Smog and air pollution have already been linked to lung diseases, but could they also be a cause of breast cancer? An intriguing new study from Montreal suggests they might.

The study, published in Environmental Health Perspectives, links the risk of breast cancer to traffic-related air pollution.

Dr. Mark Goldberg, from The Research Institute at McGill University Health Centre investigated the link by combining data from several studies.

They created two air pollution "maps" showing levels of nitrogen dioxide (NO2), a by-product of car exhaust, in different parts of Montreal in 1996 and 10 years earlier in 1986.

Then, they charted the home addresses of 383 postmenopausal women diagnosed with invasive breast cancer in a 1996-97 study onto the air pollution maps. They also looked at 416 women diagnosed with other kinds of cancers, excluding those that have been linked with occupational exposures.

They found the incidence of breast cancer was clearly higher in areas with higher levels of air pollution.

"Across Montreal, levels of NO2 varied between 5 [parts per billion] to over 30 ppb. We found that risk increased by about 25 per cent with every increase of NO2 of five parts per billion," Goldberg said in a news release.

"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."

Dr. Goldberg notes that the study doesn't prove that NO2 causes breast cancer.

"This gas is not the only pollutant created by cars and trucks, but where it is present, so are the other gases, particles and compounds we associate with traffic -- some of which are known carcinogens. NO2 is only a marker, not the actual carcinogenic agent."

The researchers note that there are limitations to their research.

"For example, we don't know how much the women in the study were exposed to pollution while at home or at work, because that would depend on their daily patterns of activity, how much time they spend outdoors and so on," says Dr. Goldberg.

Study co-author Dr. France Labrèche from the Université de Montréal notes that some studies published in the U.S. have also shown possible links between cancer and air pollution.

"At the moment, we are not in a position to say with assurance that air pollution causes breast cancer. However, we can say that the possible link merits serious investigation," she said.

"From a public health standpoint, this possible link also argues for actions aimed at reducing traffic-related air pollution in residential areas."

The study was a collaborative effort by researchers from the Research Institute of the MUHC, McGill University and Université de Montreal. It was funded by a research grant from the Canadian Cancer Society and another one from the Canadian Institutes of Health Research (CIHR).