but rather forms smaller blocky amphibole cleavage fragments (Figure 2d). Crushing the asbestos fibres does not form cleavage fragments, but forms only numerous finer fibres (Figure 2b), which retain their aspect ratio as the bundles are broken apart. Aspect ratio is the ratio of a particle’s length to its thickness or width. Asbestiform fibres typically have aspect ratios greater than 20:1; the aspect ratio of the asbestiform tremolite in Figure 2a and b is many times greater than 20:1. Chrysotile, the serpentine mineral group asbestos mineral, has a remarkable rolled sheet silicate structure, which always forms as individual fibrils and aggregates or fibre

Figure 2. PLM images of tremolite asbestos fibres from North Carolina and New York showing the distinctive morphology before and after crushing. Tremolite asbestos (a) as received and (b) after crushing. Non-asbestiform tremolite particle from New York (c) as received and (d) after crushing. (The field of view = 1 mm for all images.)


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Figure 3. TEM image of chrysotile that illustrates the unique morphology of the fibrils of this serpentine group mineral. Sectioned across the fibrillar length, alternate silicate and Mg-containing crystallographic layers roll onto themselves forming a central hole and hollow cylinders (fibrils). Image Credit: Yada (1967, Figure 7, p. 707).

bundles (Bernstein and Hoskins 2006). Individual chrysotile fibrils are exceedingly thin, about 200Å (0.02 μm) in diameter (Figure 3), and are not visible except at ultrahigh magnification. Aggregates of the fibrils make thin fibres with a diameter of approximately 0.1 μm (Figure 3). The lengths observed for chrysotile can vary from less than 1 μm for an individual fibril to well over 10 cm (many orders of magnitude difference, and the latter is visible to the naked eye) for fibre bundles (Virta 2002; Ross and Nolan 2003). Commercial chrysotile consists of fibres and bundles that usually exhibit diameters from 0.1 to 100 μm and aspect ratios from a minimum of 20 to greater than 1000 (Virta 2002). When chrysotile fibre bundles are disaggregated, as happens during milling and grinding operations, the bundles may break down to produce single fibrils. Because chrysotile only forms with asbestiform morphology, it is always classified as asbestos. By contrast, the five regulated amphibole minerals are only considered to be asbestos if they crystallize as thin hair-like fibres, i.e. with asbestiform morphology. Amphibole asbestos fibres typically vary in width from 0.1 to slightly greater than 1 μm and vary in length from a few micrometres to several centimetres (Ross and Nolan 2003). However, the vast majority of amphibole minerals commonly found in rocks occur as shown in Figure 2 with blocky, prismatic, or acicular morphologies. Acicular particles are straight and elongated, with a needle-like shape, and the particle may be bounded laterally and terminated with the crystal faces typical of the amphibole mineral group (Skinner et al. 1988). Many amphiboles are associated with the other industrial minerals, talc and vermiculite (Ilgren 2004). Prismatic and acicular amphiboles are not asbestos nor asbestiform and are not regulated as asbestos. In comparison to asbestiform fibres, prismatic particles generally have widths ≥1 μm and aspect ratios less than 10:1, and they typically exhibit crystal faces or cleavage traces. Cleavage is the property of an individual mineral to fracture


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or break, preferentially along specific planes of weakness typical of the crystallographic character of the mineral. Cleavage fragments are smaller pieces of the non-asbestiform amphibole mineral. Especially in microscopic view, they show the sharp stepped sides of the cleavage planes and blunt or angular terminations (Figure 2c and d). Cleavage fragments therefore can be distinguished from asbestos fibres based on their morphology and relatively small aspect ratios using PLM or higher magnification, e.g. transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Figure 4 illustrates two field emission scanning electron microscopy (FESEM) secondary electron images comparing a prismatic amphibole cleavage fragment (Figure 4a) to a chrysotile fibre bundle displaying a splayed end (Figure 4b). The cleavage fragment width, approximately 2.2 μm, is more than 10 times greater than the width of the chrysotile fibre bundle and more than 100 times greater than the width of an individual chrysotile fibril. Note also that the chrysotile fibre bundle (Figure 4b) exhibits curvature or splayed ends which are not inherent in cleavage fragments. The asbestiform habit can be defined microscopically by the following morphological characteristics (Perkins and Harvey 1993).

• Mean aspect ratio between 20:1 and 100:1, higher for fibres longer than 5 μm. • Very thin fibrils, usually less than 0.5 μm in width. • Two or more of the following: 䊊䊊䊊䊊

parallel fibres occurring in bundles; fibre bundles displaying splayed ends; matted masses of individual fibres; and fibres showing curvature.

The mineralogical community usually expands these optically based morphological characteristics to include additional information available by electron microscopy-related techniques such as selected area electron diffraction (SAED) to determine a particle’s crystallographic characteristics and energy dispersive X-ray spectroscopy (EDS or EDX) to determine elemental composition. In summary, there are measurable and quantifiable mineralogical distinctions between asbestiform and non-asbestiform minerals. Unfortunately, these distinctions have not been accepted by regulatory organizations and have not been incorporated into the adopted regulations as discussed below and in Part II. Commercial, regulatory, and other asbestos definitions Over the years, asbestos has typically been defined and used in at least four different manners depending on the specific context (Glenn et al. 2008).

• Commercial asbestos definitions highlight the properties of asbestos that impart commercial value, such as high tensile strength, low thermal and electrical conductivity, high heat resistance, and high mechanical and chemical durability. • Regulatory asbestos definitions are generally intended for occupational settings and identify asbestos minerals and asbestos-containing materials (ACMs) to be regulated from those that should not be regulated. • Mineralogical and geological asbestos definitions distinguish asbestiform minerals from non-asbestiform particles (e.g. elongated single-crystal minerals and cleavage fragments) based on their crystal structure, chemistry, morphology, and/or mechanism of formation employing traditional analysis techniques which are presented separately below.

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Figure 4. FESEM secondary electron images. (a) an elongated, prismatic, actinolite ‘cleavage fragment’ particle and other irregular mineral debris from an El Dorado Hills, CA, soil sample, and (b) a bundle of chrysotile fibres and individual fibres from a Canadian asbestos mine.

• Analytical asbestos definitions provide laboratories and analysts with the tools and guidelines required to characterize, distinguish, and count regulated structures to determine their concentration in air, solids, liquids, or tissues.


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These asbestos definitions are not consistent, and difficulties arise when attempting health risk assessments. For example, the commercial asbestos definition focuses on the economically desirable properties that historically made the different asbestos minerals useful for improving the physical properties of commercial products. However, the commercial definition by itself does not offer the precision that a mineralogist would use to permit accurate scientific distinction between what constitutes an ‘asbestos’ mineral fibre and what does not, or that an analyst would ask when counting the number of fibres, or the amount in weight or volume per cent in a sample. Mineralogy and geology were two of the first scientific disciplines to describe asbestos, and to those groups, the term means that the material has a specific fibrous form, i.e asbestiform (Gunter et al. 2007). ‘Fibre’ is a textural term meaning that the material looks, and more importantly behaves, like a fibre, e.g. the material can curve and bend under force in contrast to cleavage fragments, which result from splitting of single crystals on grinding for example. In contrast to the commercial and mineralogical definitions of asbestos, the regulatory definitions were created to characterize hazardous airborne particles that could be released from asbestos raw materials or manufactured products. The Occupational Safety and Health Administration (OSHA) is responsible for regulating asbestos in the US workplace. The six minerals that are classified and regulated as asbestos by the OSHA include one serpentine and five (of approximately 80) amphibole minerals. Chrysotile asbestos has been the most commonly used form of asbestos in manufactured products (Clinkenbeard et al. 2002; Ross and Nolan 2003; Lee et al. 2008b). Regulators have defined the term ‘fibre’ in various ways based on particle aspect ratio and the method used to conduct the analyses. For example, a specific particle is typically considered to be a fibre if it has an aspect ratio greater than 3:1 by light microscopy or greater than 5:1 by electron microscopy (Gunter et al. 2007). Particle aspect ratio criteria were never meant to define asbestos but were developed as counting criteria for use in industrial settings where the source of the airborne fibres was a commercial asbestos product. The regulatory community developed these counting criteria to determine whether a fibrous particle met certain health-based concerns (Gunter et al. 2007). Unfortunately, in some laboratories, aspect ratio has become the primary or sole means of asbestos fibre definition. This practise is at odds with the definitions used by mineralogists and with risk models that do not define asbestos according to simple shape characteristics (Gunter et al. 2007). The use of a dimensionless parameter such as aspect ratio does not recognize the actual length and width dimensions of the fibre or particle and is, therefore, of little or no use when discussing exposure or toxicological outcome (Wylie et al. 1993). An arbitrarily defined small aspect ratio, e.g. 3:1, will not only include all asbestos fibres, whose ratio is typically greater than 20:1, but will also include many other non-asbestos elongated mineral particles, especially in the mixed mineral dusts found in a typical natural environment (Lee et al. 2008b). Winchite and richterite are two amphibole minerals that may occur with asbestiform morphology. The amphibole mineral group is characterized by complex elemental substitution within the crystal lattice that designates the amphibole structures. Except for the five regulated amphiboles discussed above, there are a few fibrous amphibole minerals that were never classified or regulated as asbestos, because there were no known commercial asbestiform mineral deposits. These amphiboles were not incorporated in manufactured products and, as a result, were not regulated. However, some of these non-regulated amphiboles do grow with asbestiform morphologies and occur as impurities in otherwise non-asbestos ore deposits. For example, at the vermiculite mine near Libby, MT, a small percentage (less than 10%) of non-regulated winchite [(NaCa)Mg4(Al,Fe3+)Si8O22(OH)2] and richterite


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[Na(NaCa)(Mg,Fe2+)5Si8O22(OH)2] amphibole particles, which crystallized as asbestiform fibres, contaminated the vermiculite [(Mg,Ca,K,Fe2+)3(Si,Al,Fe3+)4O10(OH)2·4H2O] ore (Ross and Nolan 2003; Gunter et al. 2007). These asbestiform amphiboles caused serious pulmonary health problems and deaths among former Libby vermiculite miners (McDonald et al. 2004; Bandli and Gunter 2006). Because of the potential health effects associated with asbestos and other asbestiform minerals, it is important to accurately identify asbestiform amphibole minerals by establishing their chemical composition and structure.


Fluoro-edenite: a newly identified calcic amphibole Fluoro-edenite {NaCa2Mg5Si7AlO22F2} is a member of the calcic amphibole mineral group and was first identified in 2001 (Gianfagna and Oberti 2001). It occurs in both prismatic and fibrous morphologies in volcanic rocks on the flank of Mt Etna near Biancavilla, Sicily, Italy (Paoletti et al. 2000; Soffritti et al. 2004; Burragato et al. 2005; Gianfagna et al. 2007). Fluoro-edenite found within the Mt Calvario stone quarry occurs as fibres, with lengths ranging from 12 to 40 μm and widths ranging from 0.4 to 1 μm (note the aspect ratio) (Burragato et al. 2005). These fibres were originally identified as tremolite/actinolite but were later found to be a distinctive amphibole fluoro-edenite. A recent TEM study of amphiboles from Biancavilla also found tremolite asbestos associated with the fluoro-edenite (Andreozzi et al. 2007). The amphiboles were identified by TEM using EDS and SAED analyses. The tremolite asbestos was characterized by fibres thinner than 0.1 μm with very high aspect ratios (greater than 50:1). For example, Figure 5 shows TEM and FESEM images obtained for a tremolite asbestos fibre found in a sample of fluoro-edenite from Biancavilla. The fibre has a length of ∼17 μm and a width of ∼0.2 μm (aspect ratio = ∼85:1). For the several amphibole minerals found in this geographic site, which ranged from fibrous to prismatic morphology, TEM/EDS data demonstrated that the aluminium content of the crystals was correlated with fibre width. The asbestiform fibres have very low aluminium content (less than 0.3 aluminium atoms per formula unit) in comparison to the prismatic edenite particles, which contained much higher aluminium contents (greater than 0.5 aluminium atoms per formula unit). These findings correspond to data published in mineralogical compendia (Dorling and Zussman 1987; Deer et al. 1997; Verkouteren and Wylie 2000). Asbestiform fluoro-edenite and tremolite from Biancavilla, as well as asbestiform winchite and richterite from Libby, MT, were never associated with commercial asbestos deposits, but rather occurred as asbestiform amphibole contaminates associated with the building stone in Biancavilla or in vermiculite ore deposits in Libby. As contaminates, they received little attention until their health effects became apparent many years after initial exposure. Future investigations on the different fibres in Biancavilla, for example, will be needed to clarify the health effects of these minerals (Andreozzi et al. 2007). Erionite – a fibrous zeolite Zeolites are a common mineral group composed of hydrated aluminosilicates of alkali and alkaline earth metals that can have both fibrous and non-fibrous morphologies. One of the fibrous zeolites is erionite {[Na2K2CaMg]4–5[Al9Si27O72]·27H2O}. Erionite usually occurs as thin fibres having a woolly appearance. Fibrous zeolites, such as erionite, are not classified as asbestos or asbestiform. However, erionite was determined to induce a high


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Figure 5. TEM image and FESEM secondary electron images of a tremolite asbestos fibre in a fluoro-edenite sample from Biancavilla, Sicily, Italy. (a) TEM full particle image, (b) FESEM full particle image, (c) FESEM image of particle left end, and (d) FESEM image of particle right end.

incidence of malignant pleural mesothelioma through environmental exposures to respirable fibres of erionite in the Cappadocian region of central Anatolia in Turkey (Dogan 2003). The volcanic tuffs in this region contain respirable fibres of erionite, and these tuffs have been excavated and quarried to provide caves and building materials for homes. Although there are significant amounts of tremolite asbestos and chrysotile in the region, extensive studies have concluded that the erionite was the probable cause of the observed mesotheliomas (Emri et al. 2002; Dogan 2003; US DHHS 2005; Dogan et al. 2006; Carbone et al. 2007; Dikensoy 2008; WDNR 2009). In general, zeolite-type materials have useful physical and chemical properties and are widely employed in industry. At one time, erionite was used commercially as a metal impregnated catalyst in the hydrocarbon-cracking process because of its open crystal structure. It has now been replaced by synthetic, non-fibrous zeolites (WDNR 2009). A minor and probably unintentional use of erionite-rich blocks was as building materials (WDNR 2009). For example, there are homes made of erionite-rich blocks in Oregon and weigh-stations made of the same materials in Nevada (Dogan 2003). The use of erionite to increase soil fertility and to control odours in livestock production has also been studied (WDNR 2009). Deposits of fibrous erionite also occur naturally in weathered volcanic ash within the western USA, including Arizona, Nevada, Oregon, California, and Utah, where they present a potential environmental exposure risk (Dogan 2003).


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Synthetic inorganic fibres Asbestos is not the only inorganic fibre that has been used to impart strength, fire resistance, thermal insulation, or electrical insulation in manufactured products. Synthetic fibrous inorganic materials have become commonplace in our everyday lives. There are a wide variety of man-made fibres in the environment such as mineral ‘wool’ and fibres made of glass, ceramics, and organic polymers (Blake et al. 1998; Carpenter and Wilson 1999; Bernstein 2007). The utility of these fibrous materials continues to spur the development of new types of fibres and new applications. For example, glass fibres or fibreglass has become a major construction material for insulating residential and commercial buildings. Glass fibres have also replaced copper wire in some intercontinental telephone systems. High temperature refractory fibres are used in industrial furnace applications. Carbon and graphite fibre composites are used to make tennis racket frames and golf clubs. Synthetic inorganic fibrous materials have largely replaced asbestos and have become commonplace. Since the recognition of asbestos as a carcinogen, there has been a worldwide concern over whether any synthetic fibres are carcinogenic, and significant research into the toxicity and biological activity of fibrous materials with different chemistries has been undertaken. Similar to studies of asbestos minerals, this research has demonstrated that the potential for lung disease is strongly related to the size and biopersistence of long, thin fibres, which are small enough to be deposited deep into the lung (Blake et al. 1998; Carpenter and Wilson 1999; Bernstein 2007).

