OUTDOOR RADON DOSE CONVERSION COEFFICIENT IN SOUTH-WESTERN AND SOUTH-EASTERN UNITED-STATES

Two hundred and forty samples of radon (Rn-222) gas concentration, together with unattached and attached-to-ambient-aerosol radon progeny concentrations, were collected at 16 outdoor sites in four US states. The effective dose rate (to the lungs) was calculated with ICRP's recently recommended lung dosimetry model (ICRP Publication 66) using the measured potential alpha energy concentration (PAEC) and unattached fraction (f(p)). The measurement procedure for f(p) assumed a characteristic particle diameter of 1.1 nm for the unattached progeny. The site-average value of the conversion coefficient between radon gas concentration and effective dose rate was found to be 6.7 nSv.h(-1) per Bq.m(-3), with a standard deviation of 1.3 nSv.h(-1) per Bq.m(-3). The correlation coefficient (r) of effective dose rate with respect to radon gas concentration between these 16 sites was 0.98. Thus, the radon gas concentration alone was a good predictor of effective dose rate in outdoor air, at least when the dose calculation was based on a known PAEC and the assumption of discrete unattached and attached particle size modes. For an average outdoor occupancy of 5 h.d(-1) together with the average radon gas concentration, the annual effective dose rate averaged for outdoor exposure at these 16 sites was approximately 80 mu Sv.y(-1), with a median value of 54 mu Sv.y(-1) and a geometric standard deviation between sites of about 1.9. The average effective dose rate from outdoor radon progeny found in this study is approximately one-tenth the value of 740 mu Sv.y(-1) from indoor exposure, which is based on the EPA's value of 46.3 Bq.m(-3) for the average radon gas concentration in US homes, and ICRP's recommended dose conversion convention of 1.1 mSv per mJ.h.m(-3) (3.8 mSv.WLM(-1)). Correlations between the outdoor dose rate and a number of meteorological and environmental variables were examined: equivalent uranium concentration in soils, wind speed, terrain roughness, atmospheric stability and particle concentration.