Post-menopausal women living near busy roadways are at a higher risk for developing breast cancer than those living where the air is cleaner, according to the study. (CBC)

Women who live in urban areas with high levels of traffic-related air pollution have an increased chance of getting breast cancer, according to a new study by researchers from McGill University and the University of Montreal.

However, the researchers said their "disturbing" findings should be taken with caution. They are recommending further study into the possible link between breast cancer and air quality.

The study is published in Wednesday's edition of the journal Environmental Health Perspectives.

The researchers created detailed air pollution maps for the island of Montreal showing rates of nitrogen dioxide, one of the gases present in car exhaust.

They compared those maps to ones showing the home addresses of women diagnosed with breast cancer over the same period of time.

Study co-author Mark Goldberg said even after factoring in all the other risks for breast cancer — including smoking, alcohol use and workplace hazards — the results were startling.

Post-menopausal 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.

"We found around a 30 per cent increase in the risk of breast cancer for every increase of five parts per billion of this tracer gas, nitrogen dioxide," said Goldberg, who is a clinical epidemiologist and professor of medicine at McGill.

Body of evidence growing

Goldberg said the results should be interpreted with great caution.

He said it is not known how much exposure the women in the study would have had to exhaust in the air. The levels would vary, he said, according to where they lived, worked and how much time they spent outdoors.

Nitrogen oxide itself doesn't cause breast cancer, he added. Researchers used the gas as a marker to follow in their study.

Montreal researchers compared rates of nitrogen oxide from car exhaust to the addresses of breast cancer patients across the city. (CBC)

"Where it is present, so are the other gases, particles and compounds we associate with traffic — some of which are known carcinogens," said Goldberg.

Previous American studies have also shown possible links between cancer and air pollution. University of Montreal researcher France Labrèche said more research must be done.

"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.

Goldberg said he hopes researchers in other major world cities will follow Montreal's lead and do their own comparison maps.

He said Montreal, as big cities go, is not as traffic-congested as other urban centres.

"If you are seeing effects with lower pollution levels, imagine what you are seeing in big cities like Mexico City, Beijing and Shanghai?" he asked.

The Montreal research team based its findings on nitrogen dioxide measurements taken in 2005 and 2006, which were extrapolated to determine what the rates would have been in 1996, the year researchers could compare the addresses of the 383 women involved in the study.

The study was funded by research grants from the Canadian Cancer Society and the Canadian Institutes of Health Research.