Air Pollution and Breast Cancer Incidence in the Multiethnic Cohort Study.

Purpose: Recent studies suggested fine particulate matter (PM_2.5) exposure increases the risk of breast cancer, but evidence among racially and ethnically diverse populations remains sparse.

Materials and methods: Among 58,358 California female participants of the Multiethnic Cohort (MEC) Study followed for an average of 19.3 years (1993-2018), we used Cox proportional hazards regression to examine associations of time-varying PM with invasive breast cancer risk (n = 3,524 cases; 70% African American and Latino females), adjusting for sociodemographics and lifestyle factors. Subgroup analyses were conducted for race and ethnicity, hormone receptor status, and breast cancer risk factors.

Results: Satellite-based PM_2.5 was associated with a statistically significant increased incidence of breast cancer (hazard ratio [HR] per 10 μg/m³, 1.28 [95% CI, 1.08 to 1.51]). We found no evidence of heterogeneity in associations by race and ethnicity and hormone receptor status. Family history of breast cancer showed evidence of heterogeneity in PM_2.5-associations (P_{heterogeneity} = .046). In a meta-analysis of the MEC and 10 other prospective cohorts, breast cancer incidence increased in association with exposure to PM_2.5 (HR per 10 μg/m³ increase, 1.05 [95% CI, 1.00 to 1.10]; P = .064).

Conclusion: Findings from this large multiethnic cohort with long-term air pollutant exposure and published prospective cohort studies support PM_2.5 as a risk factor for breast cancer. As about half of breast cancer cannot be explained by established breast cancer risk factors and incidence is continuing to increase, particularly in low- and middle-income countries, our results highlight that breast cancer prevention should include not only individual-level behavior-centered approaches but also population-wide policies and regulations to curb PM_2.5 exposure.