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

Two hundred and forty samples of radon (Rn-222) gas concentration, tog ether with unattached and attached-to-ambient-aerosol radon progeny co ncentrations, were collected at 16 outdoor sites in four US states. Th e effective dose rate (to the lungs) was calculated with ICRP's recent ly recommended lung dosimetry model (ICRP Publication 66) using the me asured potential alpha energy concentration (PAEC) and unattached frac tion (f(p)). The measurement procedure for f(p) assumed a characterist ic particle diameter of 1.1 nm for the unattached progeny. The site-av erage value of the conversion coefficient between radon gas concentrat ion 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 correla tion coefficient (r) of effective dose rate with respect to radon gas concentration between these 16 sites was 0.98. Thus, the radon gas con centration alone was a good predictor of effective dose rate in outdoo r air, at least when the dose calculation was based on a known PAEC an d the assumption of discrete unattached and attached particle size mod es. For an average outdoor occupancy of 5 h.d(-1) together with the av erage 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 dev iation between sites of about 1.9. The average effective dose rate fro m 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 concentrati on 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 outd oor dose rate and a number of meteorological and environmental variabl es were examined: equivalent uranium concentration in soils, wind spee d, terrain roughness, atmospheric stability and particle concentration .