Association between long-term exposure to ambient air pollution and breast cancer risk in Mexico: a systematic review of observational studies.

Breast cancer remains a major public health concern in Mexico, and ambient air pollution has been proposed as a possible contributing factor. We conducted a PROSPERO-registered systematic review, following PRISMA guidance, to assess observational evidence on long-term exposure to PM2.5, PM10, NO2, SO2, O3, and polycyclic aromatic hydrocarbons (PAHs) in Mexican women. Searches of bibliographic databases and grey literature, including dissertations, institutional repositories, and scholarly networking sites, identified 35 records. Six studies met the inclusion criteria: four ecological or spatial studies and two retrospective clinical studies from Hermosillo, the Monterrey Metropolitan Area, Miguel Alemán, and Mexico City. Where pollutant concentrations were reported, PM2.5 and PM10 were directly assessed; other studies used residential proximity to industrial or emission sources as exposure indicators. Across studies, higher estimated exposure was consistently associated with greater breast cancer burden, less favorable prognosis, or more adverse tumor characteristics. Even so, the evidence base was limited by heterogeneity in design, indirect exposure assessment, and substantial risk of bias. Overall certainty of evidence was very low. These findings are best interpreted as preliminary and hypothesis-generating, underscoring the need for stronger, better-controlled studies before causal or policy conclusions are drawn.