A large prospective cohort study of more than 76,000 women confirmed a strong association between cigarette smoking and lung cancer but found no link between the disease and secondhand smoke.

The fact that passive smoking may not be strongly associated with lung cancer points to a need to find other risk factors for the disease [in nonsmokers], said Ange Wang, the Stanford University medical student who presented the study at the June 2013 meeting of the American Society of Clinical Oncology in Chicago.

Investigators from Stanford and other research centers looked at data from the Women's Health Initiative Observational Study (WHI-OS). Among 93,676 women aged 50-79 years at enrollment, the study had complete smoking and covariate data (including passive smoking exposure in childhood, adult home, and work) for 76,304 participants. Of those, 901 developed lung cancer over 10.5 mean years of follow-up.

The incidence of lung cancer was 13 times higher in current smokers and four times higher in former smokers than in never-smokers, and the relationship for both current and former smokers depended on level of exposure. However, among women who had never smoked, exposure to passive smoking overall, and to most categories of passive smoking, did not statistically significantly increase lung cancer risk. The only category of exposure that showed a trend toward increased risk was living in the same house with a smoker for 30 years or more.