Analytical methodologies Historical evolution of asbestos analysis One of the most important aspects of dealing with asbestos minerals is their proper identification and characterization. Many different procedures were developed over the past 100 years, commencing with personal observations of hand samples to today where we have the ability to analyse the extremely fine respirable airborne fibres. Beginning with PLM and continuing through TEM, analytical techniques were devised to identify asbestos in a bulk sample of mixed minerals, soils, or construction materials. These techniques are now used to count asbestos fibres collected on an air sample filter in an occupational setting or during an asbestos abatement project (Walton 1982; Baron 1994; Santee and Lott 2003). Historically, the development of current methods focused on the analysis of commercialgrade chrysotile asbestos found in workplaces where asbestos was being manipulated or processed (Walton 1982). Several techniques, now obsolete, were used for asbestos measurements until the late 1960s (Paulus 1942; Santee and Lott 2003). Before this time, it was not widely recognized that the fibrous nature of asbestos was intimately related to its toxicity; therefore, these early techniques typically involved collecting airborne particles and counting all ‘large’ particles (length ≥1 μm) at low magnification by optical microscopy (Paulus 1942). Thermal precipitators, impactors, impingers, and electrostatic precipitators were used to sample suspected airborne asbestos particles (Walton 1982; Baron 1994). Note that no attempt was made to accurately determine the mineral species. In the early 1960s, air filter collection of particulates and analysis were first conducted in the UK and later in the USA (Ayer et al. 1965). As studies on asbestos-induced disease increased, cellulose-based membrane filter sampling was applied and higher magnification (approximately 400× magnification) phase contrast microscopy (PCM) was initiated for counting fibres (Edwards and Lynch 1968; Walton 1982; Baron 1994). The PCM


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method involves drawing air through a mixed cellulose ester (MCE) filter to capture any particles with a focus on airborne asbestos fibres. A wedge-shaped portion of the filter is placed on a glass microscope slide and made transparent so that an area (‘field’) can be viewed by PCM. All of the fibres meeting the defined criteria for asbestos were counted and were considered to be a measure of the airborne asbestos concentration. Because the toxicity of asbestos appears to be related primarily to fibre length and width, analytical methods focus on providing information on those parameters, as well as on the total number of fibres and mineral type. In general, samples are visualized using an optical or electron microscope by trained technicians. Fibres and any other particles are typically viewed on filters at magnifications specified by the method used and counted according to the regulatory rules and capabilities of each method. The primary asbestos characterization techniques in use today include PCM, PLM, and TEM, with growing interest in SEM. These techniques are described in more detail below. Phase contrast microscopy In 1970, the first regulatory PCM method for asbestos was established to assess industrial exposures and evaluate airborne fibres in the workplace where commercial asbestos was in use (Martonik et al. 2001). In 1977, the National Institute for Occupational Safety and Health (NIOSH) issued the first PCM method – the Physical and Chemical Analytical Method (P&CAM) (NIOSH 1977), which was updated (NIOSH 7400) in 1994 following studies that showed variability in earlier determinations due in part to the qualities of the microscopes (NIOSH 1994). The NIOSH 7400 PCM method specified sample collection procedures, filter and microscope qualities, and counting protocols. The NIOSH (1977) and NIOSH 7400 PCM methods both arbitrarily count as fibres all particles visible in the microscope that are at least 5 μm long and have a minimum aspect ratio of 3:1 (Walton 1982; NIOSH 1989b, 1994). In workplaces where asbestos was mined, processed, or used, it was a safe assumption that the majority of particles fitting the simple counting rules were asbestos. Unfortunately, the dimensional criteria of the counting rules have been incorrectly used by some as a de facto definition of asbestos. There are four main advantages of PCM over other methods (OSHA 1988):

• the technique is specific to fibres and excludes non-fibrous particles from the analysis;

• the technique is inexpensive compared to electron microscopy techniques and does not require specialized knowledge to carry out the analysis for total fibre counts;

• the analysis is relatively quick and can be performed on-site for rapid determination of air concentrations of asbestos fibres; and

• the technique has continuity with the epidemiological studies that have been performed on samples over a long time span so that estimates of expected disease can be inferred from past determinations of asbestos exposures.

The main disadvantage of PCM is that it does not positively identify asbestos fibres. Other particles with fibrous morphology, or satisfying the dimension criteria which are not asbestos, may be included in the count unless differential counting is performed. It requires a great deal of experience to adequately differentiate asbestos from non-asbestos fibres. This is an important limitation when the method is used in settings where fibre concentrations with a significant non-asbestos fraction may occur. In such cases, positive identification of asbestos must be verified by PLM or electron microscopy techniques. A



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further disadvantage of PCM is that it will not resolve the finest asbestos fibres encountered during an analysis (approximately 0.02–0.25 μm diameter); so, for some exposures, more fibres may be present than are actually counted. Therefore, the PCM method is only an index of exposure and uses the assumption that the detected particles are correlated with the fibres actually causing disease (Baron 1994). The primary purpose of the standardized PCM methods was never to discriminate between asbestos and non-asbestos fibres, but only to monitor and control the airborne commercial asbestos fibres in order to reduce the incidence of disease. Since its adoption, the PCM method has become the generally accepted technique used for exposure and risk assessments from which dose response assessments are derived (Walton 1982; Bailey 2004; Berman 2006). However, it should be noted that the PCM minimum 3:1 aspect ratio was not based on any scientific definition of asbestos characteristics or the toxicological significance of the ratio but simply reflected a need to improve consistency in ‘exposure’ measurements by analysts from different laboratories. A dimensional criterion specifying a minimum aspect ratio of 3:1 for particles longer than 5 μm is not valid for the analysis of mixed mineral dusts simply because in most natural environments there are too many non-asbestos particles that would fit this simple definition. Especially in mixed mineral environments, the standard PCM methods do not provide enough information to differentiate between asbestos and non-asbestos mineral particles. The PCM technique can be extended beyond the standard methods to provide additional information concerning airborne fibres in mixed mineral environments., i.e. to address the wide spectrum of particles that may be present in airborne samples. The particles can range from short, wide fibres to very long, thin fibres, and there is a general consensus among health experts that long, thin fibres present more of a health risk than short, wide fibres. However, a controversy exists concerning the particles that fall in the middle of this length–width continuum. The risk of these intermediate-sized fibres is not well understood. Because the health effects of the intermediate-sized fibres are not established and the contribution to disease from very thin non-PCM countable fibres is not quantified, there is uncertainty as to how to handle these particles. Should the intermediate-sized particles be differentiated from the long, thin asbestos fibres? If it is found that sorting of the particle population is necessary from a risk perspective, what is the most cost-effective method to achieve this goal? Some type of screening method is necessary as an initial step in the analytical process of mixed mineral environments. The ultimate goal of the screening step would be to provide information on the size distribution of the particles and fibres. If no high-risk fibres are detected, then no additional analysis may be necessary. If an elevated population of potentially high-risk fibres is discovered, the most appropriate technique to accurately measure and unequivocally identify the presence of asbestos will need to be used. The American Society for Testing and Materials (ASTM) recently implemented a screening method based on the PCM technique for determining an index of occupational exposure to airborne fibres in mines, quarries, or other locations where ore may be processed or handled (ASTM 2006). ASTM recognized and addressed the complexity of analysing for asbestos in such mixed mineral dust atmospheres by developing a rapid screening optical protocol. This protocol preserves the information obtained in the conventional PCM analysis but adds discriminate analysis to identify samples with significant numbers of long, thin fibres. The method provides an estimate of the fraction of counted fibres that may be asbestos by classifying the fibres (longer than 5 μm and an aspect ratio of 3:1) into three groups: (1) fibres that show curvature, splayed ends, or appearance of bundles; (2) fibres that are longer than 10 μm or thinner than 1.0 μm; and (3) all other


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countable fibres. If an elevated content of long thin fibres is detected optically, the ASTM method recommends supplemental electron microscopy analysis for verification. This type of approach, which differentiates particles of different size ranges and different physical characteristics, is the first step in screening mixed mineral samples. Following the screening step for mixed mineral environments, additional analyses may be necessary to accurately measure and unequivocally identify asbestos. Although the complete chemical, morphological, and crystallographic analysis of every particle in a mixed mineral sample would be ideal, it may not be realistic because of time and cost limitations. Maximum effort needs to be focused on the identification and classification of the particles that pose the most risk. More uncertainty might be acceptable with a full identification of the particles discriminating those that pose less of a risk. The challenge for scientists and future policy makers will be to streamline and efficiently organize the steps most appropriate for analysing fibres that present the most risk in airborne samples of mixed mineral dusts. Polarized light microscopy Optical microscopy, specifically PLM, has been used to analyse rocks and minerals for well over 100 years, and it is a well-known analytical procedure (McCrone 1980; Baron 1994; Bloss 1999; Santee and Lott 2003; Gunter 2004; Gunter et al. 2007). Tables of the optical properties of the many hundreds of mineral species, as determined by oil immersion techniques, were first published in 1900 (Larsen and Bermen 1934). Originally used for examining thin sections of rocks or mounts of mineral grains, PLM is now used for determining the mineral content of materials that may include asbestos such as soils, building materials, raw materials used in various manufacturing processes, and ore/host rock samples; this is different than the PCM analysis of MCE filters discussed above. For PLM analysis of bulk samples, it is assumed that the sample being analysed has been properly collected, documented, and is representative of some larger population of material. The US Government arbitrarily classifies a material as ‘asbestos-containing’ if the concentration of asbestos exceeds 1% by weight. In 1982, the Environmental Protection Agency (EPA) issued an Interim Method (US EPA 1982) that created a uniform procedure for identification and quantification of asbestos in bulk building materials using the PLM. The method, published in the Code of Federal Regulations (currently at Appendix E to Subpart E, 40 CFR 763), required the use of ‘point counting’ or modal analysis techniques to quantify the asbestos content of the material. Alternatively, a visual estimation technique can be used for quantification (NIOSH 1989a; Crane 1992). NIOSH recommends using visual estimation of the sample by PLM to quantify the amount of asbestos. Recognizing that there were problems with established protocols for the analysis of bulk and raw materials, several states issued their own PLM methods. The California Air Resources Board (CARB) issued Method 435 for use in determining the asbestos content in serpentine aggregate in storage piles, on conveyor belts, and on surfaces such as roads, shoulders, and parking lots (CARB 1991). The State of New York issued a PLM method that utilizes a stratified point count method for quantification of the asbestos content of materials with ‘substantial amounts of asbestos’, but says a visual estimation of the content is acceptable (ELAP 1990). An optical procedure published by the European Union (EU) was specifically designed for the determination of low concentrations of asbestos in bulk materials (Schneider et al. 1997). The procedure is similar to that of OSHA in that it uses a combination of PLM with



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PCM. However, the EU method specifies a number of procedures to use in removing the matrix material, thus improving the precision and accuracy of the asbestos determination. The standard method using PLM for asbestos analyses in the USA is the EPA 600 Test Method (Perkins and Harvey 1993). This method was developed for asbestos determination in bulk materials or bulk commercial products. In general, the bulk material is examined for sample heterogeneity. Macro characteristics such as obvious layering in the material, colour, fibrous components, and general appearance are noted. The samples are ground, teased, or chemically treated to disassociate the fibres from the matrix material. Multiple grain mounts are prepared and analysed. If asbestos is observed, the type is identified by refractive index measurements, usually with the aid of central stop dispersion staining as described by Bloss (1999). After identification of asbestos type or types, it becomes necessary to quantify the asbestos content. This is done by visual area estimation or for low-level concentrations; a 400-point count is usually employed for quantification. There are appreciable errors associated with visual area estimation because of analyst bias and sample heterogeneity. In addition, the regulated limit of asbestos in bulk materials is always expressed in weight per cent, but the measured asbestos content is based on volume or area per cent, creating an additional source of error. Differences in a particle’s density and size will affect analytical results, but depending on the percentage of asbestos present, this error may or may not be significant. For example, a bulk material containing 20% by weight asbestos with an error of 5% still exceeds the Federal OSHA allowable regulatory limit of no greater than 1 wt.%, but for materials containing very low concentrations of asbestos, a relatively small error may be the difference between required abatement and no action. An example of the errors associated with the use of PLM methods is shown in the National Volunteer Lab Accreditation Program (NVLAP) proficiency testing and accreditation programme, which is administered by the National Institute for Standards and Technology (NIST) (Richmond and Faison 2003). Proficiency testing takes place bi-annually to accredit and reaccredit laboratories for asbestos identification and estimation. The results for four samples from the 231 laboratories participating in the August 2007 proficiency testing (NVLAP 2007) showed that, for the qualitative part of the analysis, 3% of the laboratories incorrectly identified asbestos in Sample 1 and 0.4% of the laboratories incorrectly identified asbestos in Sample 2. No laboratories misidentified the type of asbestos in the two samples containing chrysotile. However, the ‘acceptable range for the analysis’ overlapped the regulatory limit of 1.0%, i.e. Sample 3 was an ACM and yet within analytical uncertainty it could be reported as under the regulatory limit, whereas Sample 4 was below the 1.0% limit and yet within analytical uncertainty it could be reported as over the regulatory limit. Spindle stage-assisted PLM for the characterization of asbestos The spindle stage is an accessory to the PLM and provides the ability to rotate a particle about a horizontal axis with respect to the plane of the microscope stage (Bloss 1981, 1999; Gunter 2004; Gunter et al. 2004; Dyar and Gunter 2008). For routine PLM asbestos analysis, such as the EPA 600 method (Perkins and Harvey 1993), the spindle stage has no practical use. However, for studies requiring detailed optical characterization of the mineral species, the spindle stage is invaluable. In brief, the spindle stage allows for dimensional and optical characterization of single crystals or aggregates, with the added benefit of being able to analyse the same particle with X-ray diffraction and electron beam instruments.

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Verkouteren and Wylie (2000) studied the variations in the amphibole group tremolite to ferro-actinolite, a solid solution series (changes in Fe content of the minerals), by determining the unit cell, composition, in association with optical properties, and habit. The optical properties were analysed using a spindle stage. In the study of 35 samples classified as asbestiform, 19 contained fibres of sufficient size to measure the three principal refractive indices of a mineral particle (ie a, b, and g), whereas the remaining 16 samples each exhibited anomalous optical properties. A later study by these authors explored the anomalous properties of asbestiform amphibole said to have a ‘byssolitic’ habit (Verkouteren and Wylie 2002). They defined byssolitic as: . . . samples that occur as single fibers, sometimes loosely aligned, that have a vitreous luster and are easily reduced to a powder by hand grinding. Individual ‘byssolitic’ fibers are often tabular in cross-section with well-developed (100) faces and widths of at least a few micrometers.

