Any radiation dose a risk, panel finds

WASHINGTON – Even very low doses of radiation pose a risk of cancer over a person’s lifetime, a National Academy of Sciences panel has concluded. It rejected some scientists’ arguments that tiny doses are harmless or may in fact be beneficial.

The findings, disclosed in a report Wednesday, could influence the maximum radiation levels that are allowed at abandoned reactors and other nuclear sites and raises warnings about excessive exposure to radiation for medical purposes such as repeated whole-body CT scans.

“It is unlikely that there is a threshold (of radiation exposure) below which cancers are not induced,” the scientists said.

While at low doses “the number of radiation-induced cancers will be small … as the overall lifetime exposure increases, so does the risk,” the experts said.

Even common X-rays pose some risk of adverse health effects, the scientists found, although the panel said there was not enough information available to accurately estimate the cancer risk from X-rays. Nevertheless, the report said, there is evidence that per unit of absorbed radiation, X-rays may be more dangerous than other radiation.

The panel also said that approximately one person out of 1,000 would develop cancer from exposure to the amount of radiation from a single, average whole body CT-scan.

But the report should not scare people away from nuclear medicine, said Dr. Henry Royal, a professor of radiology at Washington University in St. Louis. He said most often the benefits of such tests and treatments outweigh the risks.

But Royal also said that procedures such as CT scans should be used to deal with specific medical problems, not as part of annual medical screenings. “You should not be exposed to radiation for superficial reasons,” Royal said in a telephone interview.

Scientists for years have debated how extremely low doses of radiation affect human health.

Pro-nuclear advocates, as well as some independent scientists, have maintained that the current risk models for low-level radiation have produced more stringent requirements than is necessary to protect public health.

It is an issue in determining decontamination requirements at abandoned reactors and at federal weapons sites.

The academy’s panel stood by the “linear, no threshold” model, which is generally the acceptable approach to radiation risk assessment. This approach assumes that the health risks from radiation exposure decline as the dose levels drop, but that each unit of radiation – no matter how small – is assumed to cause cancer.

“The scientific research base shows that there is no threshold of exposure below which low levels of ionized radiation can be demonstrated to be harmless or beneficial,” said Richard R. Monson, the panel’s chairman. He is a professor of epidemiology at Harvard’s School of Public Health.

The panel said new and more extensive data developed over the past 15 years only strengthen the conclusions of the panel’s last report, in 1990, on low-level radiation risks.

The scientists estimated that one out of 100 people exposed to 100 millisievert of radiation over a lifetime probably would develop solid cancer or leukemia, and that half of those cases would be fatal. They also said that 42 additional cancers can be expected in the same group from other than low-level radiation sources.

A millisievert is a measurement of radiation energy deposited in a living tissue. A person absorbs about 3 millisievert of radiation annually from natural sources and 0.1 millisievert from a chest X-ray. The report noted that exposure from a whole body CT scan is about 10 millisievert, much higher than a normal X-ray.