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1. RISK OVERVIEW 7 1. RISK OVERVIEW 1.1 RECENT DEVELOPMENTS IN RADIATION DOSIMETRY Estimates of health risks to exposed cohorts in the HRSA program have historically been obtained from dose assessment or retrospective dosimetry because many of the people did not have personal dosimeters and there was a lack of comprehensive workplace or environmental monitoring. The doses from external radiation were estimated using conversion factors similar to those compiled by Unger and Truby (1981), International Commission on Radiological Protection (ICRP) Publication 74 (ICRP, 1996), and International Commission on Radiation Units and Measurements Report 68 (ICRU, 2002). Doses from internally deposited radionuclides were estimated using physiologically-based pharmacokinetic (PBPK) models, such as those developed by ICRP (1979), and these methods are continually updated. There have been changes in tissue weighting factors, wT, which are defined as the fraction of stochastic risk for carcinogenesis or hereditary effects resulting from radiation exposure of organ T, relative to the risk from uniform exposure of the entire body (ICRP, 1991). Changes in wT based on incidence as a measure of health detriment are being planned by ICRP and will increase the estimated risk of several cancers (particularly cancers of the thyroid and breast), without a major effect on the estimated risk of respiratory cancers. There has been complete revision of the model for the human respiratory tract (ICRP, 1994) and of basic anatomic data on the skeleton (ICRP, 1995). Although the recent revisions can reduce uncertainty, they will not substantially change the general assessment of risk to cohorts in the HRSA program. The revised PBPK model for the human respiratory tract does not include dosimetry for inhalation of radon or the short-lived descendants of radon that are referred to as radon daughters. Historically, the risks posed by radon have been related to the time-integrated concentration of potential alpha energy from short-lived radon daughters, usually expressed in Working Level Month (WLM, or J s mâ¢ 3). The committee does not expect that this practice will be revised. Any changes in risk estimates from radon will be related to biology or observed cancer incidence, rather than dosimetry. The most comprehensive database of risks associated with external exposure from ionizing radiation is the Life Span Study of Japanese atomic-bomb survivors conducted by the Radiation Effects Research Foundation. Current estimates of risk are related to dose assessments for each person according to a system called DS86 (NRC, 1987). In 2001, a National Research Council report made recommendations regarding revisions to DS86 to reduce uncertainty in dose assessments (NRC, 2001). The revisions have been completed and will be published as DS02 (Roessler, 2003). The protocol will be used to obtain an updated estimate of dose for each person. Initial indications are that changes in dose are expected to be small and will not substantially affect cancer mortality risk factors (for example, risk per sievert) for low linear energy transfer radiations such as from gamma rays and beta particles as stated in this interim report. The committee will provide more specific evaluation of these changes in the final report after reviewing the published results from DS02.