These byssolitic particles are fibrous amphiboles but do not meet the dimensional width criteria of the asbestiform habit. Using the spindle stage, Verkouteren and Wylie (2002) characterized the anomalous optical extinction of byssolitic fibres and concluded that these properties were most likely a result of twinning (a characteristic of some mineral forms) on the (100) crystal plane. A study by Sanchez et al. (2008) examined the optical characteristics of tremolite samples with differing morphologies. The study found that fibres of narrow width, resulting from crystal growth rather than cleavage, exhibited near-zero extinction angles, similar to the results reported by Verkouteren and Wylie (2002) and distinct from larger crystalline size particles where the angle expected is greater than 10 degrees. Brown and Gunter (2003) studied the optical properties of the winchite-richterite series of amphiboles from the former vermiculite mine near Libby, MT, the NIST 1867 SRM tremolite, and a tremolite from the University of Idaho collection and found that both the NIST and Libby amphiboles were predominantly flattened on (100). Another observation on the NIST tremolite was that upon rotation around the spindle axis, some fibres, which originally appeared to be single crystals, were in fact fibre bundles. Hence, the spindle stage enables greater accuracy in describing the natural morphological expression of mineral particles. Bandli and Gunter (2001) also used a spindle stage to perform optical, single-crystal X-ray diffraction, and compositional characterization on eleven amphibole particles from the Libby mine. By employing the spindle stage mount, they obtained refractive indices, as well as unit cell data, and elemental composition on the same Libby amphibole particles with TEM techniques, thus allowing them to determine subtle correlations in physical properties. For instance, they found that, as the particles took on more ‘asbestos-like’ morphological properties, the partial birefringence (i.e. b–g) of the particles decreased, and the particles exhibited anomalous optical properties. Their SEM examination showed that the (100) face was commonly expressed by the fibrous amphiboles and suggested this crystal morphology was an expression of different atoms in the crystal structure being exposed; these compositional/structural distinctions may be related to the relative difference in adverse health effects of asbestiform amphiboles compared to non-fibrous massive amphibole analogues. More work is needed on the characterization of the fibrous amphiboles coupled with studies by medical researchers to better understand the pathology of asbestos-related diseases. The spindle stage is advantageous for accurately delineating the mineral phase. Detailed work, such as those presented by Verkouteren and Wylie (2000) and Brown and Gunter (2003), could potentially give new insights and provide a foundation for improved analytical procedures as well as provide insights into mechanisms that lead to the hazards


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and risks associated with asbestos. Bandli and Gunter (2001) also employed the spindle stage for characterizing the length, width, and thickness of a particle facilitated by looking at the particle in different orientations and also for transferring the particle to an SEM for collection of additional data. Sanchez (2007) and Gunter et al. (2007) also used this method to show the differing morphology of amphiboles from Libby, MT.


Transmission electron microscopy TEM provides particle projection images in the magnification range 1000× to 1,000,000×, which allow the determination of particle shape and identification of the crystal structure of even the smallest asbestos fibres (Walton 1982; Baron 1994; Santee and Lott 2003). Particle crystal structural data are determined through SAED and, when combined with EDS, establish elemental composition allowing accurate mineral identification (Walton 1982; Baron 1994; Santee and Lott 2003) (SAED and EDS are described in more detail below). Figure 6 shows a TEM image of a single magnesio-riebeckite asbestos fibre, with a length of approximately 10 μm and a width of approximately 0.07 μm (aspect ratio of approximately 143:1). The particle was found in an air sample from Libby, MT, and was identified as magnesio-riebeckite based on SAED and EDS measurements. The particle’s high aspect ratio, parallel smooth sides, and perpendicular ends are characteristic of asbestos fibres and show slight curvature suggesting flexibility. TEM is widely regarded in the USA as the most reliable technique for asbestos analysis owing to its high image resolution, electron diffraction, and chemical identification capabilities (Samudra et al. 1977; Walton 1982; NIOSH 1989b; Baron 1994; Santee and Lott 2003; Glenn et al. 2008). TEM can be used for the analysis of bulk air and water samples (Walton 1982; Baron 1994; Santee and Lott 2003). Airborne fibre samples for TEM analysis are typically

Figure 6. TEM image of a single magnesio-riebeckite asbestos fibre from a Libby, MT, air sample. (Fibre length ∼10 μm, width ∼0.07 μm, aspect ratio ∼143:1.)


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collected onto an MCE membrane or polycarbonate (PC) membrane filter (Baron 1994). For the MCE filter type, the filter is chemically collapsed to form a smooth upper surface on which the collected fibres are trapped. The filter is etched using a low-temperature plasma asher (Federal Register, 198740 CFR 763, Appendix A to Subpart E) exposing any fibres that are trapped on or near the surface of the filter. The filter is coated with a thin conductive carbon film embedding the particles collected on the filter surface. The filter is then dissolved using an acetone vapour technique, creating an identical carbon replica of the filter with the particles embedded into it. The carbon film replica can be transferred to a copper TEM locator grid ready for TEM analysis. This sample preparation method is referred to as the direct transfer approach because fibres are transferred to the carbon film with minimal disturbance (Baron 1994). An alternate approach, referred to as the indirect transfer technique, is to liberate the particles from the filter by either sonication, dissolving the filter in an appropriate solvent, or ashing the entire filter in a low-temperature furnace. Once the particles are liberated, they are then suspended in a measured portion of pH-adjusted distilled water, sonicated (an ultrasonic bath) briefly to disperse the particles in the suspension, and an aliquot of the suspension is then deposited onto either a PC or MCE filter for final transfer to the grid carbon film (Baron 1994). With the indirect technique, the optimum particle loading for TEM analysis can be obtained; however, the sonication and suspension process can change the apparent size distribution of the particles and fibres by breaking apart agglomerates or asbestos bundles into single fibrils potentially causing erroneous results in population-based analytical protocols (Baron 1994). Evaluations of ambient air samples for asbestos were first performed in the 1970s using electron microscopy, and the first recognized EPA TEM procedure for air samples was written by Samudra et al. (1977). The method, revised in 1984, is known as the Yamate Method (Yamate et al. 1984) and, although never officially published by the EPA, it ‘became the de facto standard analytical TEM procedure for airborne measurements in the United States’ according to a report by the Health Effects Institute (HEI-AR 1992). The first fully promulgated air protocol produced by the EPA was a TEM method for testing the cleanliness of air in schools following abatement of asbestos-containing building materials. Under the authority of the Asbestos Hazard Emergency Response Act (AHERA), the EPA developed a rapid TEM method for use in clearance testing at abatement sites (Federal Register 1987). The method specified sample collection procedures and required a direct transfer preparation method. To reduce the analysis time, the AHERA method did not require recording of fibre dimensions, but did require listing the fibres as either greater than 5 μm or less than 5 μm in length. One important change of the AHERA method (Federal Register 1987) over the Draft Yamate Method (Yamate et al. 1984) was the increase in minimum aspect ratio from 3:1 to 5:1. Many experts on the AHERA committee argued for 10:1 or 20:1 as the minimum, but the decision was not acceptable to the committee members who were anxious to accumulate and use the maximum amount of data available over the longest time exposure to estimate risk. In addition, a minimum length for asbestos fibres (0.5 μm) was specified for the first time in order to improve the reproducibility of fibre counts between different analysts and/or laboratories. Recognizing that not all airborne fibres are asbestos and that the OSHA regulated only asbestos fibres, NIOSH independently issued a TEM asbestos method in 1989, NIOSH 7402 (NIOSH 1989b), which was designed for use in conjunction with PCM [i.e. NIOSH 7400 (NIOSH 1994)] to allow the determination of the asbestos versus non-asbestos proportion of countable PCM fibres. The NIOSH 7402 method specifies a magnification comparable to the magnification used in the optical microscope and counts fibres longer than 5 μm, wider than



819

0.25 μm, and with an aspect ratio of at least 3:1. OSHA permits the use of the NIOSH 7402 method when analysing air samples for OSHA compliance purposes (when performed in conjunction with PCM). An international TEM analytical method, ISO 10312, was also developed for testing commercial mineral species (ISO 1995). The TEM methods described above are best suited for counting short fibres. There are several factors that contribute to the poor statistics for long, thin fibres when analysing an asbestos population of fibres via TEM. In an ambient airborne asbestos fibre population, the fibre length distribution can vary widely depending on the source, but typically 1–10% of the fibres are longer than 5 μm in length and only 0.1–1% of the fibres are longer than 10 μm (Chatfield 1983). As a result, the measurement of all particles, as current TEM methods require, creates an intrinsically higher uncertainty for the concentration of long, thin fibres than for the fibres less than 5 μm in length. Further, there is an increased likelihood that fibres 10 μm or longer in length will intersect a sample grid bar, making it difficult to accurately determine the fibre total length (Dehoff and Rhines 1968; Yamate et al. 1984). These problems add uncertainty to the proportion of fibres longer than 10 μm that may be missed during a routine analysis. Inherent inaccuracy in the measurement of the concentration of long, thin fibres can lead to increased uncertainty in the risk estimates. To measure the length of fibres longer than 20 μm with the same precision as the width is measured, magnifications between 1000× and 10,000× are needed. This will necessitate accurate calibration of the TEM screen through all magnifications used for analysis, not just the scanning magnification. Low magnifications are required to measure fibre length and high magnifications are required to measure fibre width as well as to characterize surface texture and the nature of the fibre ends (ASTM 2002). Therefore, to avoid the issues discussed above, SEM can be more easily used to search for fibres longer than 10 μm in length. Such measurements are accomplished much more readily in a modern digital SEM than in a conventional TEM because of the relative ease of rapidly switching between low and high magnifications, and because sample grid bars are not typically present with an SEM sample preparation (Goldstein et al. 2003). The usefulness of the TEM is limited in mixed mineral environments because of the nature of the TEM image. A TEM image is a projection of the specimen created from the electrons that pass through the specimen (see Figures 5–7) (Williams and Carter 1996). The actual shape of the particle, as seen by TEM analysis, is the projection of the overall particle shape; however, the actual three-dimensional particle shape may be very different from what is observed in the TEM image. SEM can be used to obtain more detailed information compared to TEM on the true particle shape and morphology. SEM provides complementary information to TEM for asbestos characterization and is discussed below. SAED and EDS with TEM SAED and EDS are used in conjunction with TEM during the analysis of asbestos and other mineral particles (Baron 1994; Santee and Lott 2003; Gunter et al. 2007). SAED is accomplished by focusing the electron beam on selected particles and capturing the resulting diffraction pattern photographically, which corresponds to the specific and unique diffraction characteristics of the sample’s crystal structure. SAED patterns obtained for unknown particles can be measured and matched to published diffraction data for known mineral species. EDS is a qualitative and quantitative analytical technique whereby the electron beam causes the emission of characteristic X-rays that provide an elemental ‘fingerprint’ of the


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Figure 7.


TEM image of a crocidolite fibre from a human lung tissue sample.

composition of the imaged particles. The use of SAED and EDS in conjunction with TEM for identifying commercial chrysotile asbestos fibres in occupational settings is widely practised and accepted. However, a greater level of scientific rigour must be applied when using these techniques for mineral speciation in mixed mineral environments. The SAED patterns and EDS spectra for amphibole asbestos minerals are similar to many non-asbestos minerals. The methods commonly employed leave it to the expertise of the analyst to identify and evaluate non-asbestos particle interferences (Van Orden et al. 2008). SAED analysis of chrysotile is relatively straightforward because the diffraction pattern is unique. By comparison, obtaining SAED patterns of amphibole structures is not as straightforward and can be a complex and time-consuming process depending on the level of analysis required (Longo 1990). In addition, the SAED patterns for amphiboles are often similar to other minerals. For example, an SAED pattern exhibiting a row of evenly spaced reflection spots of around 5.3Å has been improperly used by some commercial TEM laboratories analysing air samples to definitively conclude that a particular elongated structure with an aspect ratio of 3:1 or 5:1 is amphibole asbestos regardless of conflicting chemical data from EDS analysis and conflicting particle morphology (Van Orden et al. 2008). The 5.3Å spacing is insufficient for determining mineral speciation as it is not unique to amphiboles; it is found in other minerals such as pyroxenes, talc, micas, and clays such as vermiculite (Van Orden et al. 2008). Van Orden et al. (2008) have recently reported that the angle, phi or Φ, between two rows is of greater use than a 5.3Å row spacing alone for differentiating amphibole structures from non-amphiboles. Using the rows of reflection spots and phi, one can define more precisely the hkl (crystallographic) plane in the diffracting crystal precisely and increase the accuracy of precise mineral identification. Multiple SAEDs on the same particle in different orientations of the crystalline lattice would also contribute to a definitive mineral identification and are necessary



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when the particle is particularly thick or when orientation relative to the electron beam, a TEM sample grid bar, or the sample matrix interferes. It is for this reason that more scientifically rigorous TEM asbestos protocols such as ISO 13794 and ISO 10312 recommend this approach for identifications of amphiboles. EDS systems are common spectroscopic tools on both TEM and SEM instruments (Kevex Corporation 1983, 1988), and EDS is by far the most routine spectroscopic method for chemical analysis related to asbestos identification. However, EDS is perhaps the most subjective of the diagnostic tools available for electron microscopy. Results can be significantly affected by the quality of the detector, collection time, orientation of the particle relative to the detector, orientation relative to the TEM sample grid bars, orientation relative to other particulate, and particle thickness. To help mitigate these issues, it is imperative that the X-ray detector be maintained in optimum condition and that the unknowns are compared to results on standard material collected on the same detector in the same time period and on particles of comparable thickness and orientation. However, even with the use of standards, EDS should be considered only a semi-quantitative technique because overlapping peaks and the typical background noise will limit sensitivity, especially for light elements. The current mineralogical nomenclature of amphiboles was defined by Leake et al. (1997). Applying the Leake rules to EDS compositional results provides one clue to the identification of the particle. However, the Leake nomenclature applies only to amphiboles and does not differentiate amphiboles from non-amphiboles. Unknowns must be evaluated utilizing the information on non-amphibole mineral phases (Morimoto 1988). As noted by most analytical procedures, there are numerous minerals that have chemistries similar to the regulated amphibole minerals such as talc and pyroxene. Therefore, owing to the similarity of the chemistry of the five regulated amphiboles with other minerals, it is necessary, at a minimum, to examine the SAED pattern for the unknown mineral particle in addition to the EDS results. Applying TEM techniques and methods to distinguish amphibole asbestos from nonasbestos in a mixed mineral environment is a more scientifically sophisticated analytical technique than has been historically required for the identification of commercial grade chrysotile in industrial hygiene air samples. When these methods are incorrectly applied to mixed mineral environments, which do not have commercial asbestos as the primary airborne fibre, non-asbestiform amphiboles can be misidentified as asbestiform amphiboles. The amphibole mineral group contains a large number of species with a wide range of chemistries. Most mineral environments will contain a variety of minerals as well as a complex blend of fibres, cleavage fragments, and elongated rock fragments (Lee et al. 2008b). Stringent methodology utilizing several techniques and scientific rigour are required to correctly identify and quantify a specific mineral within a mineral assemblage. Misidentification can result in costly reformulation of harmless products and/or unnecessary asbestos abatement projects. The flowchart procedure shown in Figure 8 was applied to a TEM study of several mineral samples whose mineral morphology could be observed in hand specimens as ranging from asbestos fibres to non-asbestos mineral particles (Van Orden et al. 2008). The procedure involves the stepwise TEM examination of particle morphologies, such as aspect ratio, shape of the particle sides and ends, particle curvature, etc., as well as the particle SAED and EDS characteristics. Details on the proper use of this flowchart are presented in Van Orden et al. (2008) and Table 2. Table 2 presents data on several types of material particles tested using the flowchart and indicates with a reasonable degree of accuracy the classification of asbestos versus non-asbestos in mixed mineral samples. The error rate using the flowchart was estimated at 5–10% (Van Orden et al. 2008).

