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INTRODUCTION

Ionizing radiation arises from natural and human-produced sources and can affect essentially all the organs and tissues of the body. Well-demonstrated late effects include induction of cancer, developmental abnormalities, and cataracts. In recent years, increasing concern has centered on the risks of these effects at the low doses and low dose rates experienced by radiation workers and the general public. That concern is influenced, in part, by renewed speculation regarding the postulated nonlinearity of the dose-response relationship at environmental levels of exposure. Assumptions as to the shape of the dose-response curve at environmental levels by regulatory agencies have profound economic and health implications.

A large amount of additional epidemiologic data have become available since the BEIR V report (NRC 1990). New statistical methods are available to increase the analytic power of interpretation of those data. Biologic data are emerging on phenomena that could affect the shape of the dose-response curve at low doses. Low-level radiation exposure might induce genomic instability and thus result in damage to cells many cell generations after exposure. Additional evidence suggests that the clusters of damage produced in the DNA at very low doses of radiation are refractory to DNA repair. Conversely, adaptive or hormetic responses to low levels of ionizing radiation might render cells refractory to later exposures.

This report was prepared by the Committee on Health Risks of Exposure to Low Levels of Ionizing Radiations (BEIR VII), in the Board on Radiation Effects Research of the National Research Council's Commission on Life Sciences to summarize evidence that has accumulated on health risks posed by exposure to low levels of ionizing radiation since the BEIR V report and to determine whether the new information justifies a comprehensive study, which would be called BEIR VII phase-2.



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Health Effects of Exposure to Low Levels of Ionizing Radiations: Time for Reassessment? 1 INTRODUCTION Ionizing radiation arises from natural and human-produced sources and can affect essentially all the organs and tissues of the body. Well-demonstrated late effects include induction of cancer, developmental abnormalities, and cataracts. In recent years, increasing concern has centered on the risks of these effects at the low doses and low dose rates experienced by radiation workers and the general public. That concern is influenced, in part, by renewed speculation regarding the postulated nonlinearity of the dose-response relationship at environmental levels of exposure. Assumptions as to the shape of the dose-response curve at environmental levels by regulatory agencies have profound economic and health implications. A large amount of additional epidemiologic data have become available since the BEIR V report (NRC 1990). New statistical methods are available to increase the analytic power of interpretation of those data. Biologic data are emerging on phenomena that could affect the shape of the dose-response curve at low doses. Low-level radiation exposure might induce genomic instability and thus result in damage to cells many cell generations after exposure. Additional evidence suggests that the clusters of damage produced in the DNA at very low doses of radiation are refractory to DNA repair. Conversely, adaptive or hormetic responses to low levels of ionizing radiation might render cells refractory to later exposures. This report was prepared by the Committee on Health Risks of Exposure to Low Levels of Ionizing Radiations (BEIR VII), in the Board on Radiation Effects Research of the National Research Council's Commission on Life Sciences to summarize evidence that has accumulated on health risks posed by exposure to low levels of ionizing radiation since the BEIR V report and to determine whether the new information justifies a comprehensive study, which would be called BEIR VII phase-2.