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OCR for page 330
Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2 C Issues Raised by the Institute for Energy and Environment Research (IEER) A letter dated September 3, 1999, and authored by Ms. Lisa Ledwidge and Dr. Arjun Makhijani on behalf of IEER and a number of other signatories requested that the BEIR VII committee address six issues. The committee’s response to these issues follows. 1. Effects of Radionuclides That Cross the Placenta In Chapter 8 the committee considers post-Chernobyl data on the excess papillary thyroid cancers arising in radio-iodine-exposed children, some of whom received their exposure in utero. With respect to carbon-14 and tritium, brief comments are made in response to issue 3. The committee recommends that this issue be addressed as part of a larger review of maternal exposures in humans that may affect the fetus. 2. Effects of Radiation on Female Fetuses In Chapters 6 and 7, the committee considers the effects of in utero radiation, including medical radiation and radiation from the atomic bombs. In the recent paper by Delongchamp and colleagues (1997), nine cancer deaths among females exposed in utero to the atomic bombs were noted in comparison to only one among males. Minimal information exists in the medical literature with respect to sex-specific effects, and none reports a gender-specific association between radiation and cancer. Because of the current practice of minimizing radiation exposure to pregnant women, the committee considers it unlikely that this issue will be able to be addressed by future epidemiologic studies. 3. Effects of Organically Bound Radionuclides Cellular and animal data are available for the development of judgments on the tumorigenic, genetic, and developmental effects of tritiated water and organically bound tritium (Straume 1991; Straume and Carsten 1993). The tritium effects observed do not differ qualitatively from those resulting from external irradiation with X-rays or γ-rays. The evidence available indicates that the relative biological effectiveness of β-irradiation from tritium is generally greater (by two- to threefold) than that of γ-irradiation and similar to or slightly greater (one- to twofold) than X-irradiation. Higher effectiveness is seen in vitro in cellular studies when tritium is incorporated into DNA (e.g., as tritiated thymidine). Although the observed effects of tritium are largely attributable to ionization damage from beta particles, transmutation of incorporated tritium to helium also has the potential to damage DNA (NCRP 1979; Hill and Johnson 1993). However, following ingestion of organically bound tritium (OBT, including tritiated thymidine) the in vivo activity of digestive metabolic processes means that only a very small fraction of tritium is incorporated into cellular DNA. Thus, the predominant in vivo source of DNA damage from OBT is β-particle ionization, not transmutation. The observed in vivo effects of tritium will, in any event, include any contribution from transmutational damage to DNA. The same general principles also apply to in vivo effects from organically bound carbon-14. It is important to point out that the committee was not constituted to review the biokinetic aspects of doses from internal radionuclides such as tritium, carbon-14, strontrium-90, radiocesium, and radioiodine. Nevertheless the BEIR VII committee considered potentially informative epidemiologic data that relate to risks from internal radiation as part of its brief to review risks at low doses of low-LET (linear energy transfer) radiation. 4. Synergistic Effects This issue has been comprehensively addressed in Annex H of UNSCEAR (2000b). The BEIR committee endorses the recommendations made on page 217 of that report.
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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2 5. Data Integrity and Quality This issue is addressed in Chapter 8 on occupational radiation studies. The committee acknowledges that there is imprecision in exposure estimates of all epidemiologic studies, especially in retrospective studies of occupational groups. In general, however, studies of workers exposed to radiation tend to have better exposure data than studies of workers exposed to chemicals because of the concurrent estimation of exposure through the use of radiation badges. The committee notes that imprecision in the estimation of radiation exposure will tend toward an underestimation of any true association between radiation and health effects. To the extent that models based on these data are utilized to set standards of population exposure, the standards will tend to be lower than those that would be based on completely accurate data. 6. Effects on Various Populations The atomic bomb data are based on two populations in Japan at one point of time. The relation of radiation exposure to age at exposure and gender has been extensively studied and is summarized in Chapter 6. Data on occupational and medical exposure to radiation are available for a number of populations throughout the world for many decades. However, few details are presented in these studies on age at exposure and sex, except of course for sex-specific studies. The committee recommends that future studies of populations exposed to ionizing radiation include not only information on factors that may interact with radiation exposure, but also information on possible risks present in persons with varying demographic characteristics.
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