Traffic-Related Air Pollution Linked to Breast Cancer Risk

MONTREAL, Canada, October 10, 2010 (ENS) – The risk of breast cancer, the second leading cause of death from cancer in women, has been linked to traffic-related air pollution among older women in a new study by researchers in Montreal.

In the study, 383 postmenopausal women living in areas of the city with the highest levels of air pollution from traffic were almost twice as likely to develop breast cancer as women living in the least polluted areas, the scientists found.

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

Only about 30 percent of cases of breast cancer can be explained by accepted risk factors, the researchers say. Occupational studies have shown associations between the incidence of breast cancer and exposure to contaminants found also in ambient air. In this study they sought to determine whether the incidence of postmenopausal breast cancer is associated with exposure to urban air pollution.

Traffic jam in Montreal (Photo by Ann Walsh)

“We’ve been watching breast cancer rates go up for some time,” says study co-author Dr. Mark Goldberg, a researcher at the RI MUHC. “Nobody really knows why, and only about one third of cases are attributable to known risk factors. Since no one had studied the connection between air pollution and breast cancer using detailed air pollution maps, we decided to investigate it.”

The study is published in the online issue of “Environmental Health Perspectives,” a journal of the U.S. National Institute of Environmental Health Sciences.

Dr. Goldberg and his colleagues addressed the research by combining data from several studies.

First, they used the results of their 2005-2006 study to create two air pollution maps showing levels of nitrogen dioxide, NO2, a by-product of vehicular traffic, in different parts of the city in 1996 and 10 years earlier in 1986.

Then, they charted the home addresses of women diagnosed with breast cancer in a 1996-97 study onto the air pollution maps. They found the incidence of breast cancer was clearly higher in areas with higher levels of air pollution.

“We found a link between post-menopausal breast cancer and exposure to nitrogen dioxide which is a marker for traffic-related air pollution,” says Dr. Goldberg.

“Across Montreal, levels of NO2 varied between five parts per billion to over 30 ppb. We found that risk increased by about 25 percent with every increase of NO2 of five parts per billion,” he said.

The researchers counsel caution in interpreting the results.

“First of all, this doesn’t mean NO2 causes breast cancer,” Dr. Goldberg explained. “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. Nitrogen dioxide is only a marker, not the actual carcinogenic agent.”

A study of this kind can be subject to unknown errors, he cautioned. While the researchers tried to account as much as possible for them, areas of uncertainty remain.

“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,” Dr. Goldberg said.

Co-author Dr. France Labreche of the Universite de Montreal said, “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.”

“From a public health standpoint,” said Dr. Labreche, “this possible link also argues for actions aiming at a reduction of traffic-related air pollution in residential areas.”

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