Another problem is that measuring exposure to passive smoke is hard. “Living with a husband who smokes a lot with the windows closed is reported the same as living with one who smokes a little, mostly on the porch
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. In that group, the hazard ratio for developing lung cancer was 1.61, but the confidence interval included 1.00, making the finding of only borderline statistical significance.