The report's findings, based on 15 additional years of Hiroshima/Nagasaki follow-up data, extensive new research conducted at nuclear facilities and an explosion in knowledge linked to recent biogenetics research, should help put to rest the 'threshold' theory expounded by those who believe a totally 'safe' level of radiation exists.

But experts also stressed that most of the sources of radiation people receive -- from natural "background" exposures to medical X-rays and CT scans -- probably contribute very little to the odds of developing cancer.

The new findings have implications for regulations covering radiation levels at abandoned nuclear plants or nuclear weapons production facilities. In the past, the nuclear industry and some independent experts have suggested that a safe low threshold for radiation exposure exists.

The members of the academies' Committee on Biological Effects of Ionizing Radiation (BEIR) said their latest report, the seventh in a series and the latest since 1990, effectively refutes that theory.

Full-Body CT Scans

The panel members, speaking to reporters Wednesday in Washington, DC, also stressed that radiation-linked cancer risks for most individuals -- even with relatively high lifetime exposures -- remain quite low.

Even at the highest end of what experts consider low-dose lifetime exposure -- 100 millisieverts -- "about 1 person in 100 would develop cancer," said panel member Ethel Gilbert, a biostatistician at the National Cancer Institute, in Bethesda, Md.

To put that number in context, she noted that 42 other lifetime cases of cancer can be expected in that same population of 100, due to such factors as smoking, environmental toxins or other influences.

The vast majority of Americans will be exposed to lifetime radiation levels below 100 millisieverts, Gilbert noted. That dose is "a little larger than lifetime exposure of natural background levels, and it's about 1,000 times the dose you receive from a single chest X-ray," she added.

Full-body CT scans remain one potential source of concern, however.

"We estimated that whole exposure to a whole-body scan is about 10 millisieverts," committee Chairman Dr. Richard Monson, a professor of epidemiology at the Harvard School of Public Health, told reporters. "That's a relatively high dose."

Consumer interest in full-body CT scans -- which are typically paid for out-of-pocket to check for signs of disease in otherwise asymptomatic individuals -- has been fueled by the growth of private CT centers advertising this service.

"There's currently no information on whether there's an association between exposure to radiation from CT scanning and adverse health effects," Monson said. But, he added, "Prudence should always be your guideline, and exposure to unnecessary radiation should be avoided."

Radiation Can Cause Malignancy

According to panel member William Dewey, director of the Radiation Research Laboratory at the University of California, San Francisco, radiation can cause malignancy by sending electrons through cells that create pockets of energy, which, in turn, produce double-strand breaks in a cell's DNA. This can lead to "deletions in the DNA, gene losses, and those types of changes in the DNA that have been associated with cancer," he explained.

The committee members pored over 50 years of data on survivors and descendents of survivors of the Hiroshima and Nagasaki atomic bomb attacks, as well as accumulated data from dozens of studies on exposures to workers in the nuclear industry and other high-exposure sites.

They report that natural, unavoidable "background radiation" -- exposure from naturally occurring radon, cosmic rays and earth-borne radiation -- makes up 82 percent of the radiation the average American will be exposed to in a lifetime. The bulk of the remaining 18 percent of exposure arises from man-made sources, such as medical X-rays, nuclear medicine and consumer products.

The committee also looked at the effects of low-dose radiation on non-malignant disease, especially cardiovascular problems, but found "no direct evidence of increased risk."

The panel members also noted that 50 years of follow-up of Japanese A-bomb survivors suggest that "at low or chronic doses," genetic risks for offspring of exposed parents is "very small."

'No-Threshold' Hypothesis

The report's findings, based on 15 additional years of Hiroshima/Nagasaki follow-up data, extensive new research conducted at nuclear facilities and an explosion in knowledge linked to recent biogenetics research, should help put to rest the "threshold" theory expounded by those who believe a totally "safe" level of radiation exists, the experts said.

But even though he believes the data now strongly favors the "no-threshold" hypothesis, Monson said he expects that "some minds will be changed, while others will not."

More information: A copy of the committee's report in brief is available at the National Academies of Science.