Radon concentration levels in 16th and 17th ...

DOI: 10.1007/s10967-009-0537-1

Journal of Radioanalytical and Nuclear Chemistry, Vol. 280, No.2 (2009) 419–422

Radon concentration levels in 16th and 17th century’s churches and convents G. Espinosa,1* J. I. Golzarri,1 A. Angeles,2** F. Juárez,3 T. Martínez,4 M. Navarrete4 1 Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, México, D.F., México Nacional de Investigaciones Nucleares, Km. 36.5 Carretera Mexico-Toluca, La Marquesa, Ocoyoacac, 52750, Estado de México 3 Instituto de Geofísica, Universidad Nacional Autónoma de México,Circuito Institutos, Ciudad Universitaria, 04510, México, D.F. México 4 Facultad de Química, Universidad Nacional Autónoma de México, Edificio D, Ciudad Universitaria, 04510 México, D.F. México

2 Instituto

(Received February 17, 2009)

The preliminary results of indoor radon concentration measurements taken in some of Mexico City’s colonial churches and convents are presented. Measurements were taken in the churches of Santa Catarina, La Conchita, San Juan Bautista, San Antonio Panzacola, San Diego and San Mateo and the church and convent complex of El Carmen. These structures are all located in Coyoacan, a borough of Mexico City. Indoor radon concentrations in churches and convents constructed in the 17 th and 18 th centuries are interesting for several reasons. Most of these buildings were built using the stones of ancient Mexican pyramids, mainly blocks of basalt and volcanic pyroclastic rocks, and possess walls between 40 cm and 70 cm thick and naves with large volumes of air and relative low ventilation. The churches are public places with people most of the time. The indoor radon concentrations were measured using nuclear track detectors consisting of a closed-end cup containing CR-39 Lantrack® polymer as detector material. The measurements were taken over four periods of three consecutive months. The results show indoor radon concentrations of between 82 and 165 Bq.m–3, below to the United States Environmental Protection Agency (USEPA) indoor radon action level for workplaces. Using these results, the radiological risks were calculated and found to be negligible.

Introduction The inhalation of radon gas is responsible for about 47% of the natural effective dose received by the humans from natural radioactive sources.1 Outdoor radon does not represent a significant health hazard because high concentrations are never reached, but radon becomes a problem when released into an enclosed or poorly ventilated area such as private dwellings, public buildings, mines and caves visited by tourists.2–5 The aim of this work is to characterize indoor radon (222Rn) concentrations in churches and convents constructed in the 17th and 18th centuries. Mexico, as a former Spanish colony, abounds in colonial churches. As an example, the small town (50,000 inhabitants) of Cholula is home to more than 365 colonial churches. Figures 1 and 2, show colonial churches styles in Mexico. In Mexico’s older cities such as Mexico City, Puebla, Queretaro and Guanajuato, colonial churches are found in every three or four city blocks. As a start to understanding the distribution and concentration of radon in Mexico’s colonial churches, seven churches and one convent, all located within the borough of Mexico City called Coyoacan, were chosen for the study. This study represents the first evaluations of radon concentrations in Mexico City’s colonial churches. Some of the churches chosen are still used for their original purpose, while others are museums open to the public. All churches in Mexico belong to the state. Special permission from the government was sought in order to have access to the churches for the indoor radon evaluations.

Fig. 1. San Juan Bautista church

* E-mail: [email protected] ** PhD Graduate student UAEMex 0236–5731/USD 20.00 © 2009 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest Springer, Dordrecht

G. ESPINOSA et al.: RADON CONCENTRATION LEVELS IN 16TH AND 17TH CENTURY’S CHURCHES AND CONVENTS

Fig. 2. San Antonio Panzacola church

Churches description6–7 – Santa Catarina (Ch-1): Originally an open chapel consisting of a frame over bases and capitals. Towards the 17th century a chapel with a barrel-vaulted nave was constructed to complement the open chapel, and the openings and the choir were walled in. Subsequent changes include the adding of a tower and the loss of the tower’s lantern in the earthquake of 1985. The church was declared a National Monument on August 16, 1932. – La Conchita (Ch-2): A prehispanic ceremonial center existed in the plaza affectionately known as “La Conchita”. It has been asserted that upon arrival of the Spanish, Hernan Cortes ordered the construction of a provisional chapel, which would be temporarily administered by military chaplains. Religious administration was assigned to the Franciscans in 1524. The Commission of Sacred Art states of the historically and architecturally important structure that “At the beginning of the sixteenth century “La Conchita” was the official residence of Hernan Cortes, who ordered the construction of this chapel to mark the site on which the first mass was celebrated and, according to traditions, in order that Malintzin may say her prayers”. – San Juan Bautista (Ch-3): One of the most ancient Catholic temples in the Valley of Mexico, construction having begun in the 16th century. Its plateresque façade and vestibule of three naves from that time have been 420

