Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5

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2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5

INDOOR RADON MEASUREMENTS IN DWELLINGS AND WORKPLACES OF CURITIBA URBAN AREA (BRAZIL)

Flávia Del Claro1, Sergei A. Paschuk1, Janine N. Corrêa1, Zildete Rocha2, Jaqueline Kappke1, Allan F. N. Perna1, Hugo R. Schelin1, Talita O. Santos2

1 Universidade Tecnológica Federal do Paraná Av. Sete de Setembro, 3165 80230-901 Curitiba, PR [email protected], [email protected], [email protected],

2 Centro de Desenvolvimento da Tecnologia Nuclear Av. Presidente Antônio Carlos, 6627 31270-901 Belo Horizonte, MG [email protected]

ABSTRACT

Considering that radon and its progeny exposure is proved to be the main cause of lung cancer among non- smokers and occupation-time at some commercial establishments and workplaces is equal or even bigger then at domiciles and dwelling, present study has been spread to the constructed closed environment and workplaces of commerce and productive sector. The measurements were performed by the Laboratory of Applied Nuclear Physics of UTFPR in 2009 - 2011 when 120 detectors were installed at domiciles and workplaces of Curitiba, Paraná St., Brazil. Experimental setup was based at CR-39 detectors that were installed in diffusion chambers protected with filters. In collaboration with CDTN/CNEN it was performed the calibration of CR-39 detectors at the NIRS in Japan. The exposure time was set to be of 100 days. Alpha particle track development was performed using 6.25M sodium hydroxide (NaOH) solution and ethanol (2%) during 14 hours at 70°C. The counting was conducted using an optical microscope. Measured 222Rn activity levels in dwellings varied between 4.37 Bq/m3 and 320.82 Bq/m3 resulting at an average of 46.94 Bq/m3. Indoor measurements at workplaces presented the variation of radon activity concentration between 3.08 Bq/m3 and 67.50 Bq/m3 resulting at the average of 34.51 Bq/m3. Considering the recommendations of the World Health Organization, UNSCEAR and the International Commission on Radiological Protection (ICRP) concerning the radon-in-air concentration inside the dwellings that can reach 200 Bq/m3 taking into account the occupation-time of 7000 hours/year, obtained results are within normal limits and no mitigation measures have to be performed.

Keywords: Radon, CR-39 detector, Environmental measures.

1. INTRODUCTION

Among the principle sources of radon-in-air inside the dwellings it has to be mentioned the soil, water and construction materials. The knowledge about the factors that influence the entry of 222Rn in the internal structures of dwellings has been improved after the analysis and calculations involving the simplified model for houses and masonry buildings cited in the UNSCEAR Annual Reports on the Effects of Atomic Radiation (1988 and 1993) apud UNSCEAR [1].

The 222Rn can be incorporated into the internal atmosphere of a dwelling following several ways, including its release and emanation from soil through breaks in the foundation and by the diffusion through building materials. With regard to building materials as a source of radon in air, several studies [2 - 6] report that the soil contribution is about 10 times bigger than originates from common construction materials with high degree of compaction, such as stones, concrete and masonry bricks.

Nevertheless, the diffusion of 222Rn from the building materials is usually considered as a minor source when compared to gas entry directly through the foundation, in some cases it is possible that concrete or masonry bricks manufactured from materials that contain traces of natural radioactive elements may be significant sources of radiation.

In general, the entry of radon is effected by barometric pressure, ambient and outdoor temperature, differential pressure, wind speed, etc. Studies focused at the meteorological factors and their influence on the internal concentrations of 222Rn and its progeny were performed by Steinhaus (1975) apud Eisenbud and Gesel [7].

Actually the International Norms such as ICRP 65 [8], for example, states that the limit for indoor radon activity within the dwelling environment has to be below 200 Bq/m3. The houses with radon concentration in air between 200 - 400 Bq/m3, has to be observed and monitored and some remediation measures have to be taken when radon concentration is found between 400 - 600 Bq/m3.

The intervention level is considered to be equal or greater than 600 Bq/m3. The US Environmental Protection Agency (EPA) [9] suggests practical intervention in residence where the concentration of radon reaches 148 Bq/m3.

For evaluation of the risk and necessity of interventions the National Nuclear Energy Commission (CNEN) of Brazil in the Regulatory Position 3.01/007 [10] recommends that the generic level of annual dose for general public has to be considered of 10 mSv/year which corresponds to the maximum of radon activity in air of 300 Bq/m3 in agreement with recent evaluation performed by the ICRP 106 [11].

