Radiation epidemiology: a perspective on Fukushima

Abstract

For nearly 100 years, epidemiologic studies of human populations exposed to ionisingradiation have provided quantitative information on health risks. High dose deterministic(tissue reaction) effects result when sufficient numbers of functioning cells are killed,such as in bone marrow depression that can lead to death. Lower dose stochasticeffects are probabilistic in nature and include an increased risk of cancer later inlife and heritable genetic defects, although genetic conditions in the children ofirradiated parents have yet to be convincingly demonstrated. Radiation studies are ofdiverse populations and include not only the Japanese atomic bomb survivors, butalso patients treated with radiation for malignant and non-malignant disease;patients exposed for diagnostic purposes; persons with intakes of radionuclides;workers occupationally exposed; and communities exposed to environmental andaccidentally released sources of radiation. Much is known about radiation and itsrisks. The major unanswered question in radiation epidemiology, however, is notwhether radiation causes cancer, but what the level of risk is following low dose (<100 mSv) or low dose rate exposures. Paracelsus is credited with first articulating that the‘poison is in the dose’, which for radiation epidemiology translates as ‘the lower the dose,the lower the risk’ and, an important corollary, the lower the dose, the greater the difficultyin detecting any increase in the number of cancers possibly attributable to radiation. Incontrast to the Chernobyl reactor accident, the Fukushima reactor accident has to dateresulted in no deterministic effects and no worker deaths. Estimates to date of populationdoses suggest very low uptakes of radioactive iodine which was a major determinant of theepidemic of thyroid cancer following childhood exposures around Chernobyl. Theestimates to date of population doses are also much lower (and the distributionmuch narrower) than the doses for which cancer excesses have been detectedamong atomic bomb survivors after 60 years of follow-up. Studies of populationsexposed to low doses are also limited in their ability to account for importantlifestyle factors, such as cigarette smoking and medical x-ray exposures, which coulddistort findings. Studies of the Fukushima population should be and are beingconsidered for reassurance and health care reasons. Apart from as regards the extremepsychological stress caused by the horrific loss of life following the tsunami and thelarge-scale evacuation from homes and villages, such studies have limited to no chanceof providing information on possible health risks following low dose exposuresreceived gradually over time—the estimated doses (to date) are just too small.