Lung cancer is most closely associated with smoking, but lung cancer can be caused by many air contaminants. Radon is the second leading cause of lung cancer, causing 21,000 deaths each year. About 2,900 of those people are non-smokers.[1]
Cancer can be caused by things other than mutagens though. There is a certain rate of transcription errors when cells divide, and these errors can build up in the same way mutations from radiation, etc. build up. Things that kill cells and force them to divide speed up cell division, and can cause cancer that way. For instance, acid reflux and drinking can both cause esophageal cancer.
You can also get cancer from just being extraordinarily unlucky.