Study upgrades radiation risks to humans.

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. Neutron radiation -- the most biologically hazardous -- "no longer appears to be of major importance" in those Japanese exposures, according to the new report. This observation underlies the increase in the lifetime cancer risks now attributed to any given dose of gamma radiation.

One cancer for which the revised bomb-survivor data played an especially important role was leukemia. Here, even though the radiation-risk model that best fits the data remains a linear/quadratic one, the panel found the leukemia-induction risk of low-dose exposures to be four times higher than that reported in "BEIR III."

The new report estimates that exposures of 0.1 rem per year throughout a person's lifetime might contribute an average of 550 excess deaths from cancer per 100,000 people exposed -- provided each year's exposure occurs in one single dose. Such exposures are comparable to those of a head-and-body CAT scan. "BEIR V" data suggest that if the same total dose is delivered as several smaller exposures -- as from radon in the home -- the cancer risk might fall to 300 excess deaths per 100,000 persons exposed, Ellett notes.

Overall, the estimates indicate that per unit dose, childhood radiation exposures carry roughly twice the lifetime cancer risk of adult exposures. Researchers also observed a dose-dependent incidence of retardation in Japanese children receiving fetal exposure to atomic-bomb blasts -- but only if exposure occurred between the 8th and 25th weeks of gestation, and particularly between weeks 8 and 15.