Is The Linear, No-Threshold Theory Of Radiation Obsolete?

Stanley Goldsmith, M.D. Editor-in-Chief, Journal of Nuclear Medicine       YES The linear, no-threshold theory of radiation was never more than a working hypothesis, originated in the 1950s as a prudent operational guideline. But it has acquired the aura of fact even though no one has ever generated any evidence for it.   Recently, the BEIR 5 report-drawn up by a group of authorities in epidemiology, radiation, and cancer-concluded that there is a possibility that the risk at the level of 10 rad a year, which is twice the former guideline for workers, might be zero.   Johns Hopkins researchers have followed 750,000 workers at nuclear shipyards for over four decades. Among 90,000 workers exposed to nuclear areas on ships, only 26 cases of leukemia have been found, exactly the rate expected in the general population. Among workers exposed to less than 0.5 rad, fewer cases were seen than the LNT would suggest.   In parts of the world, Denver among them, where high altitude and uranium in the soil increase natural background radiation, there's no increase in the incidence of malignancies, fetal deformities, or sterility.   Some studies can be interpreted to say that low levels of radiation are good for you, a concept known as hormesis. Dr. Bernard Cohen of the University of Pittsburgh compared radon levels in homes with lung-cancer rates for 1,600 U.S. counties. He found higher levels of radon linked to lower rates of lung cancer. In the Hopkins study, nuclear workers exposed to less than 0.5 rad had far fewer cancers than nonnuclear workers.   One can't extrapolate from in vitro studies, which suggest a mechanism for these observations, to a multicellular organism, and I don't think hormesis is real. But what's real is that there is no injury from low or even moderate radiation levels.

Daniel Strom, Ph.D., C.H.P. Staff Scientist, Health Protection Department, Pacific Northwest National Laboratory       NO The notion that the linear, no-threshold theory is obsolete ignores heritable predispositions to cancer and other genetic defects. But reviews by the National Academy of Sciences and international bodies find plenty of fruit fly and in vitro evidence that LNT holds when it comes to germ-cell damage.   Two reports were issued in the fall of 1995 by the U.S. National Council on Radiation Protection and Measurements and the British National Radiological Protection Board. They asserted that the body of evidence from both laboratory animals and human studies allows presumption of a linear, no-threshold response at low doses and low-dose rates. Those who hold the public trust have no choice but to use the LNT for radiation protection for the foreseeable future.   Most epidemiologists put little credence in Dr. Cohen's "ecologic" design, because such analyses do not associate individual exposures with individual outcomes. The association was home-radon levels averaged by county with lung-cancer rates averaged by county.   Some conclude that data on atom-bomb survi-vors show that radiation protects at low doses. But there are huge error bars at low doses that include LNT in those analyses.   Even if all cancer could be prevented or cured, the world would still need stringent radiation-dose limits. The current occupational whole-body dose limit of 5 rems annually, based on cancer risk, would climb only to 15 rems, in order to prevent cataracts.   EPA's home action level for radon-4 picocuries/L-would rise to just 21 picocuries/L. That would be low enough to achieve the current limit of 50 rems to the bronchial epithelium, a worldwide standard designed to prevent acute radiation injury.