Long‐term exposure to fine particulate matter air pollution and the risk of lung cancer among participants of the Canadian National Breast Screening Study

Recently, air pollution has been classified as a carcinogen largely on the evidence of epidemiological studies of lung cancer. However, there have been few prospective studies that have evaluated associations between fine particulate matter (PM2.5) and cancer at lower concentrations. We conducted a prospective analysis of 89,234 women enrolled in the Canadian National Breast Screening Study between 1980 and 1985, and for whom residential measures of PM2.5 could be assigned. The cohort was linked to the Canadian Cancer Registry to identify incident lung cancers through 2004. Surface PM2.5 concentrations were estimated using satellite data. Cox proportional hazards models were used to characterize associations between PM2.5 and lung cancer. Hazard ratios (HRs) and 95% confidence intervals (CIs) computed from these models were adjusted for several individual‐level characteristics, including smoking. The cohort was composed predominantly of Canadian‐born (82%), married (80%) women with a median PM2.5 exposure of 9.1 µg/m3. In total, 932 participants developed lung cancer. In fully adjusted models, a 10 µg/m3 increase in PM2.5 was associated with an elevated risk of lung cancer (HR: 1.34; 95% CI = 1.10, 1.65). The strongest associations were observed with small cell carcinoma (HR: 1.53; 95% CI = 0.93, 2.53) and adenocarcinoma (HR: 1.44; 95% CI = 1.06, 1.97). Stratified analyses suggested increased PM2.5 risks were limited to those who smoked cigarettes. Our findings are consistent with previous epidemiological investigations of long‐term exposure to PM2.5 and lung cancer. Importantly, they suggest associations persist at lower concentrations such as those currently found in Canadian cities. What's new? Outdoor air pollution and its major component, fine particulate matter (PM2.5), are classified as carcinogens, owing to their ability to cause respiratory cancers. This classification is based largely on higher PM2.5 concentrations, however. Here, using a Canadian cohort in which low PM2.5 levels were assigned to virtually all participants based on remote sensing data, lung cancer incidence was found to also be positively associated with low‐level concentrations of ambient PM2.5. Lung cancer risk increased substantially for each 10 μg/m3 increase in PM2.5. The associations were modified by smoking behavior and varied across histological types of lung cancer.