The finding by the National Academy of Sciences panel is viewed as critical because it addresses radiation amounts commonly used in medical treatment and is likely also to influence radiation levels the government will allow at abandoned nuclear sites.

The nuclear industry, as well as some independent scientists, have argued that there is a threshold of very low level radiation where exposure is not harmful, or possibly even beneficial. They said current risk modeling may exaggerate the health impact.

The Associated Press reports that the panel, after five years of study, rejected that claim.

"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 chairman and a professor of epidemiology at Harvard's School of Public Health.

The committee gave support to the so-called "linear, no threshold" model that is currently the generally acceptable approach to radiation risk assessment. This approach assumes that the health risks from radiation exposure declines as the dose levels decline, but that each unit of radiation -- no matter how small -- still is assumed to cause cancer.

"It is unlikely that there is a threshold below which cancer are not induced," said the report, although it added that at low doses "the number of radiation-induced cancers will be small." And it said cancers from such low dose exposures may take many years to develop.

The panel, formally known as the Committee on Biological Effects of Ionizing Radiation, or BEIR, generally supported previous cancer risk estimates -- the last one by an earlier BEIR group in 1990.

The associated Press reports that contrary to assertions that risks from exposure from low-level radiation may have been overstated, the panel said "the availability of new and more extensive data have strengthened confidence in these (earlier) estimates."

The committee examined doses of radiation of up to 100 millisievert, a measurement of radiation energy deposited in a living tissue. A single chest X-ray accounts for 0.1 millisievert, average background radiation 3 millisievert a year and a whole body CT scan delivers 10 millisievert..

The Associated Press reports that the committee estimated that 1 out of 100 people would likely develop solid cancer or leukemia from an exposure of 100 millisievert of radiation over a lifetime with half of those cases being fatal.

The report noted that exposure from a whole body CT scan is much higher than a normal X-ray, and it raised concerns about the frequency in which such medical diagnostics should be used.

While medical radiation is often done for good reasons, said Monson, "exposure to unnecessary radiation should be avoided."