Rather frequently the studies concluded in Europe and United States have found the radon concentration levels between 2,000 and 50,000 Bq/m3, while the recommended acceptable level [8-13] is between 148 and 200 Bq/m3.

The surveys concluded in Sweden, Canada, and the United States [2] revealed very high incidence of elevated radon and its progeny levels in ordinary houses that involve potential risks for the human health. These findings have lead to a wide range of activities and intensive research aimed at limiting human exposure to radon.

The World Health Organization reported [14] that in many countries around the world the average concentration of indoor radon remains below 148 Bq/m3 as it is established by EPA [9] and thus below 200 Bq/m3 recommended by UNSCEAR [1].

Table 1 shows some results of several studies performed in different regions of Brazil concerning indoor radon concentration.

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It could be observed that the average concentration of radon in the monitored regions of Brazil does not significantly exceed the value of 200 Bq/m3. At the same time, it could be seen that some results presented in Table 1 are significantly exceeded the recommended limits of radon-in-air in dwellings established by UNSCEAR [1] on the basis of analysis of international data.

Table 1. Values of indoor radon concentration in some regions of Brazil

Radon concentration, (Bq/m3) Region Number of Mean Minimum Maximum Reference measurements Poços de Caldas-MG Rural area 30 204 50 1046 [15] Rural area 41 220 27 1024 [16] Urban area 97 61 12 920 [16]

Monte Alegre-PA Rural area 18 116 40 338 [6] Urban area 26 75 22 188 [6] Rio de Janeiro 48 40 9 200 [17] Campinas-SP 67 80 20 254 [18] Santos - SP 8 124±37 - - [19] São Paulo 63 147 33 562 [20] São Paulo 90 131 31 615 [20] Campo Largo-PR (1) 9 60 6 123 [26] Curitiba Centro-PR (1) 11 42 6 126 [21] Curitiba Portão-PR (1) 11 85 5 486 [21] Campo Largo-PR (2) 5 186 2 637 [21] Curitiba Centro-PR (2) 30 76 5 268 [21] Belo Horizonte 501 108 4 2664 [22]

2. MATERIALS AND METHODS

For evaluation the human exposure to radon (222Rn) in dwellings of Paraná St., it was used the experimental setup based on passive track etch detectors CR-39 that were installed in diffusion chambers protected with glass microfiber filters.

The measurements were performed using the diffusion chambers similar to described in [23], which has a sensitive volume of 7.1 cm³ in the form of semi-sphere with 3 cm in diameter covered by a hollow cap for the entrance of external air. The cavities in the cap are covered by glass microfiber filter to restrain the entrance of dust and aerosols. The Figure 1 shows the general view of the diffusion chamber used in the measurements.

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To decrease the plate-out of the 222Rn decay products at the surface of CR-39, the diffusion chamber is made from carbon fiber composite plastic with high electrical conductivity. Thus, the plate-out of decay products of 222Rn mainly occurs at the wall surface of the chamber. More over, the semi-spherical format of the chamber provides a better transport of the radon decay product toward the wall and their better absorption there.

The glass microfiber filters were chosen following the results [24] where the performance of different kind of polymeric and glass microfiber filters were tested. It was concluded that the best material suited for the measurements when it is impossible to control the temperature is the glass microfiber. Other kind of filter materials such as the polymeric, for example showed large fluctuations in permeability under mentioned conditions and the properties of glass microfiber paper proved to be independent of temperature.

Figure 1. General view of diffusion chamber used in radon monitoring.

The CR-39 is track etch detector very sensitive to alpha particles. It was chosen due to its stability, high optical transparency and accumulated previous experience in chemical development of alpha particle tracks. CR-39 detectors are widely used in numerous radon long-term measurements and monitoring [25-28]. Figure 2 shows an image of the LANDAUER’s track etch detector CR-39 ready to be installed in diffusion chamber.

Present radon survey was conducted at two types of environments: dwellings and commercial workplaces of an enterprise from financial branch.

The dwellings chosen for monitoring were masonry houses built with ceramic or concrete blocks with total area of 150 - 200 m2, similar architectural elements, such as windows, doors, wall coatings and flooring materials, and partially ventilated.

