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

The findings could influence the maximum radiation levels that are allowed at abandoned reactors and other nuclear sites. The conclusions also raise 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,” scientists said in the report.

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. Scientists for years have debated how much 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 that generally is 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 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.

It also said that 42 additional cancers can be expected in the same group from other than low-level radiation sources.