Air Pollution and Breast Cancer Incidence in the Multiethnic Cohort Study

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.

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.

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).

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.