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1. RISK OVERVIEW 8 1.2. RECENT DEVELOPMENTS IN RADIATION BIOLOGY Recent studies in cellular radiation biology are providing new insights as to the mechanisms whereby radiation interacts with the components of a mammalian cell and how signals can be transferred from one cell to another. That information should improve our understanding of the basic cellular changes after radiation exposure that might be involved in inducing mutations and of how the mutations are related to a risk of cancer or hereditary disorders. However, it is not necessary at this time to reassess human cancer or heritable risk in light of the new observations because radiation cancer and hereditary risk estimates are derived from results of epidemiologic studies of exposed human populations and experimental studies of irradiated laboratory animals. Basic mechanisms are empirically accounted for in those results even if they are not identified or understood. Furthermore, they are supported by information on dose-response relationships for a variety of mutational end points of the type known to be associated with carcinogenesis and hereditary effects. New knowledge of biologic mechanisms has been included in the reviews by various authoritative international and national scientific groups that summarized the risks to health arising from exposure to low doses of radiation (NCRP, 2001, UNSCEAR, 2000a). None of the reviews has led to a conclusion that suggests that modifications of the linear-nonthreshold dose-response model that forms the basis of current risk estimates are required at this time. 1.3. RECENT DEVELOPMENTS IN RADIATION EPIDEMIOLOGY Current risk estimates for radiogenic cancers of interest among the populations covered by RECA, specifically uranium miners, millers and ore transporters, and downwinders and onsite nuclear-test participants, are based primarily on the results of epidemiologic studies of uranium and other miners who were exposed to radon underground, the Japanese atomic-bomb survivors exposed primarily to external gamma rays, and populations exposed internally to radioiodine; they have been reviewed in detail (NRC, 1990; ICRP, 1991; NRC, 1999a; UNSCEAR, 2000b; IARC, 2000; 2001). Radiation protection organizations generally have based their risk estimates for radiogenic cancers on linear-nonthreshold dose response models for all but skin cancers. In so doing, they assumed that even small increases in dose above natural background radiation levels result in a proportional increase in risk. Risk to Uranium Miners, Millers and Ore Transporters The studies of miners have identified an increased risk of lung cancer associated with exposure to alpha- particle radiation from decay products of inhaled radon. The epidemiologic studies of miners generally employed relative risk models, the estimates of which are discussed below. It should be noted that a decrease in excess relative risk could occur even if absolute risk (number of cancers per 100,000 persons) stays constant
