Study Rules Out a Total Lack of Radiation Risk

Virtually no dose of radiation is totally safe, but low levels like those used for most types of medical tests produce little increased risk, a government panel said Wednesday.

Many researchers had believed that there was a “threshold” level of radiation that had to be exceeded before exposure increased the risk of cancer, but that was 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 came a day after a European report concluded that workers in the nuclear industry faced a small, but real, increase in cancer risk even when current radiation protection standards were met.

Experts cautioned that the radiation risk must be put in the perspective of the small overall risks.

The panel concluded that among 100 people exposed to the maximum amount of workplace, medical or accidental radiation allowed over a lifetime by current guidelines, 100 millisieverts, would produce one cancer. Among that same group, however, 42 others will develop cancer from other causes.

“Smaller doses result in proportionately smaller risks,” said panel member and biostatistician Ethel S. Gilbert of the National Cancer Institute.

The 100-millisievert 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 approach the guideline.

The report estimates that most people receive about 3 millisieverts 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 noted that a wealth of new data has accumulated over the last 15 years. The original guidelines were based largely on studies of survivors of the atomic bomb dropped on Hiroshima.

In the last 15 years, Gilbert 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 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 panels’ findings are similar to those of a large study of nuclear power workers published online Tuesday by the British Medical Journal.

Researchers from the World Health Organization’s International Agency for Cancer Research in Lyon, France, studied data for 407,391 nuclear industry workers in 15 countries.

Dr. Elizabeth Cardis and her colleagues estimated that exposure to 100 millisieverts of radiation would lead to a 10% increase in the risk of death from all cancers other than leukemia and a 19% increase in risk of death from leukemia.

The actual exposure of the workers, however, averaged about 19 millisieverts.

The group thus estimated that 1% to 2% of all cancers among the employees was produced by workplace radiation exposure.

The team noted that much of the data were collected in the early years of the nuclear industry, when exposures were higher than they are today.

Both groups of findings are expected to have little immediate effect on the general public and should not prove particularly alarming.

“This report suggests that there is a very small risk for the development of cancer in a patient’s lifetime when exposed to low-level radiation such as that used in diagnostic radiology procedures,” said Dr. Bruce L. McClennan, president of the American Roentgen Ray Society.

But Monson and others suggested that consumers should be very cautious about undergoing whole-body CT scans unless there was a medically sound reason for doing so.