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Single crystal

Aspect ratio ≥ 5:1 No Yes

Parallel sides


No – angular or stepped

Yes

Curved structure

No

Perpendicular ends Yes

No – tapered or irregular

Yes

Uniform diffraction contours

No – irregular or way

Yes

SAED pattern 75° ≤ Angle ≤ 90°

No

Yes

Does SAED show twinning?

No

Yes

No

Yes

Is EDXA consistent with amphibole?

Does SAED show super-lattice?

Yes

Are multiple SAED consistent w/ amphibole?

No

Yes

Asbestos

Non-asbestos

Figure 8. Flowchart showing the characteristics that can be used to determine whether a particle is asbestos (Van Orden et al. 2008).


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Table 2. Application of the TEM flowchart classification procedure to amphibole samples of known morphology. Mineral type Amosite Crocidolite


Jamestown tremolite New York tremolite NIST SRM 1867a North Carolina tremolite

Description of hand sample Commercial product, aerosolized to obtain a respirable fraction Asbestos amphibole from an ore sample, very long fibres Fibrous tremolite used in animal studies, moderate fibre length Tremolite ore sample, acicular appearance in hand sample Mixed tremolite fibres and non-fibrous tremolite particles Fibrous tremolite, moderate fibre length

Asbestos classification (%)

Non-asbestos classification (%)

95

5

100

0

70

30

2

98

11

89

84

16

Samples that appeared to be fibrous in hand specimens (i.e. amosite, crocidolite, Jamestown tremolite, and North Carolina tremolite) showed very high percentages of fibres classified as ‘asbestos’ using the TEM protocol, whereas the non-asbestos tremolite sample from New York indicates the population of fibres to be primarily non-asbestos. The NIST tremolite sample (1867a SRM) was found to be a mixture of both fibrous and nonfibrous particles (Van Orden et al. 2008).

SEM and FESEM SEM operates by focusing a beam of electrons onto the sample surface and scanning the beam over a selected area (Goldstein et al. 2003). A variety of signals are generated from the interaction of the primary beam of electrons and the specimen, including secondary electrons, backscattered electrons, Auger electrons, characteristic X-rays, and other photons of various energies. The low-energy secondary electrons are scattered from the sample surface and detected above the surface synchronously with the beam scan rate and provide surface detail and morphological information about the specimen. Asbestos and other mineral particles can be observed at high magnification and with high resolution, and, in addition, SEM can provide semi-quantitative elemental analysis information using EDS similar to the detail presented above under TEM (Chatfield 1983). SEM has evolved over the past 30 years into a reliable and effective method for the enhanced morphological characterization of asbestos fibres (Lee et al. 1977, 2008a; Middleton 1982; Dorling and Zussman 1987; Platek et al. 1992; Chisholm 1995; Hartikainen and Tossavainen 1997; Meeker et al. 2003, 2006; Harris et al. 2007; Lee and Strohmeier 2007; Strohmeier et al. 2007a, 2007b; Bunker et al. 2008a, 2008b; Huntington et al. 2008). Past concerns over the visibility of fibres in SEM images have been alleviated by the advent of digital microscopy (Platek et al. 1992; Williams and Carter 1996; Lee and Strohmeier 2007). Unfortunately, only a few standard SEM analytical methods (primarily European) for asbestos exist (WHO 1985; VDI 1991, 1994, 2004; Frasca et al. 2000; ISO 2002). The development of high-resolution FESEM instruments makes SEM an attractive technique to augment TEM analysis and offset some of its morphological limitations for complex asbestos sample characterization. FESEM instruments provide much higher magnifications (e.g. greater than 100,000× and up to more than 1,000,000× for some instruments) and higher image resolution compared to traditional SEMs and most TEMs

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(Goldstein et al. 2003). In addition, modern digital FESEMs can also produce high-resolution three-dimensional ‘stereo pair’ images on the nanometre to micrometre scale (Goldstein et al. 2003). Three-dimensional stereo pair SEM images can be used to help determine whether a given particle is asbestiform or non-asbestiform because of the ability to ‘see’ the third dimension. For example, Figure 9 compares a TEM image and FESEM secondary


(a)

(b)

Figure 9. Comparison of a TEM image and an FESEM secondary electron image of a non-asbestiform richterite particle from Libby, MT. (a) TEM image and (b) FESEM image.



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electron image of a richterite particle from Libby, MT. The TEM image shows an elongated, richterite crystal, with parallel sides, and 16:1 aspect ratio. The FESEM image illustrates the prismatic nature of this particle based on its smooth cleavage planes, angled and stepped ends, and parallel sides. Together, these TEM and FESEM data identify and validate the non-asbestiform morphology of this prismatic richterite crystal. Bunker et al. (2008b), Harris et al. (2007), Lee et al. (2008a), Lee and Strohmeier (2007), and Strohmeier et al. (2007a, 2007b) described an enhancement of the Yamate TEM airborne asbestos method (Yamate 1984) for the analysis of mixed mineral dusts by adding FESEM imaging of each particle that had an aspect ratio of 3:1 or greater. The procedure involved the transfer of the TEM sample grid into the FESEM after the TEM/ EDS/SAED analyses were completed and the relocation of the particles meeting the ≥3:1 aspect ratio criteria on a particle-by-particle basis. The developed protocol required the morphological (by TEM and FESEM), crystallographic (by SAED), and chemical (by EDS) characterization of each particle including the collection of FESEM secondary electron images of the full structure, of both structure ends, and of the particle surface. Stereo pair FESEM images of the full particle and the particle surface provide the depth perception information not obtainable from a single secondary electron image. Figure 10 shows examples of the TEM and FESEM images that were collected in these studies for a bundle of South African crocidolite fibres. The true morphology of the particle is much more evident in the FESEM secondary electron images compared to the projection TEM image. (Note: stereo glasses are required to properly view stereo pair images.) FESEM imaging also demonstrates that amphibole cleavage fragments have dimensions and morphological features very different than true asbestos, as shown in Figure 4. The overall morphology of elongated mineral particles characterized by FESEM can be described using the seven primary classifications: fibre, acicular, prismatic, bladed, bundle, columnar, and irregular. Details on the particle classification system and definitions for these particle morphology categories can be found elsewhere but are part of the Glossary (Harris et al. 2007; Strohmeier et al. 2007a). Similar classification terms were originally developed in 1977 by the US Bureau of Mines (Campbell et al. 1977) to differentiate between common mineral rock fragments and their asbestiform varieties using optical microscopy; the US Bureau of Mines classification was widely recognized and is still used for the characterization of minerals and mineral dusts. The FESEM particle relocation process and analysis are essential and complementary elements to the TEM analysis for accurate particle-by-particle examination of mixed mineral dusts. Comparison of TEM and FESEM images (Figures 11 and 12) illustrate the ability to more sharply visualize small individual particles and identify their true morphology. Although there are no standard SEM protocols for mixed mineral environments, the United States Geological Society (USGS) did note in a recent study on the use of SEM/ EDS and other techniques for characterizing environmental asbestos particles that ‘. . . the emerging practise of fully characterizing all particles of potential concern, both chemically and morphologically, will aid in developing appropriate analytical procedures . . .’ (Meeker et al. 2006); Figures 9–12 demonstrate this technique. Clearly, SEM and FESEM, in particular, are techniques that can be used in the development of accurate particle-by-particle analysis procedures. Complete characterization (e.g. chemistry, crystallography, and morphology) in mixed mineral dust samples on a particle-by-particle basis using complementary TEM/SEM protocols aids in the development of and improvement of standard analytical procedures. In addition, characterization data can assist in the interpretation of epidemiological data and assessment of potential health risks that are needed for sound regulatory policies.


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Figure 10. TEM image and FESEM secondary electron images of a bundle of South African crocidolite fibres. (a) TEM full particle image, (b) FESEM full particle image, (c) FESEM image of the particle left end, (d) FESEM image of the particle right end, (e) FESEM image of the particle surface, and (f) FESEM stereo pair image of the particle surface.

Another recent advance in imaging technology includes a high-resolution SEM instrument (Hitachi Model S-5500) that combines the benefits of FESEM and low-kV scanning transmission electron microscopy (STEM) in one instrument. The FESEM and STEM images can be acquired simultaneously, in addition to high-resolution EDS analysis and element mapping, without moving the sample. The high-resolution STEM capabilities are demonstrated in Figure 13, which shows a ‘bright field’ (i.e. transmission) STEM image of a portion of a South African crocidolite fibre with a width of approximately 0.06 μm. The STEM image reveals very fine parallel lines on the fibre surface. The physical nature of these lines is not specifically understood at this time, but the lines may represent the

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Figure 11. Prismatic euhedral single-crystal richterite particle in an air sample from Libby, MT. (a) TEM image, (b) FESEM secondary electron image, and (c) FESEM secondary electron stereo pair image. The particle was supported by other adjacent mineral debris and is projecting outward from the plane of the sample grid in the FESEM secondary electron image, and this was not evident in the TEM image. The TEM image indicates that the particle has an aspect ratio ≥3:1 and therefore should be counted as an ‘asbestos fibre’ following the simple counting rules. The FESEM secondary electron image, however, demonstrates that the particle is not asbestiform. The particle has a prismatic morphology with crystalline faces and a rough surface. Particle orientation and morphology can be demonstrated even more effectively by FESEM stereo pair imaging (Strohmeier et al. 2007a).



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Figure 12. Irregular muscovite mineral particle in an air sample from Libby, MT. (a) TEM image, (b) FESEM secondary electron image, and (c) FESEM secondary electron stereo pair image. The TEM projection image indicates a particle with an aspect ratio ≥3:1, and therefore this particle would be counted as an ‘asbestos fibre’. However, the FESEM secondary electron and stereo pair images indicate that the particle is not asbestiform; it has a sheet-like structure and is projecting outward from the plane of the sample grid. The actual shape and dimensions of the particle are quite different than those implied by the two-dimensional TEM image. In addition, EDS and SAED analyses indicated that this particle was muscovite {KAl2(AlSi3O10)(F,OH)2}, a common rock-forming mineral that has a layered sheet structure and belongs to the mica group of minerals (Strohmeier et al. 2007a).



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Figure 13. Bright-field STEM image of a portion of a South African crocidolite fibre.

inter-planar spacings in the crocidolite crystal structure. Although electron diffraction is not available on this instrument, this technology may be useful for convenient STEM and FESEM morphological and elemental characterization of asbestos and non-asbestos particles. Cross-sectional samples of mineral particles observed in an FESEM can provide information on shape, dimensions, and crystal growth that is difficult to obtain with other methods of sample preparation and analytical techniques. To do so, Huntington et al. (2008) examined cross-sections of mineral particles that were fractured across the particle length after preparation by vacuum impregnation. Three asbestos samples were analysed: NIST chrysotile 1866 SRM, NIST amosite 1866 SRM, and ore grade South African crocidolite. These samples were chosen for this study because they represented the three major types of commercial asbestos historically used in the USA (Virta 2002). The FESEM secondary electron images in Figure 14 reveal increasing fibre cross-section dimensions from chrysotile through crocidolite to amosite. The two amphibole fibre samples tended to be more variable in width than chrysotile, which is consistent with previous studies (Steel and Wylie 1981). The FESEM cross-section image (Figure 14a and b) illustrates chrysotile’s smooth rolled cylindrical crystal structure. By contrast, the cross-section images of crocidolite and amosite fibres (Figure 14c–f, respectively) appeared as aggregates with circular to rectangular cross-sections, which is consistent with previous studies (VDI 2004). The original NIOSH ‘White Paper’ Roadmap for Scientific Research on Asbestos (Middendorf et al. 2007) noted that the cost of using TEM and/or SEM for routine asbestos sample analysis would be considerably higher than PCM analysis, and the turnaround time for sample analysis would also increase substantially. The Roadmap proposal did not emphasize research in developing new SEM methods. While this argument has some merit, it should not prevent the development and regulatory adoption of advanced electron microscopy methods. When dealing with matters of public policy and protecting the public health,



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Figure 14. FESEM secondary electron images. (a) and (b) chrysotile cross-sections; (c) and (d) crocidolite cross-sections; and (e) and (f) amosite cross-sections (Huntington et al. 2008)

it is vital that the best scientific methods are used to provide accurate measurements for risk assessments. The original NIOSH Roadmap proposal stated that any routine use of electron microscopy methods for counting and sizing fibres would require an analysis of inter-laboratory and inter-operator variability. Assessment of inter-laboratory and interoperator variability should not pose a significant problem to implementing improved electron microscopy methods because various laboratory accreditation organizations and round-robin testing protocols already exist to evaluate laboratory and analyst competence for the PCM and TEM existing methods. Regarding SEM, and FESEM in particular, NIOSH did note in its revised Roadmap proposal that (NIOSH 2008):


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Ease of sample preparation and data collection for SEM analysis compared to TEM, along with some SEM advantage in visualizing EMP (elongated mineral particles) and EMP morphology (e.g., surface characteristics), suggests a re-evaluation of SEM methods for EMP characterization and mineral identification both for field and laboratory analysis.

This apparent change concerning SEM analysis is most likely the result of discussions on the use of FESEM for the characterization of asbestos and other mineral particles that were presented at the Public Meeting for Comments on the original NIOSH Roadmap proposal (Lee and Strohmeier 2007). It will be important to develop analysis techniques with improved resolution to visualize the smaller diameter fibres typical of asbestiform minerals, which pose the highest risk, and to assure the most complete and accurate fibre counts. The major challenge for scientists is to develop and streamline cost-effective screening techniques to accurately determine the fibres of highest risk (i.e. long, thin fibres) with acceptable uncertainty and operator variability. This formidable task could possibly be accomplished with particle counting protocols that allow higher uncertainty in the measurement of lower to medium risk particles (i.e. short, wide fibres). As new methods are developed, it is important to point out that reported risk estimates for occupational asbestos exposure were originally determined by PCM methods. Hence, fibre counts obtained with improved microscope resolution capabilities would not be directly comparable to current occupational exposure limits for asbestos without developing meaningful conversion factors to compare the new results with the original risk data generated using the older PCM method.

Raman spectroscopy Although it does not have widespread use for asbestos characterization, Raman microspectroscopy has been demonstrated in a number of studies to be a simple and effective analytical technique for distinguishing between the six regulated asbestos phases (Bard et al. 1997; Rinaudo et al. 2003, 2004). Raman spectroscopy is a light scattering technique that is used to study vibrational and rotational modes in molecular systems. It is highly sensitive to the structural variability and particularly the chemical substitutions typically occurring in the amphibole asbestos minerals. The accurate identification of the asbestos phase in question can be attained by analysing the Raman bands corresponding to the symmetric and asymmetric stretching modes of the different siliconoxygen linkages from standard amphibole species. The observed Raman bands are sufficiently different for all six asbestos minerals, thus presenting a molecular fingerprint that allows differentiation between various species of the serpentine and amphibole asbestos groups. The Raman technique also has the added advantage that no sample preparation is required. Unfortunately, one Raman study reported no important differences between the Raman spectra of asbestos fibres and their non-fibrous forms, except in the hydroxide stretching region (Bard et al. 1997). A more recent study indicated that UV-Raman spectroscopy was suitably sensitive for supplementing the established European technique of SEM/EDS for an unambiguous discrimination of fibrous asbestos materials (Petry et al. 2006). It should be noted, however, that the optimum spatial resolution of typical Raman instruments is only about 1 μm. Therefore, Raman microspectroscopy cannot be easily used for characterizing single asbestos fibres and at the present time is primarily limited to applications on larger fibre bundles.

