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Not all cases of lung cancer are caused by a lifetime of smoking.

An estimated 10 to 15 per cent of people stricken with the potentially deadly disease have never smoked a cigarette, cigar or pipe. And a new study suggests that non-smoking women may be more prone to lung cancer than non-smoking men.

Using statistics from both the United States and Sweden, the research team found that about 8 per cent of lung-cancer cases in males and close to 20 per cent of cases in females are among "never-smokers."

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So, why does lung cancer seem to pick on women? "We just don't know," said Heather Wakelee, the lead researcher at Stanford University school of medicine in California.

In their paper, published in the Journal of Clinical Oncology, the researchers point to a variety of factors that might trigger cancer in abstainers.

They include environmental and occupational exposures to certain substances such as asbestos and chromium.

Radon, seeping into homes from underground sources, has also been linked to lung cancer. And arsenic in drinking water has been implicated.

Yet none of these factors seem to explain the higher rates found in women. The answer to the riddle might be found in hormonal levels or other physical differences in women and men.

But it is also possible that some cases are still tied to smoking -- at least indirectly.

Secondhand smoke, for example, could explain part of the gender difference, speculated Ellen Chang, a co-author of the study. Because more men smoke than women, she said, women may be more likely to be exposed to secondhand smoke.

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