In 1994, EPA published a report entitled Estimating Radiogenic Cancer Risks (EPA 402-R-93-076), which described the Agencys methodology for deriving estimates of excess cancer morbidity and mortality due to low doses of ionizing radiation. Using this methodology, numerical estimates of the risk per unit dose were derived for each applicable cancer site, and for both low-LET and alpha-particle radiation. Subsequently, small adjustments were made to the procedure used in the 1994 document, chiefly the use of more recent vital statistics. These adjustments produced a slight increase in the estimated average risk from uniform, whole-body radiation: the low-LET nominal estimate increased from 5.1OE10-2 Gy-1 to 5.75OE10-2 Gy-1. In this document, a method is describe for estimating the uncertainties in the EPA risk projections. The uncertainty in each site-specific (or whole-body) risk estimate is treated as the product of several independent sources of uncertainty, e.g., sampling errors in the epidemiologic data underlying the risk model, or uncertainty in the extrapolation of observations at high acute doses to chronic low dose conditions. A distribution is assigned to each source of uncertainty, which defines the probability that the assumption employed in EPAs risk model with respect to this source of uncertainty either underestimates or overestimates the risk by any specified amount. The joint probability distribution for the uncertainty due to all sources combined is then calculated using Monte Carlo techniques. A detailed uncertainty analysis is performed for the risks from uniform, low-LET irradiation of the whole body, the lung, and the bone marrow. For the whole body or the bone marrow, the upper limit on the 90% confidence interval is about 2 times higher, and the lower limit is about 3 times lower, than the respective nominal risk estimate. In the case of the lung, the upper bound is also about a factor of 2 higher, but the lower bound is about a factor of 5
times lower, than the nominal estimate.