Results of three cohort studies of carcinogenic effect from occupational exposure to radon are presented in order to evaluate risk from low exposures. By 1995, a total of 910 cases of lung cancer were observed, 790 of them in the cohort of Jachymov uranium miners followed since 1948 and firstly reported already in 1971. Exposures in this 'historical' cohort were relatively high (40-800WLM). The other two cohorts of workers under present mining conditions are characteristic by exposures of about an order of magnitude lower. In previous analyses, numbers of cases in the latter two cohorts were not sufficient to demonstrate any significant findings. In the present analyses, miners were restricted to low exposure rates beginning with year 1952. A total of 195 cases have been observed in exposure categories below 50 WLM, with a significant excess already in the category 10-20 WLM. The most simple model of relative risk assumes a linear dependence on total cumulative exposure W lagged by 5 years. Although the coefficients corresponding to separate cohorts in this simple model show some differences, general temporal patterns of relative risk derived from cohorts are consistent. It is shown that time and age related modifying factors can explain a large part of differences in risk coefficients. In addition, risk coefficients corresponding to low exposure rates do not show any differences or any trend. The present analyses suggest that the assumption of linear non-threshold model is justifiable and is valid also for low exposures.
