Low-Level Radiation Risks Found to Be 4 Times Higher : Health: A science panel urges the public to minimize unnecessary exposure to medicinal X-rays as one precaution.

The risk of developing cancer from low levels of radiation may be three to four times greater than previously thought, a panel of scientists said Tuesday in a landmark report with broad public health implications.

Similarly, the report said, children run a much higher risk of developing mental retardation and learning disabilities if they were exposed to radiation while in the womb, especially between the eighth and 15th weeks of gestation.

But for most people, said Arthur C. Upton, chairman of the National Research Council committee that issued the report, the findings should provide no cause for alarm. “The take-home message,” Upton said, “is that the average citizen should not view this as a source of great concern.”

Noting that most low-dose radiation sources occur naturally and are largely unavoidable, Upton urged the public to minimize unnecessary exposure to medicinal X-rays as a way to reduce the cancer risks from low-dose radiation.

“We really don’t know that there is a safe level of radiation without any biological effects,” said Upton, who was the National Cancer Institute director during the Jimmy Carter Administration.

Upton, now chairman of the department of environmental medicine at the New York University Medical Center, also said that the findings “probably” should lead to lowered radiation exposure standards for nuclear workers, although he said that the committee had not been asked to contemplate whether exposure standards should be revised.

At a 90-minute “public symposium” at the National Academy of Sciences, Upton and four committee members went to great lengths to emphasize that the new findings should be viewed in the proper context.

For instance, they noted, one of three Americans will develop cancer at some time, with 20% of them dying from it. But the risk of acquiring cancer from one diagnostic chest X-ray might be one in a million, Upton said.

But while such questions might be “negligible to an individual,” Upton added, they are vitally important to public health officials as they reconsider exposure levels for nuclear workers as well as for the medicinal uses of low-dose radiation such as mammograms and dental X-rays.

He said that the report’s conclusions were based on “significant developments” in the understanding of radiation exposure since 1980 and on new data on the largest group ever exposed to such radiation: the survivors of the atomic bombs dropped on Hiroshima and Nagasaki in 1945.

One bit of good news, the scientists said, is that there appears to be “no demonstrable hereditary damage in the children” of the Japanese survivors. “A large apprehension has been eased,” Upton said.

The report’s sharply higher estimates of the cancer risks are largely a result of more accurate estimates of the actual radiation received by the tens of thousands of the Japanese survivors of the two bombs.

Previously, Upton said, radiation from neutrons was thought to have been an important component of the survivors’ radiation doses. Now it is believed that neutrons played a much smaller role.

That means that the cancer rate among the survivors--far in excess of the rate for comparison groups who had not been similarly exposed--resulted from lower levels of radiation than had previously been suspected.

Upton and his colleagues also emphasized that medicinal sources of radiation make up only about 15% of all low-dose radiation sources. They said that 82% of this kind of radiation comes from naturally occurring sources, including 55% from radon, 8% from outer space and 11% from within the body.

The presence of radon, a naturally occurring radioactive gas that causes lung cancer, varies greatly throughout the country. Environmental officials have advised those who live in homes with high radon readings to take steps to reduce the levels.

Warren K. Sinclair, president of the National Council on Radiation Protection and Measurements, a federally funded private group, said that the current recommended annual safety limit for radiation exposure among nuclear workers is 5 rems (a rem is a standard measurement of a radiation dose equivalent).

By comparison, according to Upton and others, a chest X-ray measures about 10 millirems, or 1% of a rem. A full-mouth dental X-ray is in the range of one to five rems while a mammogram is 100 millirems or less, they said.

Upton and the committee said that they had turned up “inconclusive” evidence about whether nuclear plant workers or people who live near such facilities have experienced excessive cancers. They called for further studies.

The 421-page report released Tuesday was funded by the White House science office and prepared by the NRC’s 18-member Committee on the Biological Effects of Ionizing Radiations.