Air pollution has long been known to harm the heart and lungs, but new research suggests it might also raise the risk of breast cancer.

Researchers at the U.S. National Institute of Environmental Health Sciences (NIEHS) and the National Cancer Institute (NCI) discovered that the largest increases in breast cancer incidence were among women who, on average, had higher levels of particulate matter pollution (PM2.5) near their home in the years before enrolling in the study.

Particulate matter is released through motor vehicle exhaust, burning oil or coal, wood smoke/vegetation burning and industrial emissions. The particles are small enough to be inhaled deep into the lungs.

“We observed an 8% increase in breast cancer incidence for living in areas with higher PM2.5 exposure. Although this is a relatively modest increase, these findings are significant given that air pollution is a ubiquitous exposure that impacts almost everyone,” study author Alexandra White, head of the Environment and Cancer Epidemiology Group at NIEHS, said in an institute news release. “These findings add to a growing body of literature suggesting that air pollution is related to breast cancer.”

For the study, the researchers used data from the NIH-AARP Diet and Health Study, which enrolled more than 500,000 men and women between 1995 and 1996 in California, Florida, Pennsylvania, New Jersey, North Carolina and Louisiana, as well as in the cities of Atlanta and Detroit.

Women in the cohort were, on average, about 62 years old. Most were white.

The researchers followed these women approximately 20 years, during which they found more than 15,800 breast cancer cases.

The team estimated annual average historical PM2.5 concentrations for each participant’s residence. They were particularly interested in air pollution exposures during a period of 10 to 15 years prior to enrollment in the study, with consideration for the length of time it takes for some cancers to develop.

Most previous studies have assessed breast cancer risk in relation to air pollution around the time of study enrollment and have not considered past exposures, the study authors noted.

“The ability to consider historic air pollution levels is an important strength of this research,” said senior study author Rena Jones, principal investigator of the study at the NCI. “It can take many years for breast cancer to develop and, in the past, air pollution levels tended to be higher, which may make previous exposure levels particularly relevant for cancer development.”

The scientists also considered how the relationship between air pollution and breast cancer varied by the type of tumor. They evaluated estrogen receptor-positive (ER+) and -negative (ER−) tumors separately, finding that PM2.5 was associated with a higher incidence of ER+ breast cancer, but not ER−, tumors.

This suggests that endocrine disruption is involved. ER+ tumors are the most common breast tumors diagnosed among women in the United States.

The findings were published Sept. 11 in the Journal of the National Cancer Institute.