Women who live in areas with more air pollution may face a higher risk of developing breast cancer, according to a new, large-scale study of more than 400,000 women across the United States. It found higher breast cancer incidences even at pollution levels below current EPA standards.
The research, recently published [September 2025] in the American Journal of Public Health, comes at a time when breast cancer diagnoses have risen sharply among women younger than 50. It marks the most extensive U.S. study so far to examine air pollution and breast cancer, the authors say, and the largest in the world to look at different tumor types.
“[Air] pollution may contribute to large numbers of breast cancer cases given high incidence rates and ubiquitous exposures,” they say. “Furthermore, the potential impact of exposure could be greater in countries or regions with higher concentrations of air pollutants.”
Breast cancer remains the second leading cause of cancer death among women in the U.S., after lung cancer. About one in eight women will develop it during their lifetime.
Even though average pollution levels were low, women in more polluted areas still faced higher breast cancer risk, especially in the Midwest and in lower-income areas, the study shows. Those in the Midwest may face greater risk because regional pollutants contain more nitrates and ammonium, both associated with breast tissue changes linked to cancer, it suggests.
Key findings include:
- Nitrogen dioxide (NO₂) from motor vehicle emissions and burning fossil fuels: Women exposed to higher levels of this pollutant had a 3% higher overall breast cancer risk for every 10 parts per billion increase (roughly equivalent to one drop of water in a 10,000-gallon swimming pool). Although NO₂ may not directly damage DNA, it can promote cancer or signal the presence of other harmful pollutants such as polycyclic aromatic hydrocarbons (PAHs), which can cause inflammation and disrupt hormones.
- Fine particulate matter (PM2.5, or tiny pollutant particles of 2.5 micrometers or less): Higher levels of these tiny particles from exhaust, industry, and fires were linked to a 14% higher risk of more aggressive hormone receptor–negative (ER-/PR-) cancers and a 15% higher overall risk among women in the Midwest. The mix of metals (arsenic, cadmium), PAHs, nitrates, ammonium, and sulfates in fine particulate matter can damage DNA and increase inflammation.
- Ozone (O₃) from vehicles, factories, and industrial solvents used in products like paints and refrigerants: Exposure to ozone was tied to a 10% higher risk of hormone receptor–negative (ER-/PR-) breast cancers, though not to overall breast cancer rates. These tumors, which lack estrogen and progesterone receptors, are harder to treat. Ozone may cause oxidative stress, damaging cells, though its cancer-causing role is uncertain. Notably, Midwest women showed about a 12% lower overall risk at higher ozone levels.
Mobile sources, mainly automobiles, account for more than half of all the air pollution in the U.S. Power plants, oil refineries, industrial facilities, factories, and natural events such as wildfires also contribute to outdoor air pollution, which has been classified as cancer-causing to humans by the International Agency for Research on Cancer.
Air pollution has been linked not only to lung cancer but also liver damage, abnormal brain development, adverse health risks for newborns, stress and obesity, and heart-related deaths, among other issues. Scientists believe it may contribute to cancer by altering genes, causing inflammation, and disrupting hormones that influence breast cancer development.
The researchers in this study used detailed air quality models to track women’s exposure at their home addresses to the three types of air pollution—nitrogen dioxide, fine particulate matter, and ozone. They examined whether these pollutants were linked to new cases of breast cancer, evaluated associations differed by tumor type or region, and then summarized the results from all five studies.
Following participants for an average of 17.5 years, the researchers linked their health data with air quality information from more than 2,600 monitoring stations nationwide. Analyses of NO₂ and ozone included 28,811 breast cancer cases, while PM2.5 analyses included 20,197 cases.
The results were consistent across different analysis methods as well as age, race, body mass index (BMI) and neighborhood socio-economic status, the researchers say. Slightly stronger effects appeared among Midwestern women and those in lower-income areas.
Other recent research supports a potential link between outdoor air pollution and higher breast cancer risk. These findings include:
- An August 2025 nationwide study of older breast cancer patients reported that chronic exposure to PM2.5, ozone, and NO₂ increased mortality, highlighting air pollution as a risk factor for poorer survival.
- A December 2024 systematic review of studies from 2013–2022 found significant links between breast cancer and multiple outdoor pollutants. The associations were strongest among hormone receptor–negative and postmenopausal women.
- An October 2024 large, multiethnic cohort study reported PM2.5 as a risk factor for breast cancer, calling for broader pollution control policies since about half of breast cancer cases cannot be explained by known risk factors. “[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 PM2.5 exposure,” the study’s authors wrote.
- A 2023 National Institutes of Health study showed that women exposed to higher levels of PM2.5 had an 8% higher breast cancer risk, especially in areas with persistently poor air quality.
Unlike many earlier studies that relied only on participants’ initial home addresses, the latest analysis tracked pollution exposure at every residence, which the researchers say reduced the risk of misclassifying people’s true exposure. At the same time, some early cases could have been missed, they say, since PM2.5 data were only available after 1999, after some cohorts were already enrolled in studies.
The researchers could not measure exposures outside the home or account for how much time participants spent outdoors, which may have affected estimates. They were also unable to assess pollution exposure during key life stages like puberty and pregnancy, when breast tissue is more sensitive.
Large, detailed studies are needed to examine how the different components and sources of PM2.5, including wildfire smoke, may influence breast cancer risk, the researchers say. They are also needed in more highly polluted regions of the world to test whether the associations hold at higher concentrations.
In addition, the researchers call for regulatory measures to reduce air pollution and its potential health harms. Based on this year’s estimated 316,950 breast cancer cases in the United States, a 3% drop in exposure to NO2, one of the main emissions from cars and burning fuels, could mean about 9,500 fewer cases, they suggest.
“It’s often not realistic for people to leave their homes and relocate in areas with better air quality in search of less health risk, so we need more effective clean air laws to help those who are most in need,” said co-author Dr. Veronica Irvin of Oregon State University’s College of Health. “We also need policies that help to reduce car traffic and promote alternative forms of transportation.”
To get the latest information about air quality near you, including PM2.5, visit the EPA’s Air Now website and enter your zipcode. When pollution levels are high, avoid exercising outdoors, or wear a mask. Learn more about investigating air quality in your community here.
Reference: White AJ, Hart JE, Quraishi SM, et al. Air Pollutants and breast cancer risk: A parallel analysis of five large US prospective cohorts. American Journal of Public Health. Published online September 25, 2025. doi:10.2105/ajph.2025.308247
