A National Academy of Sciences (NAS) report released today finds that the risks of low-dose radiation rise with the dose, and there is no safe level of radiation. That conclusion has grown stronger over the past 15 years, says the NAS committee, dismissing the hypothesis that tiny amounts of radiation are harmless or beneficial.
The Biological Effects of Ionizing Radiation--VII (BEIR-VII) panel examined the risks of exposure to both natural and man-made radiation at or below 0.1 Sieverts (Sv), which is roughly 40 times the amount the average person is exposed to each year. For typical Americans, 82% of radiation exposure stems from natural sources such as radon gas seeping from the earth; the rest comes mostly from medical procedures like x-rays.
In its last report on the topic, in 1990, a BEIR panel calculated risks by plotting cancer deaths and doses for survivors of the two atomic bombs dropped on Japan in World War II. Risks appeared to increase with the dose. Based on evidence that even a single particle of radiation can damage a cell's DNA, the panel then extrapolated this relationship to very low doses to produce what is known as the linear no-threshold model (LNT). Some scientists have challenged the LNT model, however, noting that other cell and animal studies suggest that a little radiation is harmless and could even stimulate DNA repair enzymes and other processes that protect against later insults, an idea known as "hormesis."
But the BEIR-VII report finds that the LNT model still holds. The latest cancer data on the bomb survivors, as well as fresh studies on nuclear workers and people exposed to medical radiation, all support the LNT relationship: Even a single 0.1 Sv dose would cause cancer in 1 of 100 people. Such risks should be taken into account, the report cautions, when people consider full-body computed tomography (CT) scans to find signs of disease, a recent fad at shopping malls that delivers a radiation dose of 0.012 Sv.
As for the hormesis theory, the panel found it is "not supported" by the data, although the panel says the hypothesis should be studied further. And the panel's chair, Harvard epidemiologist Richard Monson, acknowledges that the long-running debate over the LNT model won't end with this report. "Some minds will be changed; others will not," he says.