By: John C. Zink, Ph.D., P.E., Contributing Editor
In late June the National Academies of Science, Engineering and Medicine Committee on the Biological Effects of Ionizing Radiation (BEIR) released the seventh of its ongoing series of reports, known as BEIR VII. The purpose of the report was to review the current state of knowledge regarding the health effects of exposure to low linear-energy-transfer (LET) radiation (basically, x-rays and gamma rays) and to recommend updated radiation risk estimates flowing from that knowledge. Many expected the report to recognize the recent evidence for the beneficial effects of low-level radiation exposure, known as radiation hormesis (see Nuclear Reactions, May 2005), and to propose changes in the current working hypothesis that all radiation is harmful. The committee dashed those expectations.
The following statements from the committee’s summary capture the report’s significant findings:
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.
… 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.
… there is no direct evidence of increased risk of non-cancer diseases at low doses, and data are inadequate to quantify this risk if it exists.
In other words, the committee is sticking with the model which says that all radiation exposure is harmful, even in small amounts. This is the linear, non-threshold model which is the basis for radiation exposure regulations in the United States. The committee is also saying that cancer is the only discernable health effect of low-LET radiation.
The study dealt only with low-LET radiation, which comprises about 40 percent of the radiation people typically receive. The other 60 percent of a typical person’s radiation exposure is high-LET radiation, mostly from environmental radon and cosmic rays. Government agencies can regulate mostly low-LET radiation, such as that from medical x-rays and the nuclear fuel cycle. So, in spite of its lesser role, the committee studied only low-LET radiation in fulfilling its obligation as advisor to government regulators.
As with previous BEIR reports, the ongoing, long-term study of Japanese atomic bomb survivors provided much of the data considered by the committee. In addition to having another 15 years’ data on individual health histories since the last BEIR report, the committee also had access to recent work that refined the estimates of how much radiation each victim received. This source-term work led to an increased estimate of the power of the Hiroshima bomb, with a resulting increased neutron dose to the victims. Such revision would seem to indicate that the total low-LET dose to survivors was higher than previously calculated, and that expected health effects (using the current model) should be greater than those actually experienced. In spite of this, the committee apparently did not feel comfortable relaxing any of the conservatism that has always characterized radiation effects estimates.
Radiation effects are also heavily dependent upon the rate at which that radiation is received – hence, government exposure standards also limit the permissible dose rate. The Japanese atomic bomb victims received their radiation doses over a short period of time, in contrast to the normal situation. This adds the uncertainty of adjusting for the dose rate to the uncertainty already associated with estimating each individual’s actual radiation dose. The BEIR VII report acknowledges both of these uncertainties in its health effects estimates. For example, the committee’s estimate that a dose of 100 mSv (millisieverts) will result in 800 solid-tumor cancers in a population of 100,000 men, actually has a range of from 400 to 1,600 cancers. The estimate for a population of 100,000 women has a range of 690 to 2,500 cases. Nearly all the estimated effects exhibit a similar range of about 4:1. With this level of uncertainty, it should not be a surprise that the committee was unwilling to go out on a limb and propose reduced conservatism in current radiation effects estimates.
Nuclear News, published by the American Nuclear Society, reported that a study in the June 29th issue of the British Medical Journal (BMJ) seemed to support the BEIR VII basic conclusions. The BMJ paper detailed a study of nuclear workers’ health history in 15 countries, and estimated that the ratio of radiation-induced cancers to cancer from other causes tracked quite closely with the BEIR VII risk estimate.
The BEIR VII committee did acknowledge the statistical evidence for radiation hormesis, but suggested the need for more supporting research. Specifically, the report calls for research to identify the molecular mechanisms for hormetic effects at low radiation doses. This implies that the committee feels statistical evidence for such effects, alone, does not provide an adequate basis for regulatory decision-making.
The BEIR VII summary is available online at http://nationalacademies.org.