Particle size distributions of radioactive aerosols in the environment

Published measurements of Activity Median Aerodynamic Diameter (AMAD) of environmental aerosols were surveyed in order to determine a realistic default value for estimating doses to members of the public (i.e. the value to be used in dose assessments in the absence of specific information). Reported AMADs for artificially produced radioactive aerosols ranged from 0.3 to 18 μm and on fitting a single log-normal function gave a median value of 1.5 μm. The distribution did, however, show some evidence of being bimodal. Most of the measured AMADs fell into two distinct categories: aerosols released into the atmosphere by the Chernobyl accident and those produced by resuspension of radioactive material deposited on the ground or released into the sea. The distributions of measured AMADs for each category were well fitted by single log-normal functions with median values of 0.6 and 6 μm respectively. The distribution of AMADs for ⁷Be, a natural radioactive aerosol produced in the atmosphere by cosmic gamma rays, had a median value of 0.6 μm. It is concluded that for exposure of the public to radioactive aerosols in the environment a 1 μm AMAD, as recommended by ICRP in Publication 66, is a realistic rounded default value. However, public exposures arising from inhalation of material resuspended into the atmosphere are more realistically represented by a 5 μm AMAD, as recommended by ICRP for workplace exposures.