A new study has reported that exposure to urban air pollution may increase risk of postmenopausal breast cancer. Accepted risk factors explain only about 30% of breast cancers. Occupational studies have reported associations between breast cancer and exposure to certain chemicals that are also found in urban air. The authors used data from a previous case-control study conducted in Montreal during the period 1996 to 1997. A total of 383 women with invasive breast cancer were compared to 416 women with other cancers (excluding those cancers potentially associated with work-related exposures to known or suspected carcinogens). Concentrations of nitrogen dioxide (NO2) were measured across Montreal in 2005-2006 and then used to develop detailed air pollution maps for Montreal in 2006 and extrapolate the estimates to 1985 and 1996. These maps were linked to residence addresses of the study participants to estimate their exposure to NO2. NO2 is a marker for traffic-related air pollution and is not thought to cause breast cancer.

The incidence of breast cancer was found to be higher in areas with greater levels of air pollution. In a related interview, one of the authors of the study, Dr. Mark Goldberg said that, “across Montreal, levels of NO2 varied between 5 ppb to over 30 ppb. We found that risk increased by about 25 per cent with every increase of NO2 of five parts per billion. Another way of saying this is that women living in the areas with the highest levels of pollution were almost twice as likely to develop breast cancer as those living in the least polluted areas.” Further studies are needed to confirm whether NO2 or other components of traffic-related pollution are indeed associated with increased risks.

Note: Polycyclic aromatic hydrocarbons (PAHs) are the components of car exhaust that have been most consistently linked to risk of breast cancer. PAHs are chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, food and other organic substances.