Exposure assessment: implications for epidemiological studies of ionizing radiation

Quantitative estimates of ionizing radiation exposure are often available for use in epidemiological studies. However, depending on the context, the quality of the exposure estimates can vary. For example, the estimates may be specific to individuals in the study or generic values averaged over populations; unavailable for some of the potential study subjects or vary in their form between individuals; based on contemporary measurements or assessed retrospectively; based on measurements alone, on surrogate measures of exposure, or on an exposure assessment model; or, as is often the case, cover one source of radiation exposure rather than all of them. Various ways in which ionizing radiation exposures have been assessed are illustrated through reference to some studies of childhood leukaemia, concerning environmental, medical, natural and parental occupational exposures. Based on this, implications for the interpretation of radiation epidemiological studies are discussed.