Exposure to polycyclic aromatic hydrocarbons, found in petroleum products, before the age of 36, tripled the risk for women whose tumours were responsive to both oestrogen and progesterone.

The authors concede that their findings could be due to chance alone, but say they are consistent with the theory that breast tissue is more sensitive to harmful chemicals if the exposure occurs when breast cells are still active - in other words, before a woman reaches her 40s. And they point to the rising incidence of breast cancer in developed countries, which is likely to be due to a range of factors, including diagnosis of small slow growing tumours and changes in alcohol consumption.

But environmental and workplace factors are also thought to have a role, they suggest, with previously published evidence implicating exposure to certain chemicals, low frequency electromagnetic fields, and disruption of the body clock.

Source: BMJ-British Medical Journal