Identifying critical human subpopulations by age groups: radioactivity and the lung.

This work studies the effect of age on lung dose commitments arising from the inhalation of an atmosphere containing radionuclides of varying half-lives and physical sizes. This dependence is significant in many cases, approaching a ratio of 2 in several instances when the maximum dose at a given age is compared with that for an adult. In most instances, the maximum dose occurs between 4 and 8 years of age, due to a combination of small channel radii (allowing greater diffusional deposition), low lung mass, and high breathing rate. Age-dependence is largest for particles in the range 0.01 to 0.1 micrometer, a range coincident with that normally encountered in atmospheric aerosols. Resting activity appears to provide the largest age-dependence, particularly for short-lived radionuclides. The study focused on radionuclides whose half-lives were less than 2 years. Total lung doses for longer lived radionuclides are dominated by pulmonary deposition and may be computed from the data in the figures, given a knowledge of the transit times for the tracheo-bronchial region. From the present results, it appears likely that current maximum permissible levels of exposure to airborne radionuclides may require re-evaluation to ensure that children do not receive unacceptably large dose commitments. Further revision may be required upon the incorporation of age-dependent risk factors.