Surveillance and Risk Assessment Division
Centre for Chronic Disease Prevention and Control,
Population and Public Health Branch, Health Canada
This Health Canada (HC) report responds to public health concerns over environmental exposure to radioactive and other materials in Port Hope that were by-products of radium and uranium processing activities in the town since the 1930s. The study was commissioned by the Canadian Nuclear Safety Commission (CNSC). An ecological study of cancer incidence in Port Hope has already been conducted. The results showed that, compared to cancer incidence in the general population of Ontario, there was no overall evidence of an increased elevation of cancer in Port Hope. Nevertheless, some increases and decreases were observed when the data were examined by time period and for men and women separately.
The objectives of the present report included:
- extending the cancer mortality analysis to an earlier period;
- examining results for cancer and all other major causes of death; and
- comparing the cancer mortality results with the previous cancer incidence report.
The results of this study found no overall evidence of an increased elevation of cancer mortality in Port Hope. For the period 1956 to 1997, there were 836 cancer deaths observed, which was very close to the number expected (845) based on Ontario rates. Significant exposures to radiation would, a priori, have been anticipated to increase the mortality from leukemia and perhaps breast and lung cancer. Mortality rates for leukemia and breast cancer were not elevated. Lung cancer mortality was of interest because of the increased incidence observed among women (1986-1996) in the cancer incidence report. For the period 1986-1997, the standardized mortality ratio (SMR) for lung cancer among women was 1.28 (95% CI 0.89-1.77) and among men it was slightly below one. The lack of evidence of elevated lung cancer is reassuring. Further, there was no evidence that childhood leukemia mortality in particular, a known radiation-induced cancer, was increased above expectation. A consistently increased mortality from circulatory disease, including ischemic heart disease, cerebrovascular disease and diseases of the arteries was observed. For the period 1956 to 1997, there were 2,301 circulatory disease deaths observed, an increase of 15% from the 2,000 deaths expected based on Ontario rates. While there were some increases in cancer mortality when data were subdivided into finer units by time period and separately for males and females, the observed patterns were similar to Ontario rates. These findings are, on the whole, consistent with the earlier incidence report and reassuring that the patterns of cancer mortality are comparable to the province of Ontario.
The purpose and limitations of ecological studies must be kept in mind. They describe only the disease occurrence (i.e., the health status) of the community. They are not able to examine causal relationships between environmental exposure and disease occurrence. Exposure to individuals is not known and no consideration is given to the multiple risk factors of disease, population mobility impacts on environmental exposures or errors known to occur in assigning place of residence. In addition, the precision of the statistics from some causes of death is limited by the small number of deaths. Nonetheless, as there was industrial activity in Port Hope that has increased environmental levels of known carcinogens, albeit at low levels, it would be useful to periodically monitor cancer incidence and mortality in Port Hope using cancer statistics that are routinely collected or with enhanced residence information to learn whether any changes might occur. In addition, disease prevention efforts may be considered in order to reduce the elevated circulatory disease mortality which was also observed in the larger Northumberland County, rather than being limited to Port Hope.