Background: Air pollution as a complex variable

A major source of many environmental chemicals linked to increased risk for disease, including breast cancer, is the air we breathe.  Airborne toxicants include fine particulate matters (for example, PM_2.5 and the slightly larger PM₁₀); nitric oxides including, of particular importance, nitrogen dioxide (NO₂); black carbon (soot); heavy metals, including cadmium; polyaromatic hydrocarbons (PAHs); benzene, etc. [1],[2],[3]

Air pollution can develop very locally due to cigarette or other tobacco smoking. At a broader scale (within city blocks, neighborhoods, and larger regions), pollution is generated by vehicular and industrial combustion, wildfires and other biomass burning sources, and industrial manufacturing and distilling byproducts. Importantly, recent evidence has demonstrated disparities in the composition and distribution of air pollutants, with higher concentrations of many pollutants being measured in communities of color and, more generally, in communities facing higher levels of socioeconomic and environmental stressors.[4],[5],[6],[7] A recent (2021) study of breast cancer cases in the large Monterey Metropolitan Area in Mexico examined local distribution patterns and spatial associations between sources of pollution and cancer incidence. They found a clustering of breast cancer cases within the municipality in areas of the highest air pollution density.[8]

Evidence that links exposure to air pollutants and breast cancer

In the past few years, there have been many studies, both in the U.S. and internationally (e.g., France,[9] Chile,[10] Brazil,[11] Korea,[12] Taiwan[13]) and several systematic reviews and meta-analyses conducted to examine possible relationships between exposure to air pollution and risk of developing breast cancer.[14],[15],[16] The most common pollutants studied included nitric oxides, especially nitrogen dioxide, and the very small (less than 2.5 microns in diameter) particulate matter, PM_2.5.

• Most studies and analyses have concluded that a significant relationship exists between recent (usually current or the past 2-5 years) levels of NO₂ exposures and the risk of developing breast cancer. The relationships were strongest for premenopausal breast cancer and hormone receptor-positive (ER+/PR+) cancers.[17],[18]
• Among women in the Multiethnic Cohort from the Greater Los Angeles area, both traffic-related and hazardous air contaminants significantly increased breast cancer risk. Increases in risk from air pollutants were greatest for women of color. Of 16 traffic and hazardous chemicals studied, 11 significantly increased the likelihood of getting breast cancer over the 20-year study period. Five of these 11 hazardous chemicals came from traffic and fossil fuel power plant emissions (xylenes presented the highest breast cancer risk), and six of the 11 hazardous chemicals came from manufacturing, landfill, and hazardous waste emissions (vinyl acetate, vinyl chloride, ethylene dichloride, and 1,1,2,2 tetrachloroethane presented the highest breast cancer risk).[30] Though traffic emissions have declined substantially in recent decades, hazardous air pollutants remain an exposure of concern. 
• In an analysis of traffic-related pollutants in the Multiethnic Cohort, for those who lived within 500m of major highways, significant relationships were found between air nitrogen dioxide (NO2) exposures and breast cancer risk. The relationship was strongest for African American and Japanese American women compared to Latina and White women.[19] This ethnic difference may be important in interpreting other data from studies where cohorts were comprised primarily of White women. 
• The results for studies of possible relationships between exposures to particulate matter (PM₅ or PM₁₀) are more ambiguous. Recent reviews and meta-analyses found no significant relationship[20],[21] or very small effects[22] when women across cohorts were aggregated together. Geographical differences in effects were reported, with higher associations in European studies than in U.S. studies.[23] Suggested explanations included differences in air pollution composition as affected by geographical, socioeconomic, and other factors.

Similarly, White et al. found complex interactions in the U.S.-based Black Women’s Health Study.[24] Although they found no overall association between PM_2.5 levels and breast cancer incidence in a large, longitudinal (18-year follow-up period) study of Black women across the country, they found interesting and robust interactions between menopausal status and geographic location. Increased PM_2.5 exposures were associated with increased ER- tumors and cancers in premenopausal women in the Midwest but decreased ER- tumors and premenopausal cancers in the South. Differences in the composition of the particulate matter, including varying levels of industrial metals, were suggested as possible mitigating factors.[25]

Other components of industrial air pollution have been linked to an increased risk of developing breast cancer.

• A Chinese study of women from rural and urban regions found that using coal for cooking fuel was associated with a doubled risk of developing breast cancer. However, wood burning was not associated with elevated cancer risk compared with the use of ‘clean fuel’ (natural gas).[26]
• An examination of exposures to non-metallic air pollutants and breast cancer in the large, prospective Sister Study demonstrated that exposures to several toxicants, especially methylene chloride (a major industrial solvent), are linked to an increased risk of developing the disease. The effect was particularly pronounced for hormone receptor-positive (ER+/PR+) tumors and in overweight and obese women.[27]
• Exposure to higher air-borne levels of benzo[a]pyrene (BaP), a biproduct of incomplete combustion of organic solvents, was significantly associated with risk of breast cancer. The effect was strongest for women recently transitioned through menopause and for hormone receptor-positive tumors.[28]
• Atmospheric exposure to PCB153, one of the most prevalent polychlorinated biphenyls used in electronics and construction materials manufacturing, is also associated with an increased risk of breast cancer. The effect was only significant for ER+ tumors.[29]

What about indoor air pollution?

Most people spend the majority of their time indoors. People who live in highly polluted areas are likely to have some of those toxic pollutants in their homes, schools, and offices. These exposures can be exacerbated by exposure to cigarettes and other types of smoking, by releases from cooking and burning in fireplaces and woodstoves, and by using products (cleaning, personal care, fire retardants, etc.) indoors. These chemicals can be inhaled, ingested, or absorbed through the skin.

Who is most likely to be exposed to air pollution?

Women who live in high-density urban areas, areas around vehicular traffic (especially those with high traffic levels), and neighborhoods with industrial manufacturing and processing are more likely to be exposed to higher levels of air pollution.

Who is most vulnerable to the health effects of air pollution?

Geographic, socioeconomic, and ethnic factors may be important, with indications that Black and Japanese American women may be more vulnerable. Depending on the pollutant being studied, premenopausal, perimenopausal, and postmenopausal women may all be vulnerable to the cancer-associated effects of exposure to air pollution.

Infants and small children are particularly vulnerable to the disease-causing effects of indoor pollutants because they play on floors, put their hands in their mouths, and are more sensitive to many indoor pollutants.

Top tips to avoid exposures

At a personal level, controlling exposure to outdoor pollutants is difficult. Here’s what you can do: 

• Wearing a face mask with a PM 2.5 filter will decrease exposure to dangerous particulate matter that carries other chemicals in the air.
• When possible, exercise (walk, run, bike) as far away from major highways as possible to avoid high levels of diesel and other combustion products.
• Participate in community efforts to shut down or clean up polluters in your community.
• Pressure county and state governments to provide air quality monitors in your neighborhood so you can know what your air contains and so that you can use these data in community-based action.

Indoors:

• Vacuum frequently using a HEPA filter.
• Clean surfaces frequently using a damp rag and wash the resulting dust particles down the drain.
• If economically possible, install air filters, especially in rooms where you and your children spend most of your time.
• Do not smoke (cigarettes, e-cigs, etc.), especially not indoors. Do not allow others to smoke in your home, office, or school.