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Mössbauer spectroscopy Mössbauer spectroscopy is an analytical technique based on the recoilless emission and absorption of gamma rays, termed the ‘Mössbauer effect’ (Long 1984). In its most common form, Mössbauer absorption spectroscopy, a detector measures the amount of radiation a solid sample absorbs upon being exposed to a beam of gamma radiation by measuring the intensity of the beam after it is transmitted through the sample. The technique gives precise information about the chemical, structural, magnetic, and timedependent properties of certain chemical materials. However, Mössbauer spectroscopy can only be applied to specific elemental isotopes. Common isotopes measured by Mössbauer include 57Fe, 129I, 119Sn, and 121Sb (Long 1984). None the less, Mössbauer spectroscopy is a useful technique for studying the crystal chemistry of many types of minerals that contain these elements. Mössbauer spectroscopy has been used to define the ferric ion to total iron ratios (Fe3+/ Total Fe) in amphibole asbestos and to determine the iron cation site location in the amphibole crystal structure (Luys et al. 1983; Gold et al. 1997; Gunter et al. 2003, 2007; Andreozzi et al. 2007; Gianfagna et al. 2007). Mössbauer investigations were conducted because iron in amphibole asbestos has been implicated in the pathogenicity of inhaled fibres. For example, Gianfagna et al. reported differences in the ferric ion to total iron ratios for asbestiform and non-asbestiform prismatic morphologies of fluoroedenites from Biancavilla, Italy (Andreozzi et al. 2007; Gianfagna et al. 2007). The observed differences in total iron content and in the site partitioning were attributed to differences in genetic history between the fibrous and prismatic materials. Specifically, iron in amphibole asbestos is believed to enhance the absorption and catalytic surface activity of the asbestos (Gold et al. 1997). Iron can occur in either the ferrous (Fe2+) or ferric (Fe3+) valance states in amphibole asbestos, and each cation has specific site preferences in the amphibole crystal structure. In addition, the ferrous ion has been reported to be more reactive than the ferric ion in certain biologic environments (i.e. the lungs) (Gianfagna et al. 2007). Hence, the total iron weight percentage, the ferric ion to total iron ratios, and the ferric ion versus ferrous ion site preferences in the amphibole crystal structure may potentially affect the fibre’s surface chemistry and reactions at the lung tissue interface, although these molecular reactions and distinctions have yet to be precisely determined.

Part II: Asbestos and man: history, health effects, and current controversies Historical overview and origins of asbestos Asbestos was first discovered and mined in Cyprus approximately 5000 years ago and was used in the manufacture of cremation clothes, lamp wicks, hats, and shoes (Ross and Nolan 2003). The word asbestos in Greek meant ‘unquenchable’ or ‘indestructible’ (Lee and Selikoff 1979). Asbestos was also used during prehistoric times in Finland, Sudan, and Kenya to make clay pottery with increased strength (Darcy and Alleman 2004). Although asbestos-based cloth was in use in Norway, Russia, China, and Italy before 1860 (Lee and Selikoff 1979), the rapid increase in use as thermal and fire-resistant insulation occurred with the development of steam technology beginning at that time (Schreier 1989; Ross and Nolan 2003). Major asbestos sources supplying asbestos to these applications included the reopened Roman-age mines in the northern Italian Alps and the newly discovered vast chrysotile deposits in Quebec, Canada (Schreier 1989; Ross and Nolan 2003). In addition, large chrysotile deposits were discovered and



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developed in the Ural mountains in the 1880s (Ross and Nolan 2003). By 1890, the asbestos industry was rapidly expanding with hundreds of new applications being introduced (Jones 1890). Crocidolite, a commercial name, not a mineral name, was discovered in 1812 in the Northern Cape Province of South Africa, but the deposit was not developed until 1910 (Lee and Selikoff 1979). Amosite, another coined term for the asbestiform amphibole grunerite, was discovered in the Transvaal Province, South Africa in 1907, and commercial production began in 1916 (Bowles 1955). After World War I, the production and use of asbestos greatly increased. Historically, 95% of the consumption of asbestos has been chrysotile, with minor amounts of crocidolite and amosite (Ross and Nolan 2003). The commercial terms ‘crocidolite’ and ‘amosite’ originated because these were historically the most commonly used types of amphibole asbestos. In the case of amosite, the name was coined for the source of the ore: the ‘asbestos mines of South Africa’. Amosite and crocidolite, however, are no longer mined (Virta 2002). Anthophyllite and tremolite were also used in very limited quantities for speciality products, but actinolite had almost no commercial value. Tremolite and actinolite are regulated, however, because they can occur as accessory minerals in other economically important mineral deposits that are mined (Gunter et al. 2007). Other mineral fibres, which did not exhibit the physical, chemical, and thermal characteristics ascribed to asbestos described above, had little or no commercial value and were not considered to be asbestos. The total world production of all forms of asbestos between 1931 and 1999 was 166 million tonnes, of which 90–95% was the chrysotile variety (Ross and Virta 2001). Today, Russia is the world’s leading producer of chrysotile asbestos, followed by China, Kazakhstan, Canada, and Brazil (USGS 2008). The last US chrysotile asbestos mine operating near King City, CA, was closed in 2002; hence, all asbestos currently being used in the USA is imported (USGS 2008). During the twentieth century, asbestos was incorporated as functional components in thousands of commercial products. Asbestos applications included fire protection, heat or sound insulation, fabrication of papers and felts for flooring and roofing products, pipeline wrapping, electrical insulation, thermal and electrical insulation, friction products in brake and clutch pads, asbestos-cement products, reinforcing agents, vinyl or asphalt tiles, and asphalt road surfacing (Virta 2001, 2002). Figure 15, a sample of woven asbestos cloth, illustrates the flexibility and high length to width ratio of asbestos fibres. Asbestos cloth has been commonly used in welding, fire protection, and safety clothing. Despite its many desirable material properties, inhalation of asbestos fibres may pose a serious potential health risk primarily from occupational exposure to ore-grade asbestos during certain mining, milling, manufacturing, installation, and post-use abatement activities. Therefore, the commercial asbestos minerals became regulated in the workplace as their potential health effects became better understood. Recently, however, environmental exposure to the so-called ‘naturally occurring asbestos’ (NOA) has also become a major potential health concern (Lee et al. 2008b). NOA is the general all-encompassing name applied to asbestos minerals found in-place in their natural state, and typically in areas where these minerals are found in such low quantities that mining and commercial exploitation are not feasible. Although large commercial deposits of asbestos minerals are rare, there are small non-economic occurrences of asbestiform minerals. The link between asbestos minerals and disease has generated fear of public exposure when even small quantities of asbestos minerals or minerals claimed to be asbestos are discovered in the local environment. Although there is a high probability of finding non-asbestiform amphibole and serpentine minerals in many areas


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Figure 15. A sample of woven asbestos cloth. (The scale is in centimetres.)

of the USA under specific geological conditions, these minerals may be easily seen and may resemble long, thin asbestiform fibres but are not ‘asbestos’. Health effects Asbestos is classified as a carcinogen by state, federal, and international agencies, and all six types of asbestos summarized in Table 1 are considered hazardous. Humans are exposed to asbestos primarily by breathing airborne asbestos fibres, which can be deposited deep into the lungs where they may persist for long periods. Ingestion of asbestos has been proposed as a trigger for gastrointestinal cancers and other health effects; however, these links have not been demonstrated with certainty (IOM National Academy Report 2006; Plumlee et al. 2006). Medical studies have shown that there is a strong association between the diseases asbestosis, lung cancer, and mesothelioma and airborne asbestos exposure (Guthrie and Mossman 1993; Gunter 1994; Virta 2001; Clinkenbeard et al. 2002; Ross and Nolan 2003; U.S. DHHS 2005; Plumlee et al. 2006; Gunter et al. 2007). Asbestosis is a non-cancerous lung disease that results in diffuse fibrous scarring and inflammation of the lungs, which makes breathing difficult, and may eventually lead to heart failure. Lung cancer is a malignant tumour that invades and obstructs the lung’s air passages. Mesothelioma is a rare cancer that causes fibrosis and hardening of the thin membrane lining the lungs, chest, and abdominal cavity. Other adverse health effects linked to asbestos exposure include pleural effusions, pleural thickening, and pleural plaques (Plumlee et al. 2006). Pleural plaques are masses of the fibrous protein collagen that form in the lung and calcify, which can be detected on chest X-ray films (Ross and Nolan 2003; Gunter et al. 2007). Most cases of these diseases occur to people heavily exposed in the past to uncontrolled asbestos in the workplace or to household contacts of



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asbestos workers (Anderson et al. 1979; Virta 2001; Ross and Nolan 2003; U.S. DHHS 2005; Gunter et al. 2007). Epidemiological studies of large cohorts of asbestos workers, exposed to variable amounts of chrysotile and amphibole asbestos, indicate that the amphiboles are more dangerous than chrysotile, with amosite and crocidolite accounting for most mesothelioma mortality (Ross and Nolan 2003). Similarly to other respirable particulates to which humans are, or have been, heavily occupationally exposed, there is reasonable evidence that heavy and prolonged exposure to chrysotile can produce lung cancer, whereas low exposures do not present a detectable risk to health (Bernstein and Hoskins 2006). Many researchers believe that amphibole asbestos fibres pose a greater health risk than chrysotile fibres because they are less soluble and more rigid than chrysotile, allowing amphibole asbestos fibres to penetrate lung tissue and remain longer (Virta 2001; Clinkenbeard et al. 2002; Bernstein et al. 2003a, 2003b; Ross and Nolan 2003; Hoskins 2004; US DHHS 2005; Bernstein and Hoskins 2006; Gunter et al. 2007; Hoskins 2008; Ilgren 2008a, 2008b, 2008c). However, the medical community has reached no consensus regarding the exact mechanism or combination of mechanisms by which asbestos causes disease (Plumlee et al. 2006). Diseases occurring from asbestos exposure typically take many years to develop. The longer a person is exposed to asbestos and the greater the intensity of exposure, the greater the chances to develop health problems. The toxicity of any material is a function of the amount of toxicant taken up by the body, the amount reaching the particular site(s) of toxic action within the body, and the amount of toxicant that survives the body’s many clearance and mitigation mechanisms (Skinner et al. 1988; Plumlee et al. 2006). One current theory on the toxicity of asbestos fibres indicates that fibre dose, fibre dimensions (i.e. length and width), and fibre durability in lung fluid are the three primary factors determining asbestos fibre toxicity (Lippmann 1990). Dose, directly related to the intensity and duration of exposure, is probably the single most important aspect of asbestos-related disease (Gunter et al. 2007). Dose depends on three factors (Plumlee et al. 2006): (1) the amount of air an individual breathes in, (2) the concentration of fibres in that air, and (3) the clearance of the fibres from the lungs. Therefore, unlike most toxic materials, asbestos dose depends strictly on the total number of fibres inhaled, and not on the mass or volume of fibres. The potential for any of the asbestos minerals to initiate disease depends in large part on the physical characteristics that make it possible for the fibre to reach and deposit within the alveolar portions of the lung (see Figure 16). Fibre length, diameter, and composition are important factors in the deposition of fibres in the lungs and influence how long they are likely to remain in the lungs. Figure 7 shows a TEM image of a crocidolite fibre found in a sample of human lung tissue. The fibre has a length of approximately 6 μm, a width of approximately 0.1 μm, and an aspect ratio of approximately 60:1. Once a fibre has been deposited in the lung, smoking can impair the lung’s ability to clear it. Rahman et al. (2000) reported that exposure to cigarette smoke and/or kerosene soot effected the penetration and retention of asbestos fibres in the lung tissues, resulting in enhanced pulmonary inflammation reactions. A group of asbestos workers from Britain were determined to have a relative risk of lung cancer in the range 1.4–2.6 (Hodgson and Jones 1986). Smoking combined with asbestos exposure increased the relative risk of developing lung cancer by more than 10 times to 28.8 (Kjuus et al. 1986). Fibre length is believed to be important because macrophages, the cells that normally remove particles from the lungs, cannot engulf fibres having lengths greater than the macrophage diameter (Blake et al. 1998; Plumlee et al. 2006). Some macrophages may die in the process of trying to engulf the fibres and release inflammatory cytokines and other chemicals into the surrounding tissues in the lungs. These cellular interactions with the

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Olfactory area Tongue

Air intake

Air

Epiglottis (closes larynx directing flow to oesophagus behind) Oesophagus

Larynx Trachea d = 20 mm


(Right) Bronchus d = 8 mm

Thymus

Bronchioles

Terminal bronchiole

Mediastinal surfaces

Parietal Pleura Vusceral

Heart

Diaphragm

Terminal bronchiole d = 0.5 mm

Pleural cavity (space between visceral and parietal pleura)

Pulmonary arteriole Elastic connective tissue Lymphatic

Pulmonary venule

Respiratory bronchiole d = 0.5 mm Alveolar duct

Pleura Alveolar duct d = 0.2 mm

Direction of bllod flow Site of O2-CO2 exchange

Alveolus

Alveolar sacs d = 0.3 mm

Figure 16. Schematic diagram of the human respiratory system. The gross anatomy of the lung, the covering membranes (pleura), airways and air sacs (alveoli) are shown. The average diameter of portions of the air flow system is indicated: trachea, 20 mm; bronchus, 8 mm; terminal and respiratory bronchioles, 0.5 mm; alveolar duct, 0.2 mm; alveolar sacs, 0.3 mm (Skinner et al. 1988, p. 110, Figure 2.1)



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fibres appear to trigger collagen buildup in the lungs known as fibrosis in general or asbestosis if associated with asbestos inhalation. This is one normal body reaction to any trauma. The size of a human macrophage, approximately 10–15 μm, and numerous studies indicate that fibres less than 5 μm in length lack significant biological potency because of macrophage clearance (Ilgren 2004, 2008b). Fibres longer than 10–15 μm have a much higher probability of remaining in the lungs for an extended period of time. This provides a logical biological foundation for the importance of differentiating between ‘short’ and ‘long’ fibres when characterizing asbestos-containing materials relative to health effects and risk (Ilgren 2008b). Deposition of fibres in the lungs is largely controlled by the fibre diameter as aerodynamic behaviour predicts that only small-diameter fibres are likely to become airborne and reach and deposit in the deepest airways of the lungs (Plumlee et al. 2006; Ilgren 2008c). Fibres much greater than 1 μm in diameter probably cannot reach the alveolar air spaces, whereas those less than 0.5 μm diameter may result in maximal deposition in the deeper portions of the lung (Ilgren 2008c). Lastly, fibres that dissolve in lung fluid in a matter of weeks or months appear to be less toxic than more insoluble fibres (Bernstein et al. 2003a, 2003b; Ilgren 2008a). In addition to mesothelioma and lung cancer, the Institute of Medicine (IOM) of the National Academy of Sciences (NAS) has published a comparison of selected cancers in the respiratory and gastrointestinal tracts and occupational asbestos exposure (IOM 2006). In the report, they assess the ability of asbestos to be deposited throughout the respiratory tract by inhalation and the gastrointestinal tract by ingestion. In the respiratory tract, there is a potential association of pharynx and larynx cancers to occupational asbestos exposure, whereas in the gastrointestinal tract, there is a possible association of oesophagus, stomach, colon, and rectum cancers to occupational asbestos exposure. These studies did not differentiate between chrysotile and amphibole fibre types owing to the difficulty of assessing mixed fibre nature of occupational exposures. The review of epidemiological evidence came from cohort studies of occupationally exposed persons and from case-controlled studies of cancers that assessed risk factors. The IOM committee had a four-level classification of evidence for causal inference: the evidence was sufficient, suggestive, inadequate to infer causality, or suggestive of no causal association. Sufficient positive association was found between occupational asbestos exposure and laryngeal cancer, but not for oesophageal cancer. There were only suggestions for cancers of the pharynx, stomach, colon, and rectum. Sources of airborne asbestos fibres include industries that mine or manufacture asbestos products, the products themselves, or products that inadvertently contain asbestos as an impurity. Other potential sources are community development and construction sites where ACMs are used, buildings containing damaged or deteriorated ACMs, buildings with ACM that are being torn down or renovated, sites where ACMs, have previously been improperly handled or disposed, as well as any naturally occurring asbestiform minerals present in the environment. Because of potential health concerns related to asbestos exposure, asbestos in manufactured goods and processes in the USA has decreased greatly over the last 30 years, and the US government has banned any new uses of asbestos in products since 1990; however, many products sold in the USA still contain asbestos. A recent government report (US DHHS 2005) speculated that, because asbestos products were so widely used, the entire US population is potentially continually exposed, and it is possible that most of us may still be exposed to different types of asbestos (and other amphibole particles) on a daily basis. Gunter has speculated that most people who reach the age of 60–70 probably have millions of amphibole particles in their lungs owing to exposure to asbestos-containing products and environmental exposure to NOA over that lifetime (Gunter et al. 2007).


