Box 1 . New epidemiologic information and approaches used in BEIR VII risk models
1 . The information available from epidemiologic studies has been greatly increased .
• For the A-bomb survivors , cancer incidence data from the Hiroshima and Nagasaki tumor registries have become available ( 13,000 cases of solid cancer ), and the number of solid cancer deaths available for analysis has nearly doubled ( 10,000 deaths ).
• There are new data from many studies of persons exposed for medical reasons . These data have been incorporated in estimating risks of female breast cancer and thyroid cancer .
• There are new data from studies of nuclear workers exposed at low doses and dose rates , including results of analyses that combine data from many studies . These data were evaluated and found to be compatible with BEIR VII risk estimates .
1 . BEIR VII provides estimates of both cancer incidence ( including non-fatal cancer ) and cancer mortality , whereas previous reports focused on mortality data .
2 . The availability of cancer incidence data on A-bomb survivors has made it possible to develop estimates for eleven specific cancer sites .
3 . The newly implemented DS02 dosimetry system for A-bomb survivors provides a more accurate basis for evaluating the dependence of risk on dose .
up to about 100 mSv ( 0.1 Sv ) of low-LET radiation . People in the United States are exposed to average annual background radiation levels of about 3 mSv ; exposure from a chest X-ray is about 0.1 mSv and exposure from a whole body computerized tomography ( CT ) scan is about 10 mSv .
There are many challenges associated with understanding the health effects of low doses of low- LET radiation , but current knowledge allows several conclusions . The BEIR VII report concludes that the current scientific evidence is consistent with the hypothesis that , at the low doses of interest in this report , there is a linear dose-response relationship between exposure to ionizing radiation and the development of solid cancers in humans . It is unlikely that there is a threshold below which cancers are not induced , but at low doses the number of radiationinduced cancers will be small . Other health effects ( such as heart disease and stroke ) occur at higher radiation doses , but additional data must be gathered before an assessment of any possible dose response can be made between low doses of radiation and noncancer health effects . The report also concludes that with low dose or chronic exposures to low-LET irradiation , the risk of adverse heritable health effects to children conceived after their parents have been exposed is very small compared to baseline frequencies of genetic diseases in the population .
Radiation Exposure and Health Effects
The mechanisms that lead to adverse health effects after ionizing radiation exposure are not fully understood . Ionizing radiation has sufficient energy to change the structure of molecules , including DNA , within the cells of the body . Some of these molecular changes are so complex that it may be difficult for the body ’ s repair mechanisms to mend them correctly . However , the evidence is that only a small fraction of such changes would be expected to result in cancer or other health effects .
The most thoroughly studied individuals for the evaluation of health effects of ionizing radiation are the survivors of the Hiroshima and Nagasaki atomic bombings , a large population that includes all ages and both sexes . The Radiation Effects Research Foundation ( RERF ) in Japan has conducted followup studies on these survivors for more than 50 years . An important finding from these studies is that the occurrence of solid cancers increases in proportion to radiation dose . More than 60 % of exposed survivors received a dose of radiation of less than 100 mSv ( the definition of low dose used by the BEIR VII report ).
Risk Models for Cancer
An important task of the BEIR VII committee was to develop “ risk models ” for estimating the risk that an exposed individual will develop cancer . This task requires expressing the dependence of risk on radiation dose and also on sex and age at exposure . Data from epidemiologic studies were used to accomplish this task . The Japanese atomic bomb survivors were the primary source of data for estimating risks of most solid cancers and leukemia . For 2 of the 11 specific cancers evaluated , breast and thyroid cancer , atomic bomb survivor data were combined with data on medically exposed persons to estimate risks . Data from additional medical studies and from studies of nuclear workers were evaluated and found to be compatible with BEIR VII models .
Since the publication of BEIR V in 1990 , more comprehensive data on cancer incidence ( including nonfatal diseases ) in atomic bomb survivors have become available , mortality follow-up has been extended for 15 years nearly doubling the number of deaths from solid cancer , and an improved dosimetry system ( DS02 ) has been implemented . In addition , new data have become available from studies of persons exposed to radiation for medical reasons and from studies of nuclear workers exposed at low doses