Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous byproducts of combustion, from sources as varied as coal and coke-burners, diesel-fueled engines, grilled meats and cigarettes. PAH residues are often associated with suspended particulate matter in the air, and thus inhalation is a major source of PAH exposure. In the Silent Spring Institute study of environmental contaminants in house dust, three PAHs (pyrene, benza anthracene and benzapyrene) were found in more than three-quarters of the homes tested.

Like many other environmental chemicals that are associated with breast cancer risk, PAHs are lipophilic and are stored in the fat tissue of the breast. PAHs have been shown to increase risk for breast cancer through a variety of mechanisms. The most common PAHS are weakly estrogenic (estrogen mimicking), due to interactions with the cellular estrogen receptor (ER). However, the major receptor-directed pathway is a different one, with PAHs associating with a protein called the aryl hydrocarbon receptor (AhR), initiating a series of cell changes that lead to altered cell signaling and ultimately to increases in DNA mutations. PAHs can also be directly genotoxic, meaning that the chemicals themselves or their breakdown products can directly interact with genes and cause damage to DNA.

Several epidemiological studies have implicated PAH exposure in increased risk for breast cancer. One of the studies from the Long Island Breast Cancer Study Project found that women with the highest level of PAH-DNA adducts had a 50 percent increased risk of breast cancer. PAH-DNA adducts are indicators of problems in DNA repair in cells, one of the early hallmarks of tumor development. In an earlier report, researchers explored the presence of PAH-DNA adducts in breast samples taken from women diagnosed with cancer as compared with those diagnosed with benign breast disease. Cancerous samples were twice as likely to have PAH-DNA adducts as were benign samples.

Occupational exposure studies have looked at workers exposed regularly to gasoline fumes and vehicular exhaust, major sources of PAHs (as well as benzene). These occupational exposures are associated with an increased risk of breast cancer for pre-menopausal women and also for men. In the case of male breast cancer, PAHs may specifically increase the risk of breast cancer in men carrying a BRCA1 or BRCA2 mutation. 

A recent case-control study in western New York indicated that very early life exposure (around the time of birth) to high levels of total suspended particulates, a proxy measure for PAH levels, is associated with increased risk of breast cancer in post-menopausal women.

See below for a table of air pollutants, including PAHs, linked to increased breast cancer risk. A PDF of this table is also available.

Learn more about policy and research recommendations to reduce exposure to air contaminants »

For references, see State of the Evidence 2008.