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Despite a large body of work, scientists do not yet know with certainty how much exposure to asbestos can result in a person developing an asbestos-related disease. Individual susceptibility appears to be an important factor, and occupational exposure studies are further complicated by the fact that the highest asbestos-related disease rates generally occur among smokers (Virta 2001; Ross and Nolan 2003). Regardless, scientists do know that long-term exposure to relatively high concentrations of airborne asbestos is a potent cause of lung diseases. In addition, several asbestiform varieties of other amphiboles (e.g. richterite, winchite, etc.) have been identified and are suspected or documented to pose a health risk similar to the regulated asbestos minerals (Clinkenbeard et al. 2002). As more information on the health effects of other asbestiform minerals becomes available, new regulations may be developed, or existing regulations modified, to include asbestiform minerals other than those currently regulated. The current EPA approved method of health risk assessment is termed Integrated Risk Information System (IRIS) and is based on the evaluation of the risk of asbestos disease to cohorts occupationally exposed to commercial asbestos (US EPA 1993). The analytical method for these analyses is PCM augmented by TEM when the identity of fibres is in question. IRIS does not address any method for assessing non-asbestos rock fragments and does not address the relative potency of different forms of asbestos. EPA, recognizing the limitations of IRIS, commissioned an effort to modernize the health risk methodology (Hofmann and Treinies 2003). The Berman-Crump Asbestos Risk Assessment Protocol (Berman-Crump Protocol) (Berman and Crump 2003) was the result of an EPA-funded, multi-year study which demonstrated that airborne amphibole asbestos fibres that are longer than 10 μm with widths that are less than 0.4 μm are of most concern with respect to health risk and that different relative carcinogenic potencies should be applied for different mineral fibre types when estimating risk. The EPA partially funded a collaborative study between NIOSH investigators and investigators from Duke University Medical Center and University of Chicago on the role of fibre size in predicting lung cancer or asbestosis in chrysotile textile workers at a single South Carolina textile plant (Kuempel et al. 2006). The results of this study support the conclusions of the Berman-Crump Protocol and found that fibre length and width were statistically significant predictors of lung cancer and asbestosis mortality. Lung cancer was most frequently indicated by long, thin fibres (i.e. greater than 40 μm length; less than 0.3 μm width), although other sizes, including those less than 5 μm length, were also indicated. A detailed review of the results from the Berman-Crump Protocol (Berman and Crump 2003) is beyond the scope of this article. In brief, the protocol defined appropriate procedures for evaluating asbestos risk and developed optimum values for exposure– response coefficients for lung cancer and mesothelioma and a conservative set of potency estimates. To assess risk, depending on the specific occupational application, either the best-estimate risk coefficients or the conservative estimates can be utilized and can be combined with appropriately determined estimates of exposure to estimate risk in any environments of interest. The Berman-Crump Protocol was the subject of a peer review consultation meeting held in San Francisco on 25–26 February 2003 (Eastern Research Group 2003). The 11-member expert panel endorsed the overall approach to risk assessment proposed in the report, although at the time that the peer review group’s recommendations were issued, additional research and analyses recommended by the consultation panel had not been completed, the protocol had not been independently peer reviewed, and the EPA had not officially adopted the protocol. More recently, the Berman-Crump method was been published in two articles in the peer reviewed literature (Berman and Crump 2008a, 2008b).



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However, until the Berman-Crump Protocol is officially adopted by the EPA as a valid risk assessment tool or until other risk models become available, assessment of asbestos exposure risk will depend primarily on qualitative identification (i.e. the presence or absence) of asbestos in a particular occupational or environmental setting; a responsibility which necessarily falls to trained asbestos analysts/mineralogists, geologists, and other scientists. When discussing the potential health effects of asbestos minerals, we believe, along with others, that it is important to distinguish between fibres and the non-asbestiform cleavage fragment analogues of these minerals. Gamble and Gibbs (2008), Ilgren (2004), and Mossman (2003) have reviewed numerous studies demonstrating that cleavage fragments and amphibole asbestos fibres have fundamentally different properties and that these differences are biologically relevant. Amphiboles that occur in igneous and metamorphic rocks range in growth habit from blocky to acicular. Amphibole fragments separated, broken, or cleaved from rocks during weathering, crushing, or grinding can occur in a variety of shapes ranging from blocky to prismatic to acicular. Asbestos fibres on the other hand attain their shape by growth, not cleavage. Nevertheless, long, thin cleavage fragments, although rare, may resemble asbestos fibres under a microscope. Prismatic and acicular single crystals and cleavage fragments, however, do not have the strength, durability, flexibility, acid resistance, or other unique properties of asbestiform fibres. They are therefore unable to persist in the body, probably because they are short and readily cleared from the lungs (Ilgren 2004). Although the toxicity of respirable cleavage fragments is so much less than that of fibrous amphiboles by any reasonable measure, it appears that they are not biologically harmful (Mossman 2003; Ilgren 2004; Gamble and Gibbs 2008). However, there is still no general consensus within the medical community about the toxicity of different fibre sizes (i.e. length, width, aspect ratio, and aerodynamic diameter), relative toxicity of the different asbestos minerals, and the potential health effects of cleavage fragments as opposed to those of fibres (Plumlee et al. 2006). This controversy contributes to the overall complexity of the asbestos risk and regulation issues. At one time, OSHA, at the recommendation of NIOSH, attempted to remove the distinction between the asbestiform minerals and their non-asbestos analogues (e.g. cleavage fragments and single crystals), thus negating the need to differentiate between them. However, in 1992, OSHA decided to keep this distinction and has separately regulated the asbestos minerals as asbestos and their non-asbestos analogues as nuisance dust (OSHA 1992). OSHA made this decision based on epidemiological studies whose results either were inconclusive or revealed no adverse health effects from non-asbestos minerals. OSHA recognized the potential interference of non-asbestos amphiboles in the context of determining asbestos concentrations, but left it to the analyst to provide a viable method of discrimination between asbestos and non-asbestos minerals. Conversely, as mentioned above, NIOSH has continued to argue for a regulation of non-asbestos cleavage fragment particles as asbestos (NIOSH 2004, 2008, 2009; Middendorf et al. 2007). Given the OSHA regulatory position and the need in risk analysis to ensure that the measured physical properties correspond to the properties of particles with known risk profiles, we believe there is a need to differentiate reliably between the asbestos amphibole fibres and non-asbestos amphibole particles. Current analytical methods used for regulatory assessment do not provide specific guidance on how to differentiate asbestos from other non-hazardous rock fragments. Such information is required in order to properly assess the hazards of exposure and evaluate the data to be used in risk assessment.

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In February 2007, NIOSH released their ‘White Paper’ proposal outlining a ‘Roadmap’ for scientific research necessary to address current controversies related to the definition of asbestos, appropriate analysis techniques for asbestos identification, and valid risk assessment methods (Middendorf et al. 2007). The purpose of this proposed research was to develop improved worker and public health policies and practices related to occupational asbestos exposure. NIOSH continues to argue for the inclusion of nonasbestos mineral particles in the definition of asbestos. For almost two decades, NIOSH has defined airborne asbestos fibres as (NIOSH 2004, 2008, 2009; Middendorf et al. 2007): Those particles that, when examined using phase contrast microscopy, have: (1) an aspect ratio of 3:1 or greater and a length greater than 5 μm; and (2) the mineralogic characteristics (i.e., the crystal structure and elemental composition) of the asbestos minerals (chrysotile, crocidolite, amosite, anthophyllite asbestos, tremolite asbestos, and actinolite asbestos) or their nonasbestiform analogs (the serpentine minerals antigorite and lizardite, and the amphibole minerals contained in the cummingtonite-grunerite mineral series, the tremolite-ferroactinolite mineral series, and the glaucophane-riebeckite mineral series.)

NIOSH does not base the above definition of asbestos on all of the physical and chemical parameters typically used by mineralogists for identifying asbestos (e.g. tensile strength, fibrous growth habit, etc.), rather NIOSH uses only specific physical criteria (i.e. particles that meet specific dimensional criteria) and compositional criteria (chemistry) to define asbestos (NIOSH 2004, 2008; Middendorf et al. 2007). This inconsistency has led to the confusion about the toxicity of various types of particles (NIOSH 2008). Other issues raised about the minerals covered by this broad definition (Middendorf et al. 2007; NIOSH 2008) include whether other fibrous minerals, amphiboles, and zeolites, in particular, should also be included in the definition and be regulated as asbestos or whether the inclusion of ‘fibre-like’ cleavage fragments of non-asbestiform amphiboles in the definition is appropriate, which is in conflict with the current OSHA policy. Still another issue is whether the asbestos particle dimensions noted above are appropriate. NIOSH released revised versions of the White Paper Roadmap proposal in June of 2008 (NIOSH 2008) and January of 2009 (NIOSH 2009). In the revised documents, NIOSH noted that (NIOSH 2008, 2009): NIOSH recognizes that its descriptions of the REL [recommended exposure limit] since 1990 have created confusion and caused many to infer that the additional covered minerals were included by NIOSH in its definition of ‘asbestos’. NIOSH wishes to make clear that such nonasbestiform minerals are not ‘asbestos’ or ‘asbestos minerals’. NIOSH also wishes to minimize any potential future confusion by no longer referring to particles from the nonasbestiform analogs of the asbestos minerals as ‘asbestos fibers’. In a clarified REL presented in this Roadmap, NIOSH avoids referring to particles from such nonasbestiform minerals as ‘asbestos fibers’ and clarifies that particles meeting the specified dimensional criteria remain countable under the REL even if they are derived from nonasbestiform minerals.

Although the revised Roadmap proposals clarify that elongated non-asbestiform mineral particles are not ‘asbestos’, NIOSH continues to include such particles in the total REL particle count (NIOSH 2008, 2009). Therefore, for all intents and purposes, NIOSH treats such elongated non-asbestiform particles as asbestos and counts them as asbestos, even though from mineralogical and current regulatory standpoints, they are not. Such a policy can only lead to more confusion among the various groups in the different disciplines performing asbestos analysis and conducting health studies and risk assessments. Clearly,


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these controversies need to be resolved and a consensus should be established across the different scientific, regulatory, and medical communities so that the definitions of asbestos can be standardized across all government agencies and among other stakeholders. Hopefully, future research studies resulting from the NIOSH proposal will lead to a unified definition of asbestos, other improvements in techniques for asbestos identification, and models for valid risk assessment.

Current asbestos issues


Occupational exposure to asbestos Asbestos fibres have been incorporated as functional components in thousands of commercial products because of their unique physical and chemical properties. Applications have included: fire protection, heat or sound insulation, fabrication of papers and felts for flooring and roofing products, pipeline wrapping, electrical insulation, thermal and electrical insulation, friction products in brake and clutch pads, asbestos-cement products, reinforcing agents, vinyl or asphalt tiles, and asphalt road surfacing (Virta 2001, 2002). After the potential health effects of asbestos exposure became known, numerous scientific and health studies were conducted, beginning in the early- to mid-1900s, in various industries and occupational settings to assess possible asbestos exposures to factory workers and other employees (Browne 1986). Such studies continue today, and a number have been mentioned in this review. The studies cover a wide range of occupational settings where harmful asbestos exposures could potentially be encountered including, for example, such industries as:

• • • • • • • • •

chrysotile mining and milling (McDonald et al. 1980, 1993; Sébastien et al. 1989); ship construction and dockyards (Fleischer et al. 1946; Harries 1971); railway construction (Battista et al. 1999); cement plants (Gardner et al. 1986; Hughes et al. 1987); insulation materials (Balzer and Cooper 1968); drywall construction (Fischbein et al. 1979); building demolition (Wilmoth et al. 1993); automotive and naval gaskets (Blake et al. 2006; Mangold et al. 2006); automotive friction products (e.g. brakes and transmissions) (McDonald et al. 1984); • chrysotile textile plants (McDonald et al. 1983, 1984; Sébastien et al. 1989; Kuempel et al. 2006); and • various types of chemical plants (Lilis et al. 1979), among others. There have been no active asbestos mines in operation in the USA since 2002 (USGS 2008). Increased safety and health regulations, improved personal monitoring, and monitoring of working conditions have greatly increased worker safety, with present occupational asbestos exposure levels now far below the historical levels that led to disease. In most cases, current occupational exposure levels are near outdoor background levels. Ross and Nolan (2003) reported that the present health risks in modern Canadian and Russian chrysotile mines are indistinguishable from the risks associated with substitute materials, such as fibreglass, rock wool, and various composites. In addition, the importation of asbestos-containing products into the USA and the use of imported asbestos in US-manufactured products have both decreased greatly in recent years. Most

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asbestos-containing products in use today are installed under conditions regulated by OSHA, and there are almost no asbestos-containing products manufactured specifically for use by the general public (Virta 2001). Even so, continued public concerns over asbestos have recently led the US Congress to implement a total ban of asbestos-containing products.


