The female sex hormone oestrogen may be to blame for putting women at greater risk of lung cancer.

US researcher Dr Jill Siegfried, from the University of Pittsburgh, has carried out an analysis of research into why women should be a greater risk of the killer disease. In both the UK and US, lung cancer now kills more women than breast cancer. This is partly due to the fact there has been an increase in the number of women smoking since the 1960s. However, Dr Siegfried found that among people who smoke the same amount, women may be up to twice as likely to develop lung cancer as men. In addition, non-smokers who develop lung cancer are two and a half times more likely to be female than male. Adverse effect Dr Siegfried believes that the higher susceptibility of women to the adverse effects of tobacco could be associated with their higher levels of oestrogen. Oestrogen has already been linked to breast, endometrial and ovarian cancer. Evidence for this comes from two studies which found a greater risk of lung cancer among women who underwent the menopause late in life, and those who had short menstrual cycles. In both cases oestrogen levels were higher than average. Dr Siegfried believes the hormone may exacerbate the effects of exposure to tobacco smoke - and to other substances known to be linked to lung cancer such as the gas radon. She said: "The concentration of sex hormones may influence not only the metabolism of the carcinogens in tobacco smoke but also may influence the growth rate of lung tumours." Sara Hiom, a science information officer for the Cancer Research Campaign, said Dr Siegfried's hypothesis was "persuasive". She said: "There is a growing body of evidence suggesting that prolonged exposure to oestrogens results in high incidence of several forms of cancer ­ even in organs hitherto regarded as not hormone sensitive." Smoking most important Dr Tim Key, a senior epidemiologist at the Imperial Cancer Research Fund, said the impact of oestrogen was likely to be far less important than that of smoking. "The size of the risk for the individual is largely determined by how much they smoke and how long they have smoked. "Other factors such as oestrogens in women could have a small effect on the risk caused by smoking, but this type of modification of risk is difficult to establish because it is difficult to accurately measure and allow for smoking history. "The overriding priority is to reduce the proportion of women who smoke." The research is published in the journal Lancet Oncology.

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