There is virtually no dose of radiation that is totally safe, but low levels such as those used for most types of medical diagnosis produce little increased risk, a government panel said yesterday.
Many researchers had previously believed that there is a "threshold" level of radiation that must be exceeded before exposure increases the risk of cancer, but that is not the case, according to a panel of the National Academy of Sciences.
"There is some risk, even at very low doses, although the risk is small," said Dr. Richard R. Monson of Harvard University, who chaired the panel.
The finding comes a day after a European report concluded that workers in the nuclear industry face a small, but real, increase in cancer risk even when current radiation protection standards are met.
Experts cautioned that the radiation risk must be put in the perspective of the small overall risks.
The panel concluded that exposure of 100 people to the maximum amount of workplace, medical or accidental radiation allowed over a lifetime by current guidelines, 100 milliSieverts (mSv), would produce one cancer. Among that same group of 100 people, however, 42 others will develop cancer from other causes.
"Smaller doses result in proportionately smaller risks," added panel member and biostatistician Ethel S. Gilbert of the National Cancer Institute.
The 100-mSv guideline is roughly the equivalent of 1,000 chest X-rays or 10 whole-body CT scans. Most people, even those in the nuclear power industry, do not come near the guideline. The report estimates that most people receive about 3 mSv of radiation annually, most of it from cosmic rays, radon and other environmental sources.
The new study essentially confirms a 1990 report by an earlier version of the panel, called the Committee on Biological Effects of Radiation.
Some scientists, particularly those in the nuclear power industry, have argued that the 1990 report overstated the risks from very low doses. Some even argued that low doses might be beneficial.
But Gilbert pointed out that a wealth of new data has accumulated over the past 15 years. The original guidelines were based largely on studies of the effects of the atomic bomb on survivors at Hiroshima. In the past 15 years, she said, researchers have developed much better estimates of the radiation doses to which the survivors were exposed, and the number of cancers in that population has doubled, increasing the statistical certainty of the findings.
There is also a great deal of new information about medical exposures and exposure in the nuclear industry. "That has really strengthened our confidence in the findings," she said.
The BEIR panel's finding "puts to rest once and for all claims that low doses of radiation aren't dangerous," said Daniel Hirsch, president of the nuclear policy organization, Committee to Bridge the Gap. "Nuclear advocates have been making this claim for years."
The Los Angeles Times is a Tribune Publishing newspaper.