conserved to the present day, although the interior has been reconstructed. The complex initially belonged to the Dominicans, who later ceded its administration to the Franciscans. The beautiful Baroque altarpiece dating from the 17th century conserves la Capilla del Rosario (the Chapel of the Rosary). – San Antonio Panzacola (Ch-4): Novohispanic chapel of the 17th century, of Mexican Baroque style; constructed beside a former Camino Real (King’s Highway). Its façade displays a stone relief of San Sebastian Martir and an exterior niche with the image of San Antonio de Padua. Its attractiveness lies in the harmony of its proportions, a characteristic also displayed in the contiguous bridge sharing its name with the chapel. – San Diego (Ch-5): Originally a hermitage with a humble house as annex, constructed by the Franciscans in the first third of the 16 th century, around 1524, over the ruins of an ancient prehispanic temple dedicated to the god of war Huitzilopochtli. – San Mateo (Ch-6): Chapel constructed by Franciscans in the 17th century using volcanic rock from the town of Los Reyes. It is currently close to the National School of Art Restoration, the latter with more than 1500 students. – Church and convent “El Carmen” (Ch-7 and Cv-1): In 1597, the tribal chief of Coyoacan, together with Don Felipe de Guzman Itzolinque, Andres de Mondragon and Elvira Gutierrez, donated to the Carmelitas of Mexico some lands in the neighborhoods Tenanitla and Chimalistac of Coyoacan, at the time a few kilometers south of the capital city. Thus the religious order acquired extensive lands on which to establish a new college and house. The definitive foundation of the Carmelite college dedicated to San Angel took place in 1613. General geological characteristics of the places studied As previously mentioned the seven churches used in the study were all constructed in the 16th or 17th centuries and are all located within the region currently called Coyoacan. Coyoacan is one of the 16 boroughs, into which the Distrito Federal (Federal District – the location of part of Mexico City) is divided. It is located at the geographical center of the Federal District, in the south of the Valley of Mexico, and covers a surface of 54.4 square kilometers, an area which represents 3.6% of the territory of the capital. The borough of Coyoacan is located within the area bounded by the latitudes 19°18’ N and 19°21’ N and the longitudes 99°06’ W and 99°12’ W, and at an altitude of 2240 meters above sea level. The soil of the borough is largely made up of two types: soil of volcanic origin, from the nearby volcano Xitle, and soil from the lacustrine zones, originating from the lakes which used to be found in the area.


Radon measurement methodology Nuclear Track Methodology (NTM), making use of a passive closed-end cup device with Poly Allyl Diglycol Carbonate (PADC),9 known by its trade name CR-39 (Lantrack®), as detector material, was chosen for the indoor radon survey.8 This set-up possesses both high sensitivity to radon gas and also a highly efficient energy response for 5.49 MeV alpha particles. The 500µm-thick CR-39 was cut automatically by laser beam into chips of size 9×18 mm, and laser-inscribed for classification and numerical identification. The polycarbonate is covered with 100 µm-thick polyethylene film in order to provide mechanical, environmental and radiological protection during storage and transport. The measurements were taken in the main nave of the churches, at a height of 1.80 m, over a period of one year, divided into four periods of three months (from March 2006 to March 2007). The open ends of the cup-type monitors were protected with a 15µm plastic membrane (garbage bag for gardens) in order to exclude the 220Rn and radioactive 222Rn decay products present in ambient air. However, radioactive 222Rn decay products will be produced inside the cup and irradiate the CR-39 with alpha particles of energies ranging up to 5.49 MeV. After exposure the detectors were subjected to onestep chemical etching in a 6.25M-KOH solution at 60±1 °C with an etching time of 18 hours, this procedure forming part of well-established protocols for the taking of indoor radon measurements. The detection system was calibrated in the Oak Ridge National Laboratory (ORNL-USA) radon chamber. This calibration is certified each year at the ORNL and verified every six months in our radon chamber. The detectors were read automatically with a Digital Image Analysis System (DIAS)10 and the data analyzed automatically in a PC with Microsoft Excel® software. Results The results show relatively low indoor radon levels, independent of the building materials, geological

