Ontario Uranium Miner Cohort: Cancer incidence and mortality

Over 30,000 men were employed to extract uranium from deep underground mines in Ontario from 1954 through 1996. Despite economic benefits, mining uranium is a dangerous occupation with potentially fatal long-term consequences. One example is the excess of lung cancer mortality associated with radon decay products that has been well demonstrated in uranium miners worldwide.

In the past, surveillance of Ontario uranium miners played an important international role in radiation protection, providing information on the magnitude of cancer risks associated with radon exposure. The most recent information resulted from studies done more than 20 years ago. Although uranium mining no longer occurs in Ontario, this large cohort remains a valuable source of information relevant to radiation protection.

National mortality and cancer incidence data were linked with the cohort file by Statistics Canada. The mortality and cancer incidence experience of this cohort was compared to the general Canadian population by calculating standardized mortality (SMR) and incidence (SIR) ratios. Cancer risks associated with different levels of cumulative radon exposure were examined. This included an examination of factors that could potentially modify the relationship between radon exposure and lung cancer risk. Gamma radiation exposure was also investigated based on information available from the cohort, which included area measurements in the various mines at different points in time, employment data collected during routine annual chest x-rays, as well as badge readings recorded in the National Dose Registry (NDR) for exposures after 1980.

Significant elevations in lung cancer mortality and incidence, as well as silicosis and injury mortality were observed in comparison with the general Canadian population.  A strong dose-response relationship between cumulative radon dose and lung cancer was consistent across mortality and incidence.  When lung cancer histology was considered, the strongest associations with cumulative radon dose were found with squamous cell and small cell carcinomas.  No significant elevations or associations were observed for cumulative radon dose and stomach cancer, leukemia, or cardiovascular disease mortality in the cohort.  Silicosis and injury mortality elevations were predominantly observed among Elliot Lake (Blind River) miners hired prior to 1960.

The findings from this project contribute to our understanding of uranium mining, radon and gamma radiation exposure, and cancer risk with potential implications for radiation protection. This project also improves our understanding of non-lung cancer risks, cancers with long latency periods, incident cancer risks and risk of non-cancer endpoints (e.g. cardiovascular diseases). The silicosis and fatal injury results are consistent with the high silica exposures seen among uranium miners and the physical hazards of mining.

The final report was submitted to the Canadian Nuclear Safety Commission (CNSC) and has been published. Major primary analyses have been completed and additional analyses are underway through a number of sub-projects involving differing aspects of the cohort file, many of which are international collaborations such as the Pooled Uranium Miners Analysis (PUMA) project. The PUMA project comprises North American and European uranium cohorts and represents the largest study of uranium miners to date.

This study was funded by the Canadian Nuclear Safety Commission.