Where a woman works may significantly affect her risk of breast cancer, research suggests.

The case-control study found a nearly three-fold elevated incidence of breast cancer in women who worked on a farm, along with high risks in those who later worked in automotive-related manufacturing or healthcare.

However, critics say the results are based on too few women to draw definitive conclusions.

Noting that many substances, such as pesticides, organic solvents, and plastics, shown to induce breast cancer in animal models are present at high levels in the workplace, James Brophy (Occupational Health Clinics for Ontario Workers, Toronto, Canada) and colleagues explored the importance of occupation as a risk factor for the disease.

They analyzed the occupational histories of 1100 women from the Windsor-Essex area of Canada, of whom 564 were diagnosed with breast cancer.

This showed that breast cancer patients were 2.8 times more likely to have worked in agriculture than controls. The risk for those who subsequently worked in automotive-related manufacturing was also increased, at an odds ratio of 4.0, along with those later employed in healthcare, at an odds ratio of 2.3.

Many of the women who worked on farms did so during adolescence, when breast tissue is thought to be especially vulnerable to toxic exposures, the researchers explain in Annals of the New York Academy of Sciences.

Brophy et al suggest: "It might be hypothesized that agents or conditions present in agricultural settings initiate the breast cancer process at a vulnerable period (adolescence) and that subsequent exposures to agents or conditions (eg, shift work) in automotive-related industry, healthcare, or other industries may act as promotors."

However, Henry Scowcroft, from the charity Cancer Research UK, pointed out that the results are based on a small number of women.

"This is too small a group to be able to draw any firm conclusions about a possible link between occupation and breast cancer risk. Nor does it say anything at all about what might be behind the observed increase in risk for some occupations," he said.