Exposure to everyday sources of radiation, mostly medical X-Rays, raises the risk of cancer but not by much and there is no clear line between a harmless dose and a disease-causing dose, according to research presented Wednesday.

People should think twice about having unnecessary high-dose X-rays such as the full-body CAT scans being offered by some clinics, the panel advised, but otherwise should be reassured by the findings.

The report from the National Research Council updates 1990 findings based mostly on survivors of the 1945 atomic bomb attacks against Japan, about 45 percent of whom are still alive.

A low dose of about 100 millisieverts of radiation - the equivalent of 10 chest X-rays - can be expected to cause cancer in one out of every 100 people, the report finds.

“About 42 additional people in the same group would be expected to develop solid cancer or Leukemia from other causes. Roughly half of these cancers would result in death,” adds the report, available at http://national-academies.org .

Cancer is the second-biggest killer in much of the world after Heart disease.

The American Cancer Society estimates that nearly 1.4 million Americans will learn this year that they have cancer and 563,700 will die of it, but it says two thirds of cases are caused by tobacco use, poor eating, lack of exercise and Heart disease.

The report ties in with another issued on Wednesday, from the International Agency for Research on Cancer in France, which studied more than 400,000 nuclear industry workers and found they had a 10 percent increased risk of death from cancer.

The National Research Council is part of the National Academy of Sciences, an independent organization set up by Congress to guide government on matters of health and science.

This is its seventh report on radiation.

NO SAFE EXPOSURE

“The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial,” said committee chair Richard Monson, a professor of epidemiology at the Harvard School of Public Health in Boston.

“The health risks - particularly the development of solid cancers in organs - rise proportionally with exposure.”

But the report said few people are exposed to very much radiation. It recommends further study of infants and children exposed to radiation through X-rays or radiation treatment for cancer.

It also recommends more study of people who get frequent doses, such as those who get repeated CAT scans.

“I think what we can do is assure people that medical radiation as currently done for good reasons is part of medical care,” Monson told a news conference.

“But ... prudence should be the guideline and exposure to any unnecessary radiation should be avoided and what is unnecessary is up to an individual.”

Most sources of radiation are natural - gamma rays from space, and radon from the ground, air and water. “These sources account for about 82 percent of human exposure,” the report reads.

The 18 percent of human-made radiation comes mostly from medical radiation but also tobacco, televisions and smoke detectors.