Asbestos-containing products and the ‘Ban Asbestos in America Act’ Large numbers of people in the USA incorrectly believe that the use of asbestos was banned many years ago and that there is no present risk of exposure to asbestos through the use of commercial products. However, in 1991, the United States Court of Appeals for the 5th Circuit overturned portions of the regulations proposed by the Administrator of the EPA in 1989 to totally phase out asbestos in US consumer products by 1997 (Murray 2007). Although new applications for asbestos use were banned at that time, a complete ban did not occur, and asbestos is still being used in some consumer and industrial products made or imported into the USA such as building products and automotive brakes (Virta 2002; Murray 2007). In addition, asbestos may also be present as a low-level contaminant in certain products. Because the last active asbestos mine in the USA, operated by the King City Asbestos Corporation in California, was closed in 2002 (USGS 2008), the USA is totally dependent on imports to meet manufacturing needs. All of the asbestos imported and used in the USA is chrysotile, and Canada is the leading supplier, providing 86% of the market share, of asbestos for domestic consumption in the USA (USGS 2008). The USGS reported in 2008 that the USA imported 2000 tonnes of asbestos with an estimated usage of 84% for roofing products and 16% for other applications (USGS 2008). Some of the key issues surrounding asbestos use in products are the lack of uniform regulatory policies and the lack of robust, standardized measurement and risk assessment procedures (Hoskins 2004). Complete clarity and agreement among the various scientific, medical, and regulatory bodies on the definition of asbestos and in standard test methodologies are vital to ensure that asbestiform minerals are accurately identified and differentiated from common rock fragments in products and in natural mixed mineral environments. These issues would be mitigated if government agencies had uniform asbestos health standards, which they do not. Recently proposed US legislation will further exacerbate issues with asbestos, if enacted as currently drafted. The Senate unanimously passed the ‘Ban Asbestos in America Act of 2007’ on October 4 of that year (Murray 2007). This Bill bans the importation, manufacture, processing, and distribution of asbestos-containing products that contain more than 1 wt.% asbestos. A similar companion Bill was also introduced in the US House of Representatives on 2 August 2007 (McCollum 2007), but the proposed House Bill (H.R.3339) never moved beyond Committee deliberations, and the Bill was never presented to the full House for voting. The primary purpose of these Bills was to reduce the health risks posed by ACMs and products having ACM (McCollum 2007; Murray 2007). Some members of the House argued that the passed Senate Bill did not go far enough, and a revised draft version of the House Bill was introduced to the Subcommittee on Environmental and Hazardous Materials on 15 February 2008 (Schneider 2008; Subcommittee on Environmental and Hazardous Materials 2008). Following Committee deliberations (Subcommittee on Environmental and Hazardous Materials 2008), the final version of the House draft legislation was introduced as a new full House Bill (H.R.6903) on 15 September 2008 (Green 2008). It should be noted that the 100th Congress introduced most of the recent Senate and House Bills. The 111th Congress was sworn in on January 2009. Therefore, the latest versions of the Senate and House Bills



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to ‘Ban Asbestos in America’ will have to be reintroduced to Congress and voted on to be passed into law. The final forms of any future Bills are unknown at this time, but it is probable that any new Bills either will be little changed from the 2007 Senate and 2008 House versions or will be a compromise between the two. Therefore, we will briefly discuss the Bills in their most recent forms. The original version of the Senate Bill would have required EPA to ban the manufacture, distribution, and importation of all ‘asbestos-containing products’, which it defined as (Murray 2007): ‘any product (including any part) to which asbestos is deliberately or knowingly added or in which asbestos is deliberately used or knowingly present in any concentration’. Similarly, the latest House Bill defines ‘asbestos-containing product’ as (Green 2008): ‘any product (including any part) to which asbestos is deliberately added, or used, or in which asbestos is otherwise present in any concentration’. However, the version of the Senate Bill that was eventually passed (Murray 2007) only requires EPA to ban ‘asbestos-containing materials’, which are defined by Title II of the Toxic Substances Control Act (TSCA) as (40 FCR 700, 2002): ‘any material which contains more than 1% asbestos by weight’. TSCA defines ‘asbestos’ as the asbestiform varieties of (US Code of Federal Regulations 2002):

• • • • • •

chrysotile (serpentine); crocidolite (riebeckite); amosite (cummingtonite-grunerite); anthophyllite; tremolite; and actinolite.

The Senate Bill (Murray 2007) and House Bill (Green 2008) both modify the above definition for asbestos by adding: ‘any material formerly classified as tremolite, including – (i) winchite asbestos; and (ii) richterite asbestos; and any asbestiform amphibole mineral’. As mentioned above, the latest House Bill maintains the zero-per cent asbestos limit for most products, as did the original version of the Senate Bill, but specifies an exemption limit of less than 0.25 wt.% asbestos for aggregate products (extracted from stone, sand, or gravel operations) (Green 2008). This proposed limit would affect stone, sand, and gravel operations, and by extension, the construction industry, which uses these aggregate materials in large quantities (Hogue 2008). The House Bill would require the testing of all aggregate products and prohibit the sale of these materials if they contain more than 0.25% asbestos by weight. This provision would place an enormous requirement on aggregate producers to test each truckload of material leaving a sand or gravel pit for asbestos. It should be noted that consumption of aggregate products in the USA is approximately 3 billion tons per year, although only a third of this amount comes from areas where asbestos might be present (US Code of Federal Regulations 2002; Nolan 2008). To establish sampling and analysis methods to test this quantity of rock and similar products to determine that they contains less than 0.25% asbestos is not a scientifically justified approach to this problem. It would do little to protect the public health because there is no generally accepted method of predicting airborne fibre levels from the concentration of asbestos in an ore body, especially at such low levels (Nolan 2008). A better approach, as recommended by Nolan (2008) to the House Subcommittee on Environmental and Hazardous Materials, would be to monitor the concentration of airborne fibre levels at workplaces where aggregate products are produced, transported, and used.

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The Senate ‘Ban Asbestos in America Act of 2007’ (Murray 2007) defines ‘elongated mineral particle’ as: a single crystal or similarly elongated polycrystalline aggregate particle with a length to width ratio of 3 to 1 or greater

and ‘biopersistent elongated mineral particle’ as:


an elongated mineral particle that – (A) occurs naturally in the environment; and (B) is similar to asbestos in – (i) resistance to dissolution; (ii) leaching; and (iii) other physical, chemical, or biological processes expected from contact with lung cells and other cells and fluids in the human body.

Without further clarification of these definitions, the question of whether or not a product contains asbestos will become the subject of ongoing debate. At issue are: (1) the lack of an accepted definition for asbestos; and (2) the need for improved sampling and analytical methods to distinguish between asbestos and non-asbestiform elongated mineral particles. Numerous studies have demonstrated that common rock fragments do not cause asbestosrelated disease (Gunter 1994; Bernstein et al. 2003b; IOM 2006). If the definition of asbestos is unclear and overly inclusive, certain materials may be incorrectly identified as asbestos resulting in unfounded product bans and unwarranted public fears. The Senate ‘Ban Asbestos in America Act of 2007’ (Murray 2007) calls for the National Academy of Sciences (NAS), EPA, and other Federal entities to: (I) evaluate the known or potential mode of action and health effects of – (i) non-asbestiform minerals; and (ii) elongated mineral particles; and (II) to develop recommendations for a means by which to identify, distinguish, and measure any non-asbestiform mineral or elongated mineral particle that – (i) may cause disease or health effect; or (ii) does not cause any disease or health effect.

Establishing adequate asbestos definitions and analytical methods that will reliably discriminate asbestos from non-asbestiform minerals for the purposes of risk assessment is critical to that goal. However, in an effort to ban asbestos products as quickly as possible, the latest House Bill removes the call for these important studies (Green 2008; Schneider 2008), which will only lead to more confusion as to the definition of asbestos and the relative health effects of asbestos and non-asbestos mineral particles. Asbestos in public buildings and schools ACMs in the form of fireproofing, pipe insulation, drywall, spackle, floor and ceiling tile, etc. were installed in public buildings and homes throughout the USA for more than 60 years (Strenio et al. 1984; Lee and Van Orden 2008). ACM usage dramatically increased with the advent of spray-on fireproofing insulation and the corresponding increase in high-rise buildings in the early 1960s. Estimates were that at least 733,000 (and possibly many more) public and commercial buildings that contained ACM were constructed (Lee and Van Orden 2008). During that time, spray-on asbestos-containing fireproofing typically contained 5–35% asbestos fibres along with mineral wools, clay binders, adhesives, synthetic resins, and other proprietary agents (Reitze et al. 1972). In 1973, however, the EPA banned the use of spray fireproofing material containing more than 1% asbestos because occupational hazards associated with inhalation of asbestos fibres had been well established (US EPA 1973).



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By the late 1970s, the possible risk to human health resulting from the presence of ACM in buildings prompted widespread public and governmental concerns in the USA. It was believed by some that the mere presence of asbestos in buildings would result in significantly elevated airborne concentrations of asbestos giving rise to measurable risks of asbestos disease to building occupants and maintenance workers. Sawyer postulated a model for exposure that included spontaneous release of fibres from in-place materials and episodic exposures through entrainment of fibres from asbestos dust and debris as primary pathways or exposure routes for asbestos exposure to building occupants (Sawyer 1977). Inspired by Sawyer’s model, some investigators postulated in the late 1970s that the mere presence of ACMs in buildings would cause a second wave of asbestos-related disease, particularly mesothelioma, in the general population (Hogue 2008). More than 30 years have elapsed since the formulation of those postulates. However, no scientific data emerged to support them and, by the early 1990s, the EPA published its ‘Green Book’ concerning asbestos maintenance in buildings, stating that: ‘the health risk to most building occupants . . . appears to be very low’ (US EPA 1990). The reason for this is simply that in-place asbestos-containing surfacing materials do not spontaneously release or shed respirable asbestos fibres, nor, under conditions of normal usage, result in elevated airborne asbestos levels in buildings (Lee and Van Orden 2008). Many studies have been published documenting airborne asbestos levels in buildings similar to ambient air concentrations, and these studies were recently reviewed (Lee and Van Orden 2008). The review covered studies of various types of buildings in the USA, Canada, and the UK. For example, based on an extensive evaluation of 49 public buildings in five cities, the EPA, in a 1987 report to Congress, concluded that airborne asbestos levels in public buildings with ACMs were no different than outdoor air (US EPA 1988; Crump and Farrar 1989). Similarly, in 1992, a study conducted by the Health Effects Research Institute–Asbestos Research Panel (HEI-AR), under a mandate from Congress, concluded that airborne levels in well-maintained buildings with ACM were no different than ambient background levels (HEI-AR 1992). Other published studies produced essentially the same conclusions: indoor air levels are not significantly different than outdoor air levels and maintenance worker exposures are generally well below regulatory levels (Chatfield 1986; Burdett et al. 1987; Corn et al. 1991; Lee et al. 1992; Componiano et al. 2004). By 1990, the EPA had altered its guidance on asbestos to recommend that in-place ACMs in good condition be managed in place (US EPA 1990). As noted by the EPA (US EPA 1990): ‘Based upon available data, the average airborne asbestos levels in buildings seem to be very low. Accordingly, the health risk to most building occupants also appears to be very low’. In-place management of ACMs involves the proper use of building operations and maintenance (O&M) work practices and control measures that minimize the airborne release of fibres from ACMs, reducing exposures to workers and other building occupants. Much effort went into developing the recommended procedures, controls, training, oversight, and management methods for ensuring that O&M programmes are effective in meeting these objectives (US EPA 1990). Studies have demonstrated that, if proper O&M procedures are followed by knowledgeable, careful workers, the exposure to airborne asbestos during their work and exposure of other building occupants following completion of the work are generally well below regulatory levels (Kinney et al. 1992; Price et al. 1992; Shaikh et al. 1992; Corn et al. 1994; Mlynarek et al. 1996). In the mid-1970s, the fear over the effects of potential asbestos exposure in schools to our children became a dominant public indoor environmental concern in the USA and eventually led to the ban of asbestos-containing products in public schools. An entirely


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new industry sprung up in the mid-1980s for asbestos testing and removal of ACMs, not only from schools, but from public and commercial buildings as well. By 1995, more than $50–$100 billion had been spent on removal of ACMs from schools, university buildings, public and commercial buildings, and private homes (Ross 1995a, 1995b; NIOSH 2004). Unwarranted removal activity continues to this day, encouraged by those who profit from the abatement business, despite the publication of an advisory document in 1990 by the EPA that most asbestos removal is unnecessary and even counterproductive in terms of both health protection and costs (US EPA 1990). Responding to pressure from parent groups in the summer of 1993, the New York City (NYC) school system spent nearly $100 million for unwarranted asbestos removal in public schools (Wilson et al. 1994; Ross 1995a, 1995b). During this time, many schools remained closed, and parents were subjected to mass media coverage that promoted the idea that schoolchildren might develop asbestos-related cancer in the future. Further, based on fibre-in-air measurements, Ross reported that the calculated risk to NYC school children, using the most pessimistic models, was found to be less than six excess cancer deaths per million lifetimes, which is equivalent to smoking less than a dozen cigarettes in a lifetime (Ross 1995b). This incident prompted 17 world-renowned experts on the subject of asbestos to issue a public statement criticizing the city’s unnecessary and costly actions (Churg et al. 1993). They stressed that the public’s fears could have been substantially allayed through education and that science, not unreasonable emotion, should guide both the administrative and the public response in these types of situations. In support of this view, a study on airborne asbestos levels in 71 school buildings scheduled for abatement in the USA (Corn et al. 1991) and another study on 59 school buildings in Italy (Componiano et al. 2004) both indicated that typical indoor asbestos levels in schools were not significantly different than outdoor levels. In addition, the Corn study found that neither in-place ACM, the condition of ACM, nor accessibility of the ACM to disturbance correlated with airborne asbestos concentrations (Corn et al. 1991). The EPA recognizes four options for abatement: removal, encapsulation, enclosure, and O&M controls (Chrostowski et al. 1991). A study on the likelihood of releases of asbestos fibres during school abatement procedures and the associated risks reported that the risks to teachers and students in school buildings containing in-place ACM were approximately the same as risks associated with exposure to ambient asbestos by the general public and were below the levels typically of concern to regulatory agencies (Chrostowski et al. 1991). During abatement, however, the study found increased risks to both workers and nearby individuals. Careless, everyday building maintenance generated the greatest risk to workers followed by removals and encapsulation. The authors claimed that if asbestos abatement was judged by the risk criteria applied to EPA’s Superfund program, the no-action alternative would likely be selected in preference to removal in a majority of cases. However, the authors cautioned that risk managers should also take factors such as the type and size of fibre, situation-specific sampling and analysis limitations, episodic peak exposures, and the number of people exposed into consideration. Asbestos and amphibole contamination in products and materials Asbestiform and non-asbestiform amphibole particles have been found as accessory contaminants in mines and many other mineral deposits such as:

• copper (Ilgren 2004); • gold (Ilgren 2004);



• • • • • •

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iron ore and taconite (Nolan et al. 1999; Ilgren 2004); marble (Ilgren 2004); nickel (Ilgren 2004); talc (Rohl and Langer 1974; Ilgren 2004; Van Gosen et al. 2004); vermiculite (McDonald et al. 1988; Ross and Nolan 2003; Van Gosen et al. 2005); wollastonite {CaSiO3} (Maxim et al. 2008),

and others. As mentioned previously, the low, non-mining levels of asbestos ‘contamination’ typically found in these situations would be classified as NOA. However, miners and millers may potentially be exposed to asbestos at such sites, and products manufactured from these mines may unintentionally contain asbestos if the main material of interest contains a significant amount of asbestiform particles. Ilgren (2004) reviewed numerous studies on various types of mines and other mineral deposits that contained minor amphibole components. In those studies, the amphibole particles present in the mineral deposits were primarily cleavage fragments, and no attributable asbestos-related diseases were reported at those sites. However, it is particularly important to pay close attention to the presence of amphiboles at mining sites and to carefully examine mineral deposits for the presence of asbestos. In some cases, if asbestiform minerals are dispersed throughout an ore body, exploitation of the mineral resource may not be possible unless very careful mineral beneficiation procedures are followed (Ross and Nolan 2003). In cases where asbestoscontaining veins are dispersed between larger volumes of uncontaminated ore, it may be possible to selectively mine the valuable resource without disturbing the ACM. In 2000, tremolite asbestos was reported by the media to be present in children’s crayons (Schneider 2000, 2001; Schneider and Smith 2000; Seattle Post-Intelligencer 2000). The alleged tremolite asbestos supposedly occurred as a contaminant in talc, a hydrated magnesium sheet silicate {Mg6Si8O20(OH)4}, which was used as a strengthening agent in the formulation of the crayons. Talc can occur with several crystal habits from plates to fibres and usually contains other mineral particles (Rohl and Langer 1974). Figure 17 shows FESEM secondary electron images of typical ‘platy talc’ particles. The larger particles do appear plate like; however, some elongated particles (i.e. aspect ratio ≥3:1) are also visible. Reports of asbestos in crayons were highlighted in the media and fuelled negative public reactions (Schneider 2000, 2001; Schneider and Smith 2000; Seattle PostIntelligencer 2000). In each instance, a claim was made by a laboratory or ‘expert’ that amphibole asbestos was observed in these products. Following the revelation and subsequent national publicity, careful tests on crayons were performed by a number of scientists (US CPSC 2000; Beard et al. 2001; Verkouteren and Wylie 2001) who proved that there were no significant amounts of asbestos fibres in these products. Particles that had been identified by some laboratories as ‘asbestos’ were mainly non-asbestiform mineral particles (i.e. cleavage fragments) or ‘transitional fibres’ that did not fit a precise mineral category. Transitional fibres have characteristics of asbestos fibres and non-asbestos fibres within the same particle (Beard et al. 2001). Such fibres of mixed mineral assemblages have physical properties outside the range of asbestos and therefore are not regulated (Beard et al. 2001; Verkouteren and Wylie 2001). The reasons for the conflicting analysis results among the different laboratories were attributed to confusion and inconsistencies in the definition of asbestos and the analytical methods used to distinguish between fibres and cleavage fragments (Beard et al. 2001; Schneider 2001). Little to no publicity followed the findings that the particles present in the crayons were not asbestos, leaving the public completely misinformed as to the truth: that the original findings were in error and that their children’s health was not at risk. Even



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Figure 17.

