No dose of low-level radiation is too small to rule out a "small" increase in cancer risk for humans, say researchers.

They reviewed data on the topic for the National Research Council. The council is part of the National Academy of Sciences (NAS). The report updates information from the 1990 findings, which is based mostly on survivors of the 1945 atomic bomb attacks against Japan.

"The scientific research base shows that there is no threshold of exposure below which levels of ionizing radiation can be demonstrated to be harmless or beneficial," says Richard Monson, MD, ScD, in an NAS news release.

Monson chaired the committee that wrote the report. He is also associate dean for health education and an epidemiology professor at Harvard School of Public Health.

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'Very Small' Risk at Low Doses

"The health risks -- particularly the development of solid cancers in organs -- rise proportionally with exposure," says Monson.

"At low doses of radiation, the risk of inducing solid cancers is very small. As the overall lifetime exposure increases, so does the risk," says Monson.

The researchers define low-dose radiation as those ranging from close to zero to about 100 millisieverts. The radiation dose from one chest X-ray is about 0.1 millisieverts. A whole-body CT scan delivers about 10 millisieverts, say the researchers.

Researchers' Estimate

The report's predictions are based on information from survivors of the atomic bombings in Hiroshima and Nagasaki, Japan. It estimates that about one in 100 people in the U.S. would be expected to develop solid cancer or leukemia from a dose of 100 millisieverts, say the researchers.

For perspective, they say 42 in 100 people would be expected to develop cancer from other causes.

Some Sources Are Natural

Low-level radiation is used in the medical field for X-rays and in nuclear medicine to produce some of the scanned images of the body, such as bone, lung, and thyroid scans. It is also found in the ground and the universe.

About 82 percent of human exposure to radiation comes from natural sources -- soil, water, and some meats. Basic activities -- such as eating, drinking, and breathing -- are most people's primary sources, say the researchers.

People get exposed to about 3 millisieverts of natural "background" radiation a year, they say.

The other 18 percent comes from man-made sources, they note. A smaller percentage comes from consumer products such as tobacco, tap water, building materials, occupational exposure, fallout, and the use of nuclear fuel, say the researchers.

More Work Needed

More studies are needed to explore low doses of radiation and other health problems, including heart disease, say the researchers.

"Radiation exposure has been demonstrated to increase the risk of diseases other than cancer, particularly [heart] disease, in persons exposed to high therapeutic doses and also in atomic-bomb survivors exposed to more modest doses," they say.

"However, there is no direct evidence of increased risk of noncancer diseases at low doses, and data are inadequate to quantify this risk if it exists," they write.

Protection From X-ray and Gamma Radiation

Exposure to X-rays is almost entirely from dental and medical X-rays, including mammograms. Simple MRI scanning does not expose a person to X-rays; it uses radio waves and a strong magnetic field to produce body images.

One way to protect yourself from excessive radiation from X-rays is to make sure the technician performing the procedure has the proper qualifications, says the Environmental Protection Agency. Simply ask questions.

You might inquire about the necessity of having an X-ray or receive assurance the X-ray machine has been inspected recently and that it is properly calibrated.

You should be aware of steps taken to prevent exposures to other parts of your body -- for example, through the use of a lead apron.

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By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: The National Academies Report in Brief, "BEIR VII: Health Risks from Exposure to Low Levels of Ionizing Radiation," June 2005. News release, National Academy of Sciences. Environmental Protection Agency.