It is an unassailable fact that the incidence of breast cancer is on the rise. In 2004 in the UK, 37,000 women were diagnosed with the disease - a 10% increase on the previous year and 80% more than in 1971. According to Cancer Research UK, the lifetime risk of a woman developing breast cancer, which causes more than 12,400 deaths annually, is now one in nine. But why?

A report by the World Wildlife Fund this week suggested that man-made chemicals, such as those used in cosmetics, plastics and pesticides, might be to blame. Research by the Cochrane Library, also published this week, claimed that screening may, in fact, be raising the rate of incidence - for every 2,000 women screened, one would have her life prolonged but 10 would undergo unnecessary and potentially damaging treatment.

It is probable that improvements in healthcare and detection have resulted in a greater rate of diagnosis. According to Cancer Research UK, "Introduction of the national screening programme in 1988 led to a transient additional increase in breast cancer incidence in women aged 50-64 as early undiagnosed cancers were detected."

However, it notes: "The underlying increase in incidence predates screening, continues today, and is evident particularly in the older age groups." Indeed, 80% of cases occur in women over the age of 50. As this is generally just before the menopause, it suggests a possible link with hormonal status. The reasons for breast cancer remain complex. One in 20 cases is inherited, but the rise, particularly in developed nations, suggests that environmental factors or diet might be to blame.

The WWF report argues that man-made chemicals, more common in the affluent west, could be exerting a similar effect to synthetic oestrogens, also present in the contraceptive pill and HRT. However, other factors expose women in developed countries to higher rates of breast cancer: they begin menstruation earlier, have children later and are more likely to take the pill, have HRT, be overweight and taller, eat diets high in fat and do less exercise. They also have fewer children and so spend less time breastfeeding. "Levels are going up, but you can't ascribe it to any one thing," says a spokesperson for Breast Cancer Care.