Mortality among male workers at a thorium-processing plant.

The long-term health effects of exposure to thorium are of interest because of the possible increased use of thorium as an energy source in reactors using 232Th to produce 233U. Mortality is described in a cohort of 3039 men who were employed between 1940 and 1973 at a company involved in the production of thorium and rare earth chemicals from monazite sand. Based on deaths ascertained by the Social Security Administration and mortality rates for U.S. white males, the standardized mortality ratio (SMR) for all causes was 1.05 with 95% confidence limits (95% CL) of 0.96 and 1.15. Much of the excess mortality was attributable to non-occupational motor vehicle accidents (SMR = 1.64; 95% CL = 1.16 and 2.23), but SMRs were also high for lung cancer (1.44; 95% CL = 0.98 and 2.02), pancreatic cancer (2.01; 95% CL = 0.92 and 3.82), and diseases of the respiratory system (1.31; 95% CL = 0.92 and 1.83). In a subgroup of 592 men who worked for at least one year in selected jobs (indicative of highest exposure to thorium and thoron) that was followed up more intensively, the SMR for pancreatic cancer was significantly elevated (i.e. 4.13; 95% confidence limits = 1.34 and 9.63). The SMR for lung cancer was 1.68 (95% CL = 0.81 and 3.09), while that for respiratory diseases was 1.20 (95% CL = 0.52 and 2.37). Information on smoking habits in a sample of survivors suggested that smoking could have explained at least part of the excess mortality from lung and pancreatic cancer and from diseases of the respiratory system. Continued follow-up of the cohort through morbidity and mortality studies is needed to evaluate further the possible long-term effects of exposure to radioactivity and chemicals in the thorium extraction process.