A woman's risk of breast cancer after menopause triples if she is exposed to synthetic fibres and certain oil byproducts at workplace before her mid-30s, a study among Canadian patients said Thursday.
France Labreche of the National Institute of Public Health in Montreal led an investigation into the health records of 1,169 women aged 50 to 75.
Just under half had been diagnosed with breast cancer in 1996 and 1997, after having undergone menopause.
The other 613 women were diagnosed during the same period with other forms of cancer and acted as a comparison.
A squad of chemists and industrial hygienists probed the extent to which all the women had been exposed to about 300 different substances throughout their working life.
After filtering out other known causes of breast cancer, they found a strong link between higher rates of risk and exposure to several common synthetic materials, found in textile factories and other industrial settings.
Compared to the non-breast cancer group, the risk peaked before the age of 36, when still-active cells in breast tissue are thought to be more sensitive to harmful chemicals.
Women occupationally exposed to acrylic fibres ran a seven-fold risk of breast cancer, while exposure to nylon fibres nearly doubled the risk.
Among breast cancer patients, those whose tumours responded well to oestrogen treatment, but not progesterone treatment, were more than twice as likely to have breast cancer for every decade they were exposed to so-called monaromatic hydrocarbons -- a byproduct of crude oil -- and to acrylic or rayon fibres.
Oestrogen and progesterone are both naturally-occurring hormones used in breast cancer treatment.
Exposure before the age of 36 to another class of hydrocarbons found in petroleum products tripled the risk for women whose tumours responded to both types of hormone treatment.
The authors concede that the results are not conclusive, but point out that they are consistent with the theory that breast tissue is more susceptible to chemical toxins in women under 40.
They also note the rising rate of breast cancer in rich countries, which could also be due to earlier and better diagnosis and increased rates of alcohol consumption.
The study is published in a British journal, Occupational and Environmental Medicine.