Exposure to urban air pollution has been linked to increased risk of breast cancer in several studies. The incidence of breast cancer is reported to be higher in areas with greater levels of air pollution. Both the long-term time trend and geographic pattern of motor vehicle emissions have been positively associated with breast cancer rates. Studies of the links between certain occupations and breast cancer risk have reported associations concerning exposures to certain chemicals that are also found in urban air. High levels of airborne particulates have also been found to be associated with reduced breast cancer survival.

The mechanism of action by which exposure to air pollution increases breast cancer risk is unclear. It has been proposed that components of air pollution act as xenoestrogens, compounds that bind to and active estrogen receptors. Air pollution also appears to increase systemic inflammation. Research regarding air pollution and breast density (an important breast cancer risk factor) has been inconsistent. Now a large new study has reported a link between exposure to ambient air particulate matter and breast density.

Air pollution and breast cancer

It has not been established which components of air pollution are most responsible for the increased risk of breast cancer. One study of women living in Montreal reported that those living in areas with the highest levels of air pollution were almost twice as likely to develop breast cancer as those living in the least polluted areas. Another study found that the geographic distribution of motor vehicle density in 1970 in selected U.S. counties appeared to be linked to subsequent breast cancer rates in those counties from 1980 to 1995.

Since motor vehicles are the primary source of nitrogen dioxide (NO2) emissions, NO2 is often used as a marker for motor vehicle-derived air pollution. Polycyclic aromatic hydrocarbons (PAHs, chemicals formed during the incomplete burning of coal, oil and gas, garbage, food and other organic substances), are the components of car exhaust that have been most consistently linked to risk of breast cancer. PAHs are stored in and accumulate in fat, potentially resulting continual low-grade exposure to these carcinogens in nearby tissues.

Higher ambient levels of acrylamide, benzidine, carbon tetrachloride, ethylidene dichloride, and vinyl chloride were found to be associated with increased risk of hormone receptor positive (ER+/PR+) breast cancer in one study, whereas higher ambient levels of benzene were associated with ER-/PR- disease.

Another study reported that reported that high levels of airborne particulates were associated with reduced survival among breast cancer patients in California. Ambient particulate matter consists of all the solid and liquid particles suspended in air at a given location. It includes soot, smoke, and other components of factory and automobile exhaust, as well as dust, pollen, and liquid droplets. Women with breast cancer living in areas with higher levels of ambient particulates were found to have shorter survival than those living in areas with lower exposures in this study. The greater the exposure, the higher the risk of early death.

Latest research finds link between ambient particulates and high breast density

The study referenced at the beginning of this news story was designed to investigate the association between breast density and exposure to particles up to 2.5 microns in diameter (PM2.5) and ozone. The study included women undergoing screening mammograms at imaging facilities within the Breast Cancer Surveillance Consortium during the period 2001 through 2009. A total of 279.967 women at least 40 years of age with known residential zip codes were included. Mammographic breast density was determined using the American College of Radiology's Breast Imaging-Reporting and Data System (BI-RADS) classification system. Air particulate matter and ozone estimates for grids across the U.S. during the relevant years were obtained from the Environmental Protection Agency (EPA). These estimates were available for the majority of women (94%) for the year before the mammogram.

Women classified as having extremely dense breasts were found to have higher average PM2.5 and lower ozone exposures than women with non-dense fatty breasts (8.97 vs. 8.66 μg/m3 and 33.70 vs. 35.82 parts per billion, respectively). Further analysis showed that women with heterogeneously dense breasts were more likely to have higher exposure to PM2.5 than women with scattered fibroglandular breasts. Women with extremely dense were less likely to have higher levels of ozone exposure than women with scattered fibroglandular breasts. The authors conclude that exposure to <2.5 μm ambient air particulate matter and ozone could partially explain geographical variation in breast density. Further studies are needed to determine the causal nature of these associations, according to the authors.

Please see our article on breast density for more information.