DOI: 10.1007/s10967-009-0540-6
Journal of Radioanalytical and Nuclear Chemistry, Vol. 280, No.2 (2009) 431–437
Indoor radon levels and gamma-radiation in dwellings of the Metropolitan Zone of Guadalajara O. Y. Morales,1 T. Martinez,1* P. González,2 M. Navarrete,1 L. Cabrera,1 A. Ramírez1 1 Facultad
de Química. Departamento de Química Inorgánica y Nuclear, Universidad Nacional Autónoma de México, México DF, 04510, México 2 National Institute of Nuclear Research, Ocoyoacac, Edo. de México , 05045, México (Received February 17, 2009)
The present study in the Metropolitan Zone of Guadalajara (MZG) in the state of Jalisco was carried out along 2007. 222Rn (radon) and 220Rn (thoron) levels were measured in single-family dwellings in normal living conditions (open house condition). Measurements were accomplished by Electret Passive Environmental Radon Monitors (E-PERM) in different configurations. At the same time and for the same period, the gamma dose rate was measured by a LiF: Mg, Cu, P+PTFE thermoluminiscent dosimeter. A Quality Assurance Program was improved. Results show a lognormal distribution of annual concentrations and seasonal variations. Mean annual concentrations of radon were lower than US EPA and EEC action values. 220Rn levels appear to be higher than the typical value determined by the UNSCEAR Reports (1993 and 2000). Performance of different E-PERM configurations accomplishes the US EPA guidelines and yield useful results for diverse situations. Gamma rate dose was higher than that of the Metropolitan Zone of Mexico.
Introduction
Experimental
As it is well known, the interest regarding the exposure of human lives to high levels of natural radiation has been growing because of the radon radioactive dose counts for the main natural radioactive portion absorbed by humans.1 Radon contributes 54%, (1.3 mSv.y1) to the 2.4 mSv.y–1 effective annual average dose due to natural radiation sources. Indeed, the decay products of 222Rn (radon-progeny) and radon itself count for the main source. So, the dose emanating from the decay products of 220Rn (thoron) and 220Rn itself cannot be neglected since their range has been estimated to be 10–20%2 or even more (56.6%)3 compared to that originating from short-lived radon progeny. In order to obtain the radon and decay products annual average equivalent dose being absorbed by the Mexican population during the last years, the radon group of the Faculty of Chemistry at the National University of Mexico has undertook studies of indoor radon (222Rn and 220Rn) exposures in the Metropolitan Zone of Mexico City4–6 (this activity is closely linked to the work performed within the framework of an International Atomic energy Agency (IAEA) supported project. Some project objectives are the following: to improve the diverse configurations of passive integrated devices to determine annual mean indoor radon (222Rn and 220Rn) levels and seasonal variations in the MZG; to estimate the radon decay products concentration time evolution (potential alpha energy concentration, PAEC); the equilibrium factor in one MZG typical dwelling and the effective equivalent dose derived from them.
