[US NRC study finds against hormesis]
Science, Vol 309, Issue 5732, 233 , 8 July 2005
[DOI: 10.1126/science.309.5732.233]
News of the Week
EPIDEMIOLOGY: Radiation Dangerous Even at Lowest Doses
Jocelyn Kaiser

A new National Research Council (NRC) report* finds that although the risks of low-dose radiation are small, there is no safe level. That conclusion has grown stronger over the past 15 years, says the NRC committee, dismissing the hypothesis that tiny amounts of radiation are harmless or even beneficial.

The risk of low-level radiation has huge economic implications because it affects standards for protecting nuclear workers and for cleaning up radioactive waste. The Biological Effects of Ionizing Radiation VII (BEIR VII) panel examined radiation doses at or below 0.1 sieverts (Sv), which is about twice the yearly limit for workers and 40 times the natural background amount the average person is exposed to each year. For typical Americans, 82% of exposure stems from natural sources such as radon gas seeping from Earth; the rest is humanmade, coming mostly from medical procedures such as x-rays.

In its last report on the topic in 1990, a BEIR panel calculated risks by plotting cancer cases and doses for survivors of the two atomic bombs dropped on Japan in World War II. Risks appeared to increase linearly with the dose. Based on evidence that even a single "track" of radiation can damage a cell's DNA, the panel extrapolated this relationship to very low doses to produce what is known as the linear no-threshold model (LNT).

Some scientists have challenged this LNT model, however, noting that some epidemiological and lab 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 (Science, 17 October 2003, p. 378).

 Risky business. A new review verifies that even radiation levels well below those encountered by nuclear workers can raise cancer risk.

But the 712-page BEIR VII report finds that the LNT model still holds. The panel had the latest cancer incidence data on the bomb survivors, as well as new dose information. Committee members also reviewed fresh studies on nuclear workers and people exposed to medical radiation, all of which supported the LNT relationship. The model predicts that a single 0.1-Sv dose would cause cancer in 1 of 100 people over a lifetime. Such risks should be taken into account, the report cautions, when people consider full-body computed tomography scans, a recent fad that delivers a radiation dose of 0.012 Sv.

At the same time, notes panelist Ethel Gilbert, an epidemiologist at the National Cancer Institute in Bethesda, Maryland, "we can't really pinpoint" the risk at the lowest doses.

The BEIR VII panel examined the latest evidence for a threshold. But it found that "ecologic" studies suggesting that people in areas with naturally high background radiation levels do not have elevated rates of disease are of limited use because they don't include direct measures of radiation exposures. The panel also concluded that animal and cell studies suggesting benefits or a threshold for harm are not "compelling," although mechanisms for possible "hormetic effects" should be studied further.

Toxicologist Ed Calabrese of the University of Massachusetts, Amherst, a vocal proponent of the hormesis hypothesis, says the panel didn't examine enough studies. "It would be better if more of the details were laid out instead of [hormesis] just being summarily dismissed," he says. The panel's chair, Harvard epidemiologist Richard Monson, acknowledges that the long-running debate over the LNT model won't end with this report, noting that "some minds will be changed; others will not."


* Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII
Phase 2 books.nap.edu/catalog/11340.html

Volume 309, Number 5732, Issue of 8 Jul 2005, p. 233.
Copyright © 2005 by The American Association for the Advancement of
Science. All rights reversed.