Women who worked on farms were almost three times more likely to develop breast cancer than non-farm workers, suggests a new Canadian study that raises questions about the relationship between work history and the disease.

The 18-month study looked at 564 women around Windsor, Ont., aged 55 or under who were diagnosed with breast cancer, including 154 who had worked on farms.

Women who had worked on farms were 2.8 times more likely to develop breast cancer than non-farmers randomly selected from the region's population, according to the study in Thursday's issue of the Annals of the New York Academy of Sciences.

"We feel fairly comfortable that there is something going on in a farming setting, agricultural setting, that may be contributing to this risk," said study author James Brophy, the executive director of the Occupational Health Clinic for Ontario Workers in Windsor.

Exposure to pesticides or other farm contaminants such as diesel fumes or growth hormones may explain the increased risk, Brophy speculated. 

The link was found after Brophy and his colleagues factored out traditional breast cancer risk factors such as genetics, smoking, age, number of children and the use of hormone replacement therapies. About half of women with breast cancer don't have any of these known risk factors.

No need for farmers to worry, critic says

Cancer specialists point to "windows of vulnerability" — times such as childhood and adolescence when toxic chemicals may disrupt the hormone system, leading to increased susceptibility to cancer later in life.

There is evidence some jobs increase the risk of cancer. For example, when workers are exposed to asbestos, they can have a higher risk of lung cancer.

But Dr. Stephen Narod, an internationally recognized breast cancer expert with Women's College Research Institute in Toronto, said criticized the study's conclusion that women on farms face a three-fold increase in risk.

"When we looked more closely at the paper, we found they did a certain statistical manipulation," said Narod. "That inflated the estimate."

Narod said better designed studies than Brophy's haven't found a particular pesticide or organic chemical is consistently related to the risk of breast cancer.

He advised women on farms not to worry.

Farming history

The results of the Windsor study raise questions about how the kinds of jobs a woman has throughout her life and the substances she is exposed to on the job may affect her risk of developing breast cancer.

Among women who worked on a farm and then went to work in car manufacturing, the risk increased four times, but not for those employed in the auto industry who lacked a history of working on a farm, the researchers reported.

The statistical findings do not show a cause-effect relationship between cancer and farming, and more research is needed to understand the link.

Women participating in the study often did not know what they were exposed to while working on farms as children.

To learn more, the researchers plan to interview 2,000 women, with the aim of identifying what exposures may be behind the link; for example, whether risks differ between corn farmers and cattle ranchers. 

Scientific research points to "windows of vulnerability," times such as childhood and adolescence when chemicals may disrupt the hormone system, leading to increased susceptibility to cancer later in life, Brophy said.