THE ESTIMATION OF DDREF FROM HUMAN DATA AND THE CONSEQUENT PROJECTED OVERALL RADIATION RISK TO A WORKER POPULATION

The effects per unit dose of ionising radiation are presumed to be smaller at low occupational dose rates and levels of dose than those observed in the atomic bomb survivor population used to estimate cancer risk from radiation. The numerical parameter describing this reduction is the DDREF, dose and dose rate effectiveness factor. At present, available information is limited to that from animal experiments with estimates ranging from 2 to 10.

The likelihood of radiation induced cancer is believed to vary both with time since exposure and with age at exposure; also, the effects differ with cancer type. Empirical models of cancer risk ftave been published by the BEIR group (BEIR V Report, NAS, 1990), their results being expressed as excess relative risk per unit dose in relation to the above parameters and five cancer groups. ('Excess relative risk' is the calculated coefficient by which the underlying mortality rate for a given age, sex and year needs to be multiplied to obtain the absolute number of excess deaths.)