Air pollution may contribute to development of lung cancer in never-smokers

There was a study 5-10 years ago on Cannabis use and lung cancer, showing #1 cigarette smokers, #2 non-smokers and #3 cannabis smokers. Seems to be a ratio of healing properties combined with carcinogens that determine some of this? Then of course genetics, that seems broad as well.

There are many alternative explanations (aside from 'healing powers'), including that people with pre-existing lung issues which correlate with lung cancer (such as emphysema) are less likely to smoke marijuana.