Long-term clearance of accidentally inhaled 60Co aerosols in humans.

Long-term body retention was measured in six workers who accidentally inhaled 60Co aerosols during manipulation with a high-activity 60Co source. Improved whole body counting and calibration techniques provided good conditions to follow body clearance over 5 y. A two-detector profile scanning arrangement was used to measure the activity distribution in vivo over the lung region. The observed whole body retention followed a two-exponential time function between 10 and 1,850 d. Based on the profile measurements, the shorter exponent of 25-78 d was associated with activity leaving the pulmonary region while the long-term exponential should be interpreted as the clearance of the slowest component of the systemic burden with a biological half-time of 500-1,100 d. These observations classify the 60Co aerosols encountered in this incident as inhalation class W. The agreement of the measured retention pattern with the ICRP inhalation model was investigated assuming different aerosol size distributions characterized by an activity median aerodynamic diameter = 0.25 micron, 1 micron, and 4 microns. It was found that the ICRP model with an activity median aerodynamic diameter = 4 microns could describe the retention of our investigated cases reasonably well.