Inhalation of Radiation.

Low Doses Yield High Risks

Through studies of large groups exposed to radiation, epidemiologists try to quantify the relationship between doses received and resulting carcinogenic effects. Such information is used in the establishment of radiation protection standards. Many radiation exposures today, particularly those of workers in certain types of nuclear facilities, occur when radioactive materials are taken inside the body. But except for a few cases, few human epidemiological studies of the health effects of internal exposures have been conducted. So a team of investigators led by epidemiologist Beate Ritz of the University of California at Los Angeles launched a retrospective study of former nuclear employees to assess the long-term health effects of radiation exposures primarily due to the inhalation of airborne radioactive materials [EHP 108:743-751]. They found that low internal radiation doses may increase the risk of certain cancers.

The researchers quantified the doses to nearly 2,300 workers who had worked at various times between 1950 and 1994 at Rocketdyne/Atomics International, a nuclear research and development facility in Simi Valley, California. The investigators relied primarily on data derived from analysis of specific radionuclides in worker urine and feces samples. They also performed external measurements of the radiation emitted by the radioactive materials in the subjects' bodies.

In conducting their analyses, Ritz and colleagues separated the workers into four groups, depending on the dose they were estimated to have received. The four groups ranged from those who were not exposed at all to those receiving a maximum dose of 30 millisieverts or more. A comparison of the adjusted rate-ratios for cancers among these groups showed that the workers who received the highest doses died at a substantially higher rate from leukemias and lymphomas than did those who were not exposed. The same relationship was true for workers who died from cancers of the mouth, throat, esophagus, and stomach. Substantiating these observations was the fact that workers in the zero-dose range had the lowest rates of death and those within the two intermediate dose ranges had progressively higher rates of death with increasing dose. Again, this was true both for leukemias and lymphomas and for cancers of the mouth, throat, esophagus, and stomach. The researchers also examined lung, bladder, kidney, and prostate cancer incidence, but found no elevations in mortality rates. Although the link to increased leukemias and lymphomas had been reported in two earlier studies, the relationship to mouth, throat, and esophagus cancers had not previously been reported for workers exposed to internally deposited radionuclides in this low-dose range.

Still, due to the small number of cases in each cancer group, the authors are careful to acknowledge that their estimates are imprecise. They also caution that these findings need to be confirmed by further follow-up of the present group. Nonetheless, each such study is important because it contributes information about the potential carcinogenicity of specific radionuclides prevalent in the nuclear materials work environment.