HEALTH EFFECTS OF IONIZING RADIATION 3.1 Overview The purpose ofthis section is to provide a basis for enumerating possible and probable health outcomes among the exposed population of the Marshall Islands with respect both to the nature of the radiation injury (i-e., deterministic; stochastic) and the types and levels of radiation present. The clinical effects of exposure to radiation are dose-related expressions of underlying biological damage induced by radiation at cellular and molecular levels. Deoxyribonucleic acid (DNA)in cell nuclei is considered to be the primary site of radiation-induced chemical reactions that can result in clinical effects, but reactions also may occur among molecules in the membrane and othercellular structures. The resulting clinical manifestations of this biological damage may be categorized as deterministic or stochastic in nature. With few exceptions these effects are non-specific and indistinguishable by existing clinical or biological technologies from clinically similar lesions caused by other agents. Deterministic effects, also known as threshold effects, are due to biological damage that results in immediate or early cell death or sterilization and thereby, to cell depietion. There are threshold ranges for the doses of radiation that are required to cause lethal damageto cells sufficient to produce clinical evidence of cell depletion. The severity as well as the incidence of deterministic effects is related directly to the dose of radiation received. Deterministic effects may be clinically evident as acute signs and symptoms during the early (1 - < 60 days) post-exposure period in individuals exposed to radiation above threshold dose levels. These effects include the Acute Radiation Syndrome (ARS), acute local lesions including radiation burns, and decreased fertility. Expression of other effects of acute radiation injury also may be delayed for periods ranging from about 60 days to years after exposure, depending on the cell system affected and the degree of damage. These include fibroatrophy, cataracts, temporary infertility or permanentsterility, and hypothyroidism. Stochastic or non-threshold radiation-induced effects are associated with incomplete or SUE ZU EO