The detectors were installed at the ground floor inside the dwellings in the rooms occupied more than few hours per day, at the distance bigger than 50 cm from the walls, at the height approximately equal to sitting adult or a child in standing position.

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Figure 2. General view of CR-39 track etch detector.

The same approach was used for radon measurements inside the commercial workplaces where within the same enterprise 19 rooms were chosen for survey that attend these criteria.

The exposure time of CR-39 detectors in air was about 3 months. This time was chosen reflecting previously performed calibration and considering that expected radon activity in dwellings will not be bigger than 600 Bq/m³. The upper limit for statistical errors of 10% as well as the superposition (overlapping) of alpha particle tracks also were considered during the planning of measurements.

After the chemical development of exposed CR-39 detectors, obtained number of alpha particle tracks discounting the background was used to calculate the activity concentration of radon in air considering the calibration factor and efficiency of the system. The track counting was performed manually using the optical microscope with magnification of 100x and glass overlay mask, which permitted to identify the CR-39 surface area of 1cm2.

Before the measurements, in collaboration with the Center for Development of Nuclear Technology (CDTN/CNEN) it was performed the calibration of radon detectors at the National Institute of Radiological Sciences (NIRS) in Japan [29] where 30 diffusion chambers with mounted CR-39 detectors were exposed within controlled Environmental Storage Room in the atmosphere with varied concentrations of 222Rn. After that, exposed detectors were returned to the laboratories of CDTN and UTFPR where they were etched and read. The comparison between radon activities used for calibration and obtained number of Bq ⋅cm2 ⋅h alpha particle tracks together with statistical errors resulted in value of (405 ± 30) m3 that has to be multiplied by the density of counted alpha particle track per 1 cm2 and per hour of exposition of CR-39 detector to receive the radon activity concentration in studied dwelling.

The etching process of alpha particle tracks in CR-39 detectors was performed using 6.25M sodium hydroxide (NaOH) solution and ethanol (2% in volume) during 14 hours at the temperature of 70°C [28] stabilized by a water bath.

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The parameters of etching were adjusted during the preliminary study when 15 detectors CR- 39 were irradiated in the atmosphere with high radon concentration. After that, exposed detectors were etched using mentioned 6.25M NaOH solution at 70°C during the time that varied from 10 minutes to 14 hours.

Etched alpha particle tracks could be easily observed using an optical magnification of 100x starting from 330 minutes of etching. But the optimal track size of 50 µm was obtained after 14 hours of etching. Figure 3 shows the sample of etched detector surface obtained within mentioned above optimal for manual counting conditions.

Figure 3. Microscopic image of etched alpha particle tracks in CR-39.

3. RESULTS AND CONCLUSIONS

Considering the calibration and efficiency of used detection system, it was calculated the concentration of radon in air for each dwelling and workplace chosen for present survey together with associated errors. Figure 4 shows the frequency distribution of obtained radon activities in monitored dwellings of Curitiba urban area. The average concentration of radon within dwellings of Curitiba was obtained of 47.0 +/- 3.5 Bq/m3.

Figure 5 shows the frequency distribution of obtained radon activities at monitored workplaces of Curitiba urban area where detectors were exposed during 104 days. The average concentration of radon within studied workplaces was obtained of 34.5 +/- 3.7 Bq/m3.

It could be concluded that obtained values of radon activity in dwellings and in workplaces are within the normal range established by International norms and regulations that were discussed previously.

The result of present survey show that monitored dwellings and workplaces are healthy considering the risk associated with radon activity in air. No one from investigated places requires any sort of mitigation measures.

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50 45 40 35 30 25 20 15 10 5

Number of studied dwellings Number 0 0-20 20-40 40-60 60-80 80-100 >100 Radon activity concentration, [Bq/m3]

Figure 4. Frequency distribution of radon activity in air at monitored dwellings of Curitiba urban area.

10 9 8 7 6 5 4 3 2 1 Number of studied workplaces Number 0 0-20 20-40 40-60 60-80 80-100 >100 Radon activity concentration, [Bq/m3]

Figure 5. Frequency distribution of radon activity in air at monitored workplaces of Curitiba urban area.

ACKNOWLEDGMENTS

The authors are very thankful to CNPq, CNEN and Fundação Araucária (Paraná St.) for financial support of this work as well as to colleagues from the Center of Nuclear Technology Development (CDTN/CNEN) for assistance in the measurements.

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