FESEM secondary electron images of platy talc particles from R.T. Vanderbilt Co., Inc.

so, crayon manufacturers voluntarily removed talc from their crayon formulation (Schneider 2001). Similarly, asbestos alleged to be present in children’s play sand (Germaine 1986; Schneider 2000), proved to be incorrect after more careful study (Langer and Nolan 1987). These examples illustrate the great importance of proper identification and characterization of mineral particles alleged to be asbestos thought to be present in materials.



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Incidence of occupational asbestos-related disease has been reported in the case of the former vermiculite mine in Libby, MT, where a portion of the amphibole contaminant was asbestiform (McDonald et al. 1986a, 1986b; Amandus and Wheeler 1987; Peipens et al. 2003; Rohs et al. 2008). Libby is a small town in northwest Montana within the Rainy Creek Complex – an igneous alkaline-ultramafic body. Libby was at one time the site of the world’s largest vermiculite mine, accounting for almost 80% of the world’s vermiculite production (McDonald et al. 1986b; Virta 2001). Vermiculite, a silicate mineral {(Mg,Fe,Al)3(Al,Si)4O10(OH)2·4H2O} that expands ∼10–20 times its original volume when heated, has insulating and absorbent properties and is fire resistant (Peipens et al. 2003; Rohs et al. 2008). The mineral has found use in the construction industry as an insulation and filler material and in agriculture as a soil additive and carrier agent for fertilizers and other chemicals (Germaine 1986; Langer and Nolan 1987; Virta 2001). The unprocessed vermiculite ore reportedly contained an estimated 0–5% amphibole, both asbestiform and non-asbestiform varieties (Wylie and Verkouteren 2000; Virta 2001; Bandli et al. 2003; Gunter et al. 2003; Meeker et al. 2003; Peipens et al. 2003; Ross and Nolan 2003; Bandli and Gunter 2006). Epidemiological studies conducted in the 1980s found a high incidence of asbestos-related disease among the mine workers (McDonald et al. 1986a, 1986b; Amandus and Wheeler 1987). National attention refocused on the small town in late 1999 when the media reported a high incidence of asbestos disease among Libby residents (Ross and Nolan 2003). Within days of the first media reports, the EPA began an investigation and remediation effort. The area is now designated as a Superfund site. The Superfund action at Libby ranks among the largest and most costly in the history of the EPA (Ross and Nolan 2003). A 2003 study in the Libby area conducted by the USGS stated that: ‘the ultimate resolution . . . will be years in coming, and the final costs . . . may be enormous’ (Meeker et al. 2003). Confirming this prediction, a report issued by the EPA Office of Inspector General in December 2006 stated that the EPA still cannot verify the effectiveness of its 7-year, $100 million effort to remediate asbestos in over 700 Montana homes (Renner 2007). The mineralogy in the Libby area is complex. Several researchers had incorrectly identified the predominant asbestiform amphibole present in Libby vermiculite as tremolite, but recent studies on the amphibole minerals in Libby have indicated that winchite and/or richterite (two unregulated minerals) are most likely the predominant species of asbestiform and non-asbestiform amphiboles present in the area (Wylie and Verkouteren 2000; Gunter et al. 2003; Meeker et al. 2003; Bandli and Gunter 2006). In spite of these recent studies, Bandli and Gunter (2006) reported that the inaccurate names tremolite and actinolite are still cited in EPA literature and in the popular press as the asbestos material present in Libby vermiculite and in the Libby area. Currently, government agencies do not regulate all asbestiform minerals or cleavage fragments, so it is crucial to understand the precise mineralogy of any potential ACM in order to properly regulate potentially ACMs and protect the public health. One USGS study noted that the amphibole minerals in Libby continue to present formidable challenges to the analysts, to anyone attempting to classify these materials using existing regulatory definitions, and particularly to those attempting to extrapolate those morphological features and chemical compositions to understand potential health risks (Morimoto 1988). Regardless of whether they are asbestiform, average ambient concentrations of amphibole particles longer than 10 μm and thinner than 0.4 μm reportedly range from 0.0002 fibres per ml to below detectable limits in the community (US EPA 2006). Estimates of asbestiform concentrations range from 1 to 10% of the ambient concentrations


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(Lee and Van Orden, to be published). Thus, amphibole concentrations at Libby are similar to background concentrations for total asbestos (primarily chrysotile) in other urban environments. In fact, a recent EPA 2-year study (US EPA 2009) reported that current asbestos levels in Libby’s air are low enough that they do not pose a significant cancer risk. The potential asbestos exposure pathways in Libby include individuals with occupational exposure, family members exposed through worker contaminated work clothing, community members exposed through ambient environmental levels, and residents exposed through the use of vermiculite as insulation or as a soil additive. Because of the multiple exposure scenarios, causes of disease among the non-mining population are confounded and in dispute (MSHA 2001, 2002; Price 2003). Concerns also continue the workers who processed Libby vermiculite in manufacturing plants scattered throughout the USA, environmental contamination surrounding those plants, and the customers of those plants who used vermiculite products as insulation in their homes or in their gardens (Wright et al. 2002; Ross and Nolan 2003). Much of the debate surrounding Libby stems from the lack of a coherent national policy and scientific consensus on the definition of asbestos, the methods of identifying and classifying asbestos fibres and rock fragments, and the use of risk models that are based on exposures to commercial asbestos fibres in a situation where only a portion of the counted particles have the characteristics of asbestos fibres. Naturally occurring asbestos (NOA) in non-mining areas Public, business, and government concerns involving what is called naturally occurring asbestos (NOA) are currently widespread throughout the USA (Lee et al. 2008b). NOA contamination in various geographic locations has generated great fear and worry and has prompted, in some instances, very costly and controversial remedial actions. The case of purported NOA in El Dorado County, CA, is an example that illustrates the types of problems that can occur as a result of the various issues related to NOA. El Dorado County, CA, is located within the Great Valley ophiolite belt, which includes numerous outcrops of serpentinite and other ultramafic rock as a result of tectonic activity (Ross and Nolan 2003). The region formerly contained numerous chrysotile asbestos mines. El Dorado County attracted many new residents in recent years, so many that the population has increased nearly six-fold since 1960 (California State Senate 2005). During excavation for housing sites in El Dorado County in 1998, tremolite asbestos was allegedly reported at some of the sites, which alarmed homeowners and lowered home values. The local media produced a series of articles that suggested that the county residents’ exposure to asbestos was endangering their health and the county has been in turmoil over the issue of NOA ever since. The media stories have focused on asbestos found in homes near mining and construction activities, animals from the region that had high levels of asbestos in their lungs, and testing at three schools and a community centre: Rolling Hills Middle School, Silva Valley Elementary School, Oak Ridge High School, and El Dorado Hills Community Centre (California State Senate 2005). In 2003, the EPA conducted a series of tests at schools and other public areas in the community of El Dorado Hills to assess potential asbestos exposure (Ecology and Environment, Inc. 2005; US EPA 2005). The testing included simulated activities that can create dust such as baseball, basketball, and soccer games at schools, running and biking on nature trails, playground activities, and gardening. The study found asbestos fibres in almost all of the air samples collected during these tests and indicated that personal



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exposure levels were significantly higher during most sports and play activities compared to levels in the samples taken nearby, outside the areas of activity. The results of the study led to extensive mitigation efforts in the county. The EPA El Dorado study was later challenged, however, by a scientific report. After a careful and thorough particle-by-particle review of the EPA data and additional analysis of split samples, the materials identified as asbestos by the EPA through its contract laboratories were not asbestos, based on chemistry and morphology, but were amphibole cleavage fragments and therefore should not be considered a major health risk (RJ Lee Group, Inc. 2005; Lee et al. 2006, 2007, 2008a). The conclusions made in the critical review were supported by a number of mineralogical and asbestos experts (Langer 2005; Wylie 2005; Ross 2007). A lengthy debate ensued, highlighting the lack of consensus on the definition of asbestos, the relative risk posed by cleavage fragments, and the methods for distinguishing them. The most recent study in the El Dorado area was conducted in 2006 by the USGS on behalf of the EPA (Meeker et al. 2006). The study found that the types of amphiboles in the El Dorado Hills area are not easily characterized using standard commercial asbestos test methods. In fact, the USGS report stated that if the EPA study had been conducted as an enforcement action, it would be inappropriate to classify the amphibole particles in El Dorado Hills as an ‘actionable material’ because: (1) the majority of the particles were prismatic, not fibrous; and (2) approximately 40% of the particles were magnesiohornblende, a non-regulated amphibole. Figure 4a shows an FESEM image of an elongated prismatic actinolite mineral particle that was found in an El Dorado Hills soil sample. Although the particle is an amphibole mineral, it is clearly not a fibre and not asbestos. Figure 18 shows FESEM images of another particle found in an El Dorado Hills soil sample. This particle was also an actinolite, but the particle had an irregular morphology with attached irregular mineral debris and is not an asbestos fibre. The observed morphologies of the elongated mineral particles shown in Figures 4a and 18 are typical of those found in El Dorado Hills (RJ Lee Group, Inc. 2005; Lee et al. 2006, 2007, 2008a). The USGS study (Meeker et al. 2006) noted that the emerging practise of fully characterizing all particles of potential concern, both chemically and morphologically, will aid in developing appropriate analytical procedures, interpretation of epidemiological data, and development of regulatory policies to deal with situations such as the one in El Dorado Hills. Again it was concluded that the health, mineralogical, and regulatory communities consider a thorough evaluation of the existing asbestos definitions and analytical methods for application to NOA problems. As a result of the EPA testing at Oak Ridge High School, completed mitigation efforts in the school district cost over $1.7 million and in excess of $1.8 million for a new elementary school built in El Dorado Hills (Barber 2005). Data from the adjoining community of Folsom, CA, indicate that the cost will be in excess of $5 million to mitigate alleged NOA concerns during the construction of a new high school (Barber 2005). The necessity for these costly remediation actions is still very much in debate as a result of unanswered questions over testing methods and risk assessments. The manner in which NOA has been addressed in El Dorado Hills has had a tremendous impact on the local government and the schools, but the potential impact extends to the entire State of CA and the nation as similar conditions and events arise elsewhere. Summary As a naturally occurring mineral, asbestos has existed in the environment for millions of years. However, asbestos, whether it exists naturally in rocks and soil, in the workplace, or


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Figure 18. FESEM secondary electron images of an irregular actinolite particle in a soil sample from El Dorado Hills, CA. (a) Full particle image, (b) full particle stereo pair image, (c) particle left end image, and (d) particle right end image.

in manufactured products, is still asbestos and poses a serious potential health hazard if fibres are released into the air. Rational regulations and public policies based on sound scientific, engineering, and medical practises are needed to ensure that the public health is properly protected. However, the latest scientific and medical research available does not justify the claim that exposure to any amount of a substance labelled as an asbestiform fibre presents an unacceptable health risk. Ross and Nolan (2003) pointed out that if this were true, rocks containing any concentration of fibrous mineral could not be used for any kind of mining or similar types of activities, and thus would restrict the use of vast areas of geologic terrain needed for sustainable commercial and economic development. Good regulatory policies must weigh the health risks of action and inaction as well as the financial costs. Over-regulation and flawed public policies based on incorrect science can be extremely costly, while minimally addressing health risk, and diverting attention from more socially important endeavours. Unnecessary mitigation efforts have a negative impact on local US government’s ability to provide quality educational facilities, and public funds would be better directed towards more important social programmes and services. Thoughtful regulations, legal reform, and better education of the media and general public are needed to bring a scientific basis to public policies regarding potential exposure to asbestos and other types of airborne mineral particles, whether the asbestos is in products or the environment.


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Important areas for continued future research on asbestos include:

• reaching a consensus on the relative health effects of chrysotile versus amphibole asbestos;

• determining whether significant differences exist between the health effects of asbestiform mineral particles and cleavage fragments;

• determining whether low-level environmental exposure to amphibole asbestos presents a significant health risk;

• increasing our understanding of how mineral particles behave in the human lung and initiate disease;


• developing valid health risk assessment tools and models; and • developing reliable, cost-effective analytical testing methods that are reproducible, follow sound scientific and laboratory practises, and clearly distinguish between asbestiform and non-asbestiform particles.

Regarding the development of new analytical methods, the use of FESEM as a complementary tool to TEM or PCM appears to be a promising technique. Most importantly, accomplishing these objectives should lead to the development of a correct, comprehensive, and consistent definition of asbestos that will cover all areas dealing with asbestos including commercial interests, government regulations, the fields of mineralogy and geology, and analytical methodologies. To achieve the above research objectives and better educate the public requires increased cooperation and collaboration between the scientific, health, and regulatory communities. Glossary Acicular (particles): Needle-shaped or needlelike. The term is applied in mineralogy to straight, greatly elongated, free-standing (individual) crystals that may be bounded laterally and terminated by crystal faces. The aspect ratio of acicular crystals is in the same range as those of ‘fibre’ and ‘fibrous’, but the thickness may extend to 7 mm (Skinner et al. 1988). Acicular crystals or fragments are not expected to have the strength, flexibility, or other properties of asbestiform particles. Actinolite: A monoclinic calcic amphibole with the ideal composition Ca2(Mg,Fe2+) 2+ 5Si8O22(OH)2. Actinolite is the intermediate member of the Mg–Fe series tremolite2+ actinolite-ferro-actinolite. The composition is Mg/(Mg+Fe ) ≥0.5 and