By Raloff, J.
Science News , Vol. 136, No. 26-27
Study upgrades radiation risks to humans
Low doses of X-rays and gamma radiation pose a human cancer risk three to four times higher than previously estimated, according to a National Research Council (NRC) report due out in January. Its findings -- representing the first major reevaluation of radiation's human hazards in a decade -- also indicate some fetuses exposed to radiation face a higher-than-expected risk of mental retardation. In the same report, a follow-up of atomic-bomb survivors indicates radiation's ability to induce serious genetic damage is somewhat smaller than suggested in animal studies.
Researchers presented the newly revised estimates this week at a Washington, D.C., symposium and will publish them in the NRC's fifth report on the biological effects of ionizing radiation ("BEIR V"). The estimates reflect an additional 14 years' worth of data (primarily on 76,000 Japanese atomic-bomb survivors) that were unavailable to the "BEIR III" panel in 1980. In addition, "BEIR V" researchers used a different risk model to extrapolate from high doses to low ones.
"BEIR III" suggested that low doses of ionizing radiation -- those in the range of 1 to 10 rads -- posed less cancer risk per unit of exposure than did far higher doses. The new report, however, suggests lower-dose exposures are proportionately just as potent as higher-dose exposures in inducing human cancers other than leukemia, says William H. Ellett, staff director of the "BEIR V" panel. Substituting a linear dose-effect relationship for the earlier "linear/quadratic" model increased the apparent carcinogenicity of low-dose exposures by a factor of 2.5, Ellett says.
Recently reduced estimates of the neutron exposures suffered by survivors of atomic explosions over Japan also contributed to the upgraded potency of certain forms of ionizing radiation. …