\ od aig‘Le . intéynal sources of radiationtare ‘os and Sr, which concentrate in muscular and bony tissue respectively, and 239 pu, deposited inthe lung. 5.3.2 Criteria for Evaluating Hazard Control phir Guidelines for safe exposures to radioactivity on the atoll are given in terms of the maximum annual dose received by an individual and are also evaluated in rerms of long-term health effects. The main objective of radiological cleanup is to reduce the radioactivity of the Atoll to levels at which the population can be expected to have annual exposures below the value of these guidelines. 5.3.2.1 Long Term Health Effects. Quantitative evaluation of low levels of absorbed radiation on human health continues to be a subject of medical research. Present knowledge is based on the response to high levels of radiation of research animals, of persons undergoing medical treatment with radioactivity, and of a few victims of radioactivity accidents. Direct determination of the human health response to low levels of radiation, such as are discussed in this report, is complicated by the requirement to study radiation effects on large populations for statistically meaningful results, by the long time delay between radiation exposure, and appearance of such effects as neoplasms, by difficulty in distinguishing between effects attributable to radiation and effects not related to radiation, and because such effects as cancer susceptibility are widely varying functions of age, sex, genetic constitution, diet, personal habits, socioeconomic factors, and other variables (BEIR, 1972). Because of this, present risk estimates are based predominantly on conservative extrapolations from data obtained at high doses. The data upon which health risk estimates are based exhibit statistical variations so that, usually, the uncertainty in estimating a particular risk value is expressed by a range of values for the risk. In view of the many uncertainties related to this study, the risk models adopted result from very conservative assumptions. Forlong term exposures to low levels of radiation, such as may apply to some aspects of residence on Enewetak Atoll, the model assumes a linear relationship between dose and effect, with no threshold. The assumption of "no threshold" implies that zero dose is the only dose that yields no adverse health effects. The less conservative assumption that a threshold dose exists, below which no health effects will be observed, has not been used. The health effects of radiation on a population can be divided into ‘ two categories: somatic and genetic effects. Somatic effects relate to the body or its organs while genetic effects are evidenced only in the 5-4