Ambient air pollution and mortality in older patients with breast cancer

Background Although emerging studies link air pollution to mortality in patients with breast cancer, large-scale evidence remains limited. We aimed to evaluate associations between chronic exposures to three key regulated air pollutants, fine particulate matter, ozone, and nitrogen dioxide, and mortality in a nationwide cohort of older patients with breast cancer. Methods We constructed a cohort of patients with primary diagnosis of breast cancer aged 65 years or older between 2000 and 2016 using the Surveillance, Epidemiology, and End Results-Medicare database. High-resolution ambient concentrations of annual fine particulate matter, warm-season ozone, and annual nitrogen dioxide were estimated using hybrid models and linked to patients' residential zip codes as proxy exposures. A 3-pollutant Cox model was fitted to estimate hazard ratios for mortality associated with each pollutant, adjusting for demographics, tumor characteristics, cancer treatments, comorbidities, lifestyle factors, meteorological variables, and neighborhood-level characteristics. Results Among 593 333 patients with breast cancer, a 1-µg/m3 increase in annual fine particulate matter, a 1-part per billion increase in warm-season ozone, and a 1-part per billion increase in annual nitrogen dioxide were associated with hazard ratios for mortality of 1.0048 (95% CI = 1.0026 to 1.0070), 1.0021 (95% CI = 1.0013 to 1.0029), and 1.0022 (95% CI = 1.0014 to 1.0030), respectively. This finding translated to 49 annual excess deaths attributable to fine particulate matter, 21 to ozone, and 22 to nitrogen dioxide within the cohort. Effects were substantially larger at low exposure levels. Fine particulate matter and nitrogen dioxide had greater effects in younger patients, individuals who received chemotherapy or radiation, and individuals with disease diagnosed at later stages. Conclusion Our findings identified air pollution as a risk factor for mortality in older patients with breast cancer. Protective measures and air pollution control strategies may help reduce exposure and improve outcomes.