The study was carried out in four municipalities of the MZG: Guadalajara down town, Tlaquepaque, Tonalá and Zapopan (Fig. 1). Guadalajara is the capital city of the state of Jalisco and is located in the Southwest part of the Mexican plateau. Its altitude varies from 1540 m in Tlaquepaque to 1660 m above the sea level in Tonalá, while average temperature varies from 18.8 °C in Guadalajara center to 23.5 °C in Zapopan.7 Climate is semi-hot and sub-humid with rains falling down from summer up to the beginning of autumn. Average annual precipitation is in the range of 886 mm to 906 mm. Winter and spring are dry seasons. The area subsoil dates from the tertiary age and is composed by igneous, extrusive and sedimentary rocks, andesite, basalt and volcanic tuff. Dominant soils are regosol utric and Haplic feozem associated with luvisol. Tlaquepaque subsoil dates from to the quaternary period and is composed by pumitic tuff or pomaceous stones (ashes).7 The air survey was tested indoors (in a randomly chosen 25 single family dwellings,8 in the main living area, bedroom or living room). All dwellings were monitored in normal living conditions (open house conditions). 222Rn levels were measured by means of a smallvolume (50 mL “dome top”) chamber or “L” chamber being loaded with long-term electrets, “LLT” E-PERM configuration (Electret Passive Environmental Radon Monitor).9,10 Detectors were placed according to the Environmental Protection Agency (EPA) recommendations as described in detail in previous works.5,6
* E-mail:
[email protected] 0236–5731/USD 20.00 © 2009 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest Springer, Dordrecht
O. Y. MORALES et al.: INDOOR RADON LEVELS AND GAMMA-RADIATION IN DWELLINGS OF THE METROPOLITAN ZONE OF GUADALAJARA
Fig. 1. Chart of Monitored Area in the Metropolitan Zone of Guadalajara (MZG)
The total sampling period was one year from winter 2006 to autumn 2007, with readings at the end of each year’s season. 222Rn levels were also measured with “H” chamber (1000 mL) being loaded with a shortelectret, “HST” E-PERM configuration (just before placing the E-PERM for radon exposure), with a fivedays exposure period in each one of the four seasons along the sampled year. 220Rn levels were measured with “S” chambers (210 mL with on-off mechanism) being loaded with blue short-term electrets, one of them for 220Rn and the other one for 222Rn measurement (“SSTB” configuration).11 Both of the chambers were placed according to the EPA recommendations, as described in detail in previous work,6 and deployed during five days in each seasons along the year 2006–2007. Simultaneously and during the same time period, the gamma exposure rate was measured by a LiF; Mg, Cu, P+PTFE thermoluminiscent dosimeter12,13 developed at the National Institute of Nuclear Research, ININ to estimate the annual average and to rectify the radon concentration reading for electrets radiation response. It was also necessary to rectify estimated radon concentration by a factor for the altitude of the 432
measurement site.14 The field’s team filled out a form with detailed information of house’s characteristics and householder’s lifestyles. A Quality Assurance Program10,15–18 was devised that includes: (a) known exposure measurements; (b) duplicate detectors (10% of the total number of detectors deployed); (c) blank or control detectors for voltage drift during a time period similar to the one used for those deployed in homes; the voltage drift was not more than one volt per month over a three-months period; (d) routine instrument check and calibration. Results and discussions Figure 2 shows the annual mean indoor 222Rn concentration being measured by means of an E-PERM “LLT” configuration for each of the 25 dwellings in normal occupancy conditions from 24 to 243 Bq.m–3. It also shows the minimum and maximum average calculated in agreement with the control graphic statistic method,19 with a confidence level of 99.3% (3σ). The annual arithmetic and geometric mean (this latter attenuate the data dispersion) for all of the dwellings are 116.5±56.6 Bq.m–3 and 103 Bq.m–3, respectively, and
O. Y. MORALES et al.: INDOOR RADON LEVELS AND GAMMA-RADIATION IN DWELLINGS OF THE METROPOLITAN ZONE OF GUADALAJARA
the first one equal to that obtained in 1999 by ESPINOSA et al. with track detectors.20 Highest concentrations occurred in two dwellings located near the industrial zone during the winter season under the most adverse environmental conditions. Figure 3 shows seasonal variations measured with E-PERM “LLT” configuration (long-term) with a 17% average spring concentration lower than the autumn concentration. This seasonal variation was 55% for the same season when measured by the-PERM “SSTB” configuration (short-term). Frequency distribution of 222Rn measurements with E-PERM “LLT” configuration showed 70% of values below 148 Bq.m–3 and 91.6% below of 200 Bq.m–3, the accepted levels for the European Economic Community (EEC).5 Only 8.3% (two dwellings) showed the highest concentration between 221 and 250 Bq.m–3. Frequency distribution of 222Rn measurements with “S” chamber and short-term electrets, “SSTB” configuration showed that 100% of radon values are lower than 148 Bq.m–3. When radon is measured without a clear-cut discrimination of disturbances (like that allowed by an “L” ionization chamber), the influence of 220Rn is evident. 222Rn values21 are three times lower when accurate discrimination is made. Both frequency distributions are compatible with a log-normal distribution.
Table 1 shows 222Rn and 220Rn indoor concentrations when measured with varied E-PERM configurations. 222Rn concentration measured with short-term electrets with “SSTB” and “HST” configurations shows different mean values at p
