Breast Cancer Risk in California Linked to Poor Air Quality

What happens when the air we breathe becomes toxic? Emerging research suggests that toxic air — polluted with dust, smoke, and soot — could be increasing our risk of disease. 

A recent study published in the Journal of Clinical Oncology found that exposure to air pollution was linked to higher rates of breast cancer in women who lived in urban areas in California.  This research adds evidence of the link between air pollution and breast cancer. 

Pollutants from cars, construction sites, power plants, and wildfires settle in the air, polluting it with fine particles (called particulate matter or PM2.5). These particles — which are more than 30 times smaller than a strand of human hair — can be inhaled by humans, which can cause serious health problems. 

In 1999, the Environmental Protection Agency (EPA) began monitoring PM2.5 levels in the environment. Since then public health researchers have been able to study the link between these particles and cancer risk. The research has led to the understanding that long-term exposure to fine-particle air pollution is a risk factor for breast cancer. Through this monitoring, researchers have also learned that some groups of people are exposed to more fine-particle air pollution than others. 

In a podcast interview, study author Anna Wu, PhD, noted that research shows that people from racial and ethnic minority groups live in poorer communities that are exposed to higher levels of environmental pollutants. Wu, a professor of public health at the Keck School of Medicine at the University of Southern California, and her research team focused on over 58,000 women from diverse groups who lived in California to further investigate how these different factors impact breast cancer risk. 

Over the course of a 19-year period, 6% of the women in the study developed breast cancer. 

The researchers found that the risk of breast cancer increased by 28% when women’s exposure to fine-particle air pollution increased by 10 micrograms per cubic meter. This increased risk was across all types of breast cancer.

The women in the study, who mainly lived in urban areas, were exposed to higher levels of pollutants on average, compared to the rest of the U.S. The average person in the U.S. is exposed to 8.5 micrograms per cubic meter of fine-particle pollution annually, but in this study, participants experienced exposure levels 2.5 times higher (23.6 micrograms per cubic meter). 

While previous studies on air pollution and breast cancer risk have largely been focused on white women with higher incomes, the women that made up this study were from diverse racial, ethnic, class, and educational backgrounds. About 75% of the women in this study were African American or Hispanic. Breast cancer rates were similar across the diverse group of women, suggesting that the urban areas where these women lived had a larger impact on breast cancer risk than their race or ethnicity. 

Scientists believe that PM2.5 can be absorbed into breast tissue, possibly causing cell changes that may cause cancer. But the authors noted that more research is needed to better understand how fine-particle air pollution damages cells to increase breast cancer risk.

Alexandra White, PhD, a researcher at the National Institute of Environmental Health Sciences, wrote in an accompanying editorial that the study raises questions about how exposure to air pollution outside the home may impact breast cancer risk. 

The January 2025 fires in the Los Angeles area also raise questions about how wildfire events may impact air pollution and breast cancer risk. Recent research has shown that wildfires are increasing the levels of fine-particle pollution in the U.S. "Wildfires are a really important growing concern. We know that wildfires are increasing in both intensity and duration and frequency, and we really don't understand the long-term health impacts of wildfires," said White in a podcast interview. 

Fine-particulate matter from wildfires is a concern, she says, because "it’s a combustion byproduct, so it’s thought to have more of the components that are, we hypothesize, more relevant for breast cancer like polycyclic aromatic hydrocarbons or metals. These components are thought to act as endocrine disruptors, which may be particularly relevant for breast cancer."

Several websites produce local Air Quality Index (AQI), which can help you assess the air quality of where you live, including AirNow (U.S.), European Air Quality Index (Europe), and IQAir (global).

If the air quality is bad in your area, consider limiting outdoor activities, wearing a well-fitted N95 or KN95 mask, and keeping your windows and doors closed to keep indoor air clean.