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IA cohort study of nuclear industry workers was initiated in 1990 to determine the possible health effects of low-level radiation. A total of 5,527 deaths were ascertained among 176,000 male workers who had been retrospectively and/or prospectively followed for an average of 7.9 years during the observation period 1986-1997. Statistical analyses were made mainly on the prospective follow-up outcome of 120,000 workers followed for an average of 4.5 years. The standardized mortality ratio (and its 95% confidence interval) was 0.94 (0.90, 0.97) for 2,934 cases of all causes combined and 0.86 (0.82, 0.91) for 1,305 cases of non-malignant diseases combined, which suggested a healthy worker effect. For 1,191 cases of all cancers combined, it was 0.98 (0.93, 1.04), indicating no difference in mortality from that of the general population. In tests for trend of death rate with increasing radiation dose, no significant correlation was found for all cancers combined. For site-specific cancers, most cancers including leukemia showed no positive correlation with dose, except for cancers of the esophagus, stomach and rectum and multiple myeloma. External causes showed a significant correlation with dose. A separate questionnaire study indicated that these positive findings could be ascribed in part to lifestyle characteristics of the workers. For leukemia only, we attempted to estimate the excess relative risk per unit dose of radiation, which, with reservations because of its wide confidence interval, was within the range of variation of the risks reported in other radiation epidemiological studies. This population must be studied for a longer time and with a consideration of the possible effects of confounding factors.

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To examine confounding on the risk assessment of the radiation workers at nuclear facilities in Japan, a questionnaire survey of their characteristics such as life-style and occupational history was performed for 54,369 male and 470 female workers who were currently engaged in the job and valid answers were obtained from 48,281 males and 428 females. In order to know whether these characteristics were different among different dose groups, the Mantel extension statistical test was performed only for male respondents, with cumulative radiation doses stratified into 5 classes. Increasing trend according to the increasing doses was statistically significant for the percentages of tobacco smokers and of heavy smokers. It was also the case for heavy alcohol drinkers. Percentages of workers who were engaged in jobs dealing with specific toxic materials were also increasing in the higher dose groups. On the other hand, percentage of workers who underwent the X-ray examination of the upper digestive tracts or other radiological examinations tended to be lower in higher dose groups. These results indicate that characteristics of radiation workers such as life-style are different among dose groups and thus may play a role as a confounding factor in the statistical relation between the radiation doses and cancer mortalities.