`Safe' level of radiation exposure needs to be reevaluated, data show

Tighter radiation exposure limits may lie ahead, based on new estimates of the effects of atomic bombs that were dropped on Hiroshima and Nagasaki. The revisions indicate that survivors received much lower doses of radiation than previously thought. When combined with the survivors' health records since then, the new estimates suggest that it takes less radiation to induce cancer than previously thought.

The resulting radiation risk assessments are under study by groups such as the US National Research Council's Committee on the Biological Effects of Radiation. ``We're expecting a moderately large shoe to drop'' when studies are completed, says Warren K. Sinclair, president of the US National Council on Radiation Protection and Measurements.

US agencies are likely to wait for the council's results - expected later this year - before deciding if the new risk estimates warrant changes in exposure standards, says Ray Cooper of the council's Board of Radiation Effects Research.

Other countries are moving more quickly. Britain's National Radiological Protection Board recommended last month that the British government reduce by 70 percent the annual maximum legal exposure level for those working with radioactive materials. It also asked that the maximum allowable exposure for the public be halved.

Mr. Cooper says that any change in standards will most likely affect people working directly with radioactive materials or processes. But he says it's possible that if a change is large enough, it could affect a broader segment of society by requiring larger evacuation zones around nuclear power plants, for example.

Studies of the 90,000 Japanese who survived the atomic bombs arouse such interest because they account for about half the data scientists use to determine effects of radiation on humans, Dr. Sinclair says.

The revised dose estimates come from the US-Japanese Radiation Effects Research Foundation. It found that the Hiroshima bomb's yield was 20 percent higher than the original estimate. The Nagasaki bomb's yield fell nearly 5 percent. Housing provided about twice the shielding allowed for in previous estimates, while the body was found to be a less effective shield for its organs.

As a result of these and other changes, the dose of neutrons from the Hiroshima bomb fell to about 10 percent of its previous level, with the gamma ray dose 2 to 3.5 times higher. For Nagasaki, the neutron dose estimate fell by half; the gamma ray dose was trimmed slightly.

Mr. Cooper says several uncertainties remain as scientists sort through the implications of the new dose estimates.

One involves the relative damage done by neutrons and gamma rays. Animal tests are used to get at this problem. But questions remain on how applicable the results are to humans, he says. ``Neutrons are more damaging'' he says, but estimates range from 10 to 20 times more. Refining that figure will help pin down the relative risks from neutrons and gamma rays.

Another is the extent to which scientists can extrapolate the risks from high exposures to levels perhaps a million times less. ``Most radiation is at pretty low levels,'' Cooper says. ``And the data come from people who received very high doses. Only theory exists for extrapolating downward from that high a level.''