Why Do We Assume That Non-Smokers With Lung Cancer Contracted It From Second-Hand Smoke?

Medical websites list the many ways to get lung cancer and many will state that doctors cannot say for certain where the cancer came from.
What I mean by "we" is that often when the subject a non-smoker dying of lung cancer comes up, its followed with sentiments such as "...and thats why we gotta get smoking banded in saloons." Not that smoking is "good for you," but this is usually the first reaction, and not something like "...thats why Los Angeles is dangerous, there's too much smog and pollution."
So Christopher Reeve's wife was stuck in rooms with second hand smoke for too much time of her life??? It must not have anything to do with the environment, which we are all to blame for? Its because of smokers?

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