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Long-term exposure to air pollution may increase women's risk of breast cancer, a large cohort study suggests.

Breast cancer risk increased incrementally by 28% with increasing exposure to fine particulate matter (PM_2.5). The association was similar across racial/ethnic groups and irrespective of hormone receptor status. A meta-analysis of this and 10 other prospective cohorts showed a trend toward increased breast cancer risk with greater PM_2.5 exposure.

"As about half of breast cancer risk 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," concluded Anna H. Wu, PhD, MPH, of the University of Southern California Keck School of Medicine in Los Angeles, and co-authors in the .

The consistency of analysis and the accompanying meta-analysis suggest that the epidemiologic literature is "converging on PM_2.5 as a risk factor for breast cancer," asserted the author of an .

"These findings support additional policy-level interventions to reduce outdoor air pollution concentrations, particularly given the projected increases in PM_2.5 because of wildfires," stated Alexandra J. White, PhD, of the National Institute of Environmental Health Sciences in Durham, North Carolina. "Increased awareness of the relationship between air pollution and breast cancer for both physicians and patients could facilitate more routine capturing of information related to a patient's residential histories."

"Although exposure to outdoor air pollution is largely not directly modifiable or treatable, patients' comprehensive residence information can be used to estimate air pollutant exposure concentrations and other residence-based environmental exposures. In doing so, we can pave the way for a better understanding of the environmental contributors to breast cancer etiology."

Multiple studies have examined associations between air pollutants and breast cancer. A of more than 1 million breast cancers showed an association between breast cancer mortality -- but not incidence -- and PM_2.5 and PM₁₀. Routine monitoring of air pollutants, including PM₁₀, has occurred since the 1980s, but monitoring for PM_2.5 only since the early 2000s, Wu and co-authors noted in their introduction. The weaker association for PM_2.5 could have resulted in part from misclassification of PM_2.5 before the 2000s, with modeling based on PM₁₀ or back-extrapolation of more recent data on PM_2.5.

Three prospective studies published in 2023 -- one each from , the , and -- showed associations between breast cancer and PM_2.5. The consistency of the findings underscored a need for further investigation of PM_2.5 exposure and breast cancer, the authors continued.

The analysis included 58,358 California female participants in the (MEC) and followed them for an average of 19.3 years. A showed no associations between breast cancer and several air pollutants, including PM_2.5 and PM₁₀. Breast cancer incidence did increase significantly among females living within 500 meters of major roads, prompting a new investigation focusing on gaseous pollutants from traffic emissions.

The analysis included study participants followed from 1993-2018. During that time, 3,524 invasive breast cancers were detected among the participants. African Americans and Latinos accounted for 70% of the breast cancers.

Exposure estimates of PM_2.5 were derived from high-resolution satellite images. Exposures of other pollutants came from routinely collected monitoring data (PM₁₀, nitrogen dioxide, nitrogen oxides, carbon monoxide, and ozone).

Cox proportional hazards modeling showed a significant association with breast cancer incidence only for PM_2.5. The calculations yielded a hazard ratio of 1.28 for every 10 µg/m³ exposure (95% CI 1.08-1.51) of PM_2.5. Individual analyses showed a statistically significant association among white participants (HR 1.67, 95% CI 1.16-2.42) and nonsignificant associations among Black (HR 1.14, 95% CI 0.89-1.46) and Latino (HR 1.34, 95% CI 0.94-1.92) participants.

A test for heterogeneity showed no significant impact of race, ethnicity, or hormone receptor status on the results. An analysis of family history of breast cancer and PM_2.5 showed evidence of heterogeneity (P=0.046).

The investigators performed a meta-analysis of MEC and 10 other prospective cohorts. The results showed that breast cancer incidence increased with exposure to PM_2.5 (HR 1.05 per 10 µg/m³ increase, 95% CI 1.00-1.10, P=0.064).

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