55. Radon and Its Progeny in Office Buildings
Shinji Tokonami, Masahide Furukawa and Yuji Yamada
Keywords: radon, office buildings, dose assessment
It is well known that the inhalation of radon progeny gives large radiation dose to general public. Indoor radon surveys have been carried out in many countries. For more accurate dose assessment, it is important to understand the level and behavior after taking human activities into account. In order to investigate the behavior of radon and its progeny in occupational environments, measurements were made in actual office buildings. Long-term measurements with passive radon detectors have been conducted over five years at five sites. Continuous measurements were also done at two sites every season. The measurements were made for a week. From results of the continuous measurements, it can be recognized that their concentrations varied with time drastically based on the working hours.
Radon concentrations have been measured with passive radon detectors in several typical office buildings over five years in Tokyo. Since long-term measurement with passive radon detectors is often used for radon surveys in dwellings, the same procedure was also adopted in this survey. A passive radon detector using an electrostatic collection method was placed at each site. The prototype of the passive detector was developed by T. Iida et al., and the device manufactured by Aloka Co., Ltd was used here. The air exchange rate of the device was set to 0.67 h-1 so as to minimize the entry of thoron (220Rn) into the device. After radon gas goes inside of the device through a membrane filter, 218Po atoms are formed. Since most of them are positively charged, they can be collected on a negative electrode using an electric field. The collection efficiency of 218Po atoms depends on humidity in the device. A desiccant of P2O5 is placed in the device so as to maintain high sensitivity. A solid-state nuclear track detector is placed on the electrode to detect alpha particles emitted from radon progeny. Although cellulose nitrate (CN) films (LR115, Type 2) used to be mounted as the detecting material, the quality of the CN film was not stable and large uncertainty in the experimental results was found in our preliminary tests. Therefore, The CN films were replaced with CR-39 detectors (commercial name: BARYOTRAK) because the CR-39 detectors have a very low background and high reproducibility. The detectable range of alpha particles is wide, both alpha particles of 6.0 MeV for 218Po and those of 7.7 MeV for 214Po can be detected. The CR-39 detector and desiccant in each device were replaced every two months. After two months exposure, the CR-39 is chemically etched for 24 h in a 6N NaOH solution at 60Åé. The number of alpha tracks is counted using an optical microscope with 100 magnification.
For continuous measurement, two instruments were used. One is commercially called ALPHAGUARD. It can measure radon concentration continuously together with three meteorological parameters, temperature, relative humidity and air pressure. The other is used to measure the equilibrium equivalent radon concentration (EERC) using the Pylon AB-5 and AEP-47 as the sampling head. The conversion factor is experimentally determined at an adequate flow rate (1 L/min here). It can provide the EERC continuously every 60 min. The continuous measurements were carried out at two sites for a week every season.
Table 9 shows the average radon concentration at each site based on the long-term measurement with the passive radon detector. The concentrations seemed to be consistently constant at each site. Although seasonal variations of the concentration were analyzed, no significant difference was found. However, the radon concentrations were somewhat higher than expected after taking the result of the latest indoor radon survey (15.5 Bq/m3 as the annual mean) into account. Actually, the radon concentration should be determined with the human activities for more accurate dose assessment. In these office buildings, air conditioning was automatically supplied during the working hours (normally 9 a.m. to 5 p.m., Monday to Friday). When taking the airtight construction of these buildings into account, the radon concentration might be considerably enhanced when the air conditioning was off. While the air conditioning was on, on the other hand, ventilation would reduce the indoor radon level. In order to verify these hypotheses, radon and its progeny concentrations were continuously measured for a week at two office buildings together with the long-term measurement. Figure 19 shows the time variation of the concentration for a week in winter on the 6th floor at site A. The building had 8 floors above ground. While people were working, the concentrations were low. After work, the concentrations rose steadily because the air conditioning was stopped. Once the air conditioning began to work in the morning, the concentration level went down again. This pattern seemed to reflect activities in a week, and the same pattern was also found in summer. From this measurement, average radon concentrations throughout the day and during working hours were estimated to be 59.6 Bq/m3 and 30.7 Bq/m3, respectively; 44.5 Bq/m3 was the average EERC throughout the day and 16.2 Bq/m3 was that during working hours. The equilibrium factors were estimated to be 0.75 (throughout the day) and 0.60 (during working hours). There were large differences in their concentrations between the two when taking the ratios. On the other hand, there was a minor difference in the equilibrium factor. Other continuous measurements were made on the 10th floor at site B in summer. The building had 44 floors above ground and accommodated a lot of office workers. The continuous measurement gave 32.9 Bq/m3 as the average radon concentration throughout the day and 10.4 Bq/m3 as that during working hours; 15.6 Bq/m3 was the average equilibrium equivalent radon concentration (EERC) and 4.4 Bq/m3 was that during working hours. Evaluating the results as the equilibrium factor (F), they were 0.47 (throughout the day) and 0.42 (during working hours). These equilibrium factors seemed to be typical for an indoor environment. In terms of the differences between the two, the same conclusion could be drawn as that in the measurements at site A. Table 10 summarizes the physical parameters related to dose assessment at the two sites.
It is obvious that the radiation doses in occupational environments might be overestimated if the radon concentrations are obtained with ordinary passive radon detectors in long-term surveys. More data should be accumulated so as to evaluate the concentration using suitable modeling of the behavior of radon and its progeny in an occupational environment.
Tokonami, S., et al.: Healthy Buildings 2000, 3, 81-84, 2000.
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