Ambient Polycyclic Aromatic Hydrocarbon Exposure and Breast Cancer Risk in a Population-Based Canadian Case-Control Study

Abstract

Background: Polycyclic aromatic hydrocarbons (PAHs) represent a class of ubiquitous pollutants which are emitted through combustion of organic materials and are recognized as established human carcinogens and endocrine-disrupting chemicals. There is a lack of epidemiological research investigating ambient PAH exposures and breast cancer risk. Additionally, PAHs have seldom been modelled at the population-level for epidemiological application, and current methods for assessment of long-term exposure are limited. This thesis evaluated associations between long-term residential exposure to ambient PAHs and breast cancer risk, both pre- and post-menopausal, in the Canadian setting. Methods: Using the National Enhanced Cancer Surveillance System (NECSS), a population-based case-control study conducted within eight of the Canadian provinces between 1994-1997, annual fluoranthene exposures were estimated using the GEM-MACH-PAH chemical transport model on the basis of geocoded residential histories throughout a 20-year exposure window. Odds ratios (ORs) and 95% confidence intervals (CIs) controlling for potential confounders were estimated using unconditional logistic regression. Separate analyses were conducted for Ontario and national samples given a finer-resolution exposure surface available for Ontario, as well as additional risk factor information collected in Ontario. Results: Positive associations were observed between long-term fluoranthene exposure and premenopausal breast cancer, with inconsistent findings for postmenopausal breast cancer. For premenopausal breast cancer, adjusted ORs of 2.48 (95% CI: 1.29, 4.77) and 1.59 (95% CI: 1.11, 2.29) were observed when comparing the second highest category of exposure to the lowest, amongst the Ontario and national samples, respectively. For postmenopausal breast cancer, adjusted ORs were 1.10 (95% CI: 0.67, 1.80) and 1.33 (95% CI: 1.02, 1.73). Associations for the highest level of exposure, across both samples and menopausal strata, were non-significant. Results were robust in sensitivity analysis restricted to those with more complete residence history, confounders controlled for, and simultaneous modelling of criteria air pollutant measures. Conclusion: This study provides support for the hypothesis that ambient PAH exposures are associated with an increase in risk for breast cancer, especially for premenopausal women.