An Introduction to the Estimation of Risks Arising from Exposure to Low Doses of Ionising Radiation

Exposure to ionising radiation leads to a radiation dose. The main health effect of low levels of radiation dose is an increase in the chance of developing cancer. The radiation risk factor quantifies the level of risk caused by a given amount of radiation dose, i.e. there will be an X% risk of getting cancer per unit of dose received. This document provides an introduction to the risks from exposure to low doses of radiation and explains the derivation of the radiation risk factors used in radiation protection. The examples are focused on radioactive waste management situations but the concepts can be applied to other situations. The estimated values of risk factors are largely based on epidemiological studies of the Japanese atomic-bomb survivors, many of whom received medium to high doses of ionising radiation, and are supported by studies of other populations such as patients given medical exposures and workers receiving exposures at work. The International Commission on Radiation Protection (ICRP) used these estimates of radiation risk as the basis for their radiation protection system and made adjustments to allow for lower doses and lower dose rates, in order to estimate the risks of developing various cancers following exposure to ionising radiation from typical situations (e.g. medical, occupational or environmental exposures). Current ICRP recommendations (ICRP, 2007) for radiation protection assume an overall fatal cancer risk from low dose ionising radiation of about 5% per Sv. The studies of the Japanese atomic-bomb survivors include many features of good epidemiological studies and the findings from studies of other populations are in reasonable agreement with them. Therefore HPA has confidence in the risk factors used by ICRP. Additionally, as a measure of their acceptance, the ICRP system of radiation protection and standards is applied internationally as well as in the UK. It is possible to estimate the accuracy of radiation risk models from the evidence provided by the epidemiology studies. HPA’s view is that, when considering the risk of all cancers in a population of all ages exposed to radiation at background dose levels and above, it is reasonable to assume that the estimates of risk used for protection purposes are accurate to within a factor of 3 either way for some radionuclides and for external exposure. For certain radionuclides the evidence suggests that the accuracy of risk estimates is likely to be around a factor of 10 either way. This work was undertaken under the Environmental Assessment Department’s Quality Management System, which has been approved by Lloyd's Register Quality Assurance to the Quality Management Standards ISO 9001:2000 and TickIT Guide Issue 5, certificate number 956546.