characteristics of the soil and the air volume inside the churches. Table 1 displays the average values recorded for each season and the annual arithmetical mean. The lowest values were recorded in San Mateo church for all four seasons (82 Bq.m–3), and the highest value in San Juan Bautista church (161 Bq.m–3) in summer. This latter church also recorded the highest yearly average of 154 Bq.m–3, a concentration relatively close to 148 Bq.m–3, the recommended indoor radon levels for houses (USEPA), but a long way from the 400 Bq.m–3 recommended for workplaces.11 Radiological risk The effective dose and the derived risks are associated mainly with the inhalation of short-lived polonium (218Po and 214Po), a radon progeny alpha emitter. A comprehensive analysis of the radiation dose should consider detailed information on this aerosol as well as the degree of disequilibrium between radon and its progeny for each site and for each season. The literature reports values of the equilibrium factor ranging between 0.36 and 0.52, which suggests that an average value of 0.4 could be an acceptable in order to estimate radon progeny exposure from radon concentration measurements.12,13 Given the average indoor radon concentration and building occupancy rates and employing the WISE Uranium Project calculator,14 it is possible to calculate dose rates for individuals exposed. Values used in these calculations are from ICRP-65 (1993).1 Table 2 shows values for the radiation dose per hour, per year and the health risk for an individual exposed to radon and its decay products, assuming radon concentrations of 100 Bq.m–3 and 150 Bq.m–3 and an occupancy time of 2000 hours per year (40 hours per week for 50 weeks). In Table 2 it can be observed that there is not a significant radiological risk inside the 16th and 17th century churches studied. The main factors contributing to this low risk value in these old buildings are ventilation habits (open windows and doors for more than 12 hours a day) and broken glasses in the cupola windows.

Table 1. Indoor radon levels recorded in the locations studied Church name Sta. Catarina La Conchita San Juan Bautista San Antonio Panzacola San Diego San Mateo El Carmen Church El Carmen Convent

Code Ch-1 Ch-2 Ch-3 Ch-4 Ch-5 Ch-6 Ch-7 Cv-1

Spring

Summer

Autumn

Winter

(Bq.m–3) 103 ± 10.1 151 ± 12.3 153 ± 12.4 114 ± 10.7 95 ± 9.7 90 ± 9.5 102 ± 10.1 132 ± 11.5

(Bq.m–3) 98 ± 9.9 148 ± 12.2 161 ± 12.7 120 ± 11.0 100 ± 10.0 97 ± 9.8 111 ± 10.5 156 ± 12.5

(Bq.m–3) 95 ± 9.7 139 ± 11.8 158 ± 12.6 119 ± 10.9 98 ± 9.9 95 ± 9.7 105 ± 10.2 140 ± 11.8

(Bq.m–3) 94 ± 9.7 134 ± 11.5 144 ± 12.0 105 ± 10.2 87 ± 9.3 82 ± 9.1 98 ± 9.9 111 ± 10.5

Annual mean (Bq.m–3) 97.50 ± 4.94 142.75 ± 5.98 154.00 ± 6.20 114.50 ± 5.35 95.00 ± 4.87 90.75 ± 4.77 104.00 ± 5.10 134.75 ± 5.80

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* Table 2. Radiological risk assuming occupancy of 2000 hours per year Radon concentration Dose per hour Dose per year Excess lifetime cancer risk

100 Bq/m3 314.0 nSv 628.1 µSv (0.126 WLM)

150 Bq/m3 471.1 nSv 942.2 µSv (0.188 WLM)

0.142% (1 : 703.2)

0.213% (1 : 468.8)

Conclusions Indoor radon levels were measured in seven 16th and 17 th century colonial churches and one convent in Mexico City. No high radon concentrations were found, no matter the construction materials and size of the enclosed spaces. As a consequence the radiological risks were correspondingly low, even assuming a high occupancy factor of 2000 hours per year. The importance of including churches in such studies lies in the large number of people attending religious ceremonies and events. A benign year-round climate permits the favorable ventilation habits of the places measured which we consider to be the main factor contributing to the low indoor radon levels. The present study represents the first time indoor radon concentration measurements have been carried out in colonial churches and convents, and opens the way to future indoor radon evaluations in old public buildings and museums, as per the 90/143 Euratom recommendations.

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The authors wish thanks to D. AGUILAR and A. GARCIA for their technical help. This work was partially supported by PAPIIT-DGAPAUNAM project 1N107707.

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