Cancer and three mile island: no significant increase in five-mile radius. (Science Selections).

The 1979 accident at the Three Mile Island (TMI) nuclear power plant was the all-time worst at any commercial nuclear power plant in the United States. Small quantities of xenon and iodine isotopes were released during the accident and shortly afterward. Although exposures were later determined to be minimal, the state of the science at the time implicated low-level radiation in cancers and raised fears about increases in radiation-induced cancers among residents living near TMI. However, there has been no statistically significant elevation in overall cancer deaths among people living within a five-mile radius of TMI at the time of the accident, according to a 20-year follow-up study published this month by Evelyn O. Talbott and colleagues from the University of Pittsburgh [EHP 111:341-348].

The study compared deaths among the TMI cohort over the period 1979-1998 to those in both a local population outside of the five-mile radius and Pennsylvania's population as a whole. The team drew their data from the TMI Population Registry, compiled by the Pennsylvania Department of Health. The department had gathered radiation exposure information on 93% of the population living within five miles of the nuclear plant--nearly 36,000 people--in comprehensive door-to-door surveys shortly after the accident.

The radiation exposure data took into account where people were--whether indoors or outdoors, upwind or downwind, and so forth--during the largest exposure on the day of the accident, as well as over the 10 days following the accident, when additional radiation escaped. This allowed scientists to later determine whether each individual had received a low, medium, medium/high, or high dose of radiation. Thus, the Pittsburgh researchers were able to compare risk related to an estimated gradient of exposure within the cohort, allowing a more accurate correlation between exposure and health effects.

After the accident, scientists estimated "likely" and "maximum" whole-body gamma radiation doses for the TMI cohort. Individual exposures were estimated to range from 1 to 170 millirems (mrem). The likely dose was estimated to be 9 mrem. The maximum dose--which refers to the worst possible exposure, in which a person would have been outdoors and directly downwind of the plant--was estimated to be 25 mrem. By comparison, a chest X ray is a 10-mrem exposure, but to the chest only. And the average annual background radiation dose in the United States is 300 mrem, according to the Committee on the Biological Effects of Ionizing Radiation (BEIR).

Although the Pittsburgh team found no statistically significant rise in cancer deaths, there was a slight increase in overall mortality among the TMI population, with the deaths due primarily to heart disease. However, the heart disease is not thought to be related to radiation exposure, but rather is most likely due to the population having well-established heart disease risk factors (such as smoking and socioeconomic status) that were not measured. Another possible factor is the stress of living within the plant's shadow following the accident.

The study is particularly important because it is one of only a very few on the health effects of low-level radiation, particularly at these lowest levels of exposure. The study is also valuable for the particularly high integrity of its data: 20-year follow-up mortality data exist for 98.2% of the TMI cohort. The authors recommend, among other things, continued monitoring of the childhood population near TMI and continued follow-up on mortality for the TMI cohort beyond the original cutoff of 31 December 1998, especially as some cancers have a longer latency period.