Women who have lived or worked on a farm are about three times more likely to develop breast cancer as women who have never worked on a farm, according to a new Canadian study.

The study set out to determine if a woman's occupation affects her risk of breast cancer.

A team of researchers tracked the histories of 1,000 women from southwestern Ontario. About half of the women had breast cancer; the other half was a control group of comparable women who did not have cancer.

The team looked at nearly all women with breast cancer from 2000 to 2002 in the Windsor, Ont. area. The results show a significant increase in the risk of the disease for women who worked on a farm at some point in their lives.

The paper, which is to be published Thursday in the Annals of the New York Academy of Sciences, says women who worked on farms are 2.8 times more likely to develop breast cancer than those who have never been employed in the agricultural industry.

The study also found the risk of breast cancer may linger even after a woman has left the industry to work in other occupations.

If a farm worker went on to find a job in the health-care industry, her breast cancer risk increased by 2.3 times. If she went on to find work in the auto-manufacturing industries, she was four times more likely to have the disease.

There was no extra risk of breast cancer for women who never worked on farms and then went on to find employment in the auto or health-care industries.

The link to farming became evident after other traditional breast cancer risk factors such as genetics, smoking, age, number of children and hormone replacement therapies were eliminated.

"We think something is definitely at play in the agricultural environment," said Jim Brophy, one of the study's authors.

"Something is going on the farm that may be increasing the cancer risk of women with breast cancer."

The regular spraying of chemicals to control weeds and pests on farms may explain the link, Brophy told CTV's Avis Favaro.

A number of products used in past decades are now suspected carcinogens.

Many of the women in the study said they don't know what they were exposed to in childhood or adolescence on their family farms. Researchers are now trying to link the type of crops the women worked around to the pesticides that were commonly used at the time.

In addition to pesticides, farms are also home to other environmental contaminants, like diesel fumes, antibiotics and growth hormones, all of which could contribute to the higher cancer rates.

Though there have been some previous studies on the link between farming and breast cancer, "there is a significant gap in our understanding of work-related exposures and breast-cancer risk," it says in the paper's introduction.

Brophy says scientists all over North America are making efforts to close that gap.

"The major cancer studies going on in North America ... are focusing specifically on farms in the rural states, because there has been this seemingly large increase in cancer in this normally healthy population."

Ann Chambers, a professor of oncology at the University of Western Ontario's Schulich School of Medicine and Dentistry, warned the study warrants further research.

Chambers, who specializes in breast cancer, told the Toronto Star that the link found in the study does not necessarily mean there's a causal relationship between cancer and farming.

"The real danger that the public has in this sort of thing is that you see an association and then they think, `Aha, working on a farm causes cancer,'" she told the Star.

"And the study statistically can't say that. It says there is an association which warrants further study to understand what the cause is."

Brophy, who is currently expanding his sample size to 1,000 patients, told the Star that further research must be conducted to see if the link holds true outside of the intensively farmed Windsor area.

For now, though the Canadian Breast Cancer Foundation says it looks forward to further research to better explain the farming link.

"This study is the first of its kind in Canada. People will be looking for answers coming out of it," said Sharon Wood of the Breast Cancer Foundation of Canada.

The current study's results offer a starting place for scientists struggling to explain the rising rates of breast cancer in industrialized countries like Canada, and how the causes of the disease may begin early in life.