Increasingly epidemiological evidence supports that environmental factors are associated with breast cancer (BC) outcomes after a BC diagnosis. Although evidence suggests that air pollution exposure is associated with higher mortality in women with BC, studies investigating potential mechanisms have been lacking.

We evaluated women with BC (N = 151) attended at the National Cancer Institute–Mexico from 2012 to 2015. We calculated 1-year average exposures to particulate matter < 2.5 μm (PM_2.5) at home address before diagnosis. We used linear and logistic regression models to determine the associations between PM_2.5 exposure and BC aggressiveness (tumor size, molecular phenotype).

Average annual PM_2.5 exposure of this population was 23.0 μg/m³ [standard deviation (SD)]: 1.90 μg/m³]. PM_2.5 levels were positively correlated with tumor size at diagnosis (r = 0.22; p = 0.007). Multivariable linear models had a similar inference [risk ratio (RR): 1.32; 95% confidence interval (95% CI): 1.04, 1.674]. We did not observe differences in this association by age or menopause status. Further, women with triple-negative BC (TNBC) had significantly higher PM_2.5 levels compared with other phenotypes (p = 0.015). Multivariable-adjusted logistic regression models assessing the association between PM_2.5 and tumor size had a similar inference (RR 1.41; 95% CI 1.05, 1.89) overall for all ages and also for women who were ≤ 50 years old at diagnosis (RR 1.63; 95% CI 1.036, 2.57).

Our findings suggest a significant association between long-term PM_2.5 exposure and BC aggressiveness based on tumor size and phenotype, as well as a worse outcome.