I was wondering what are the lead causes of lung cancer besides smoking?

All are minor: Asbestos in the workplace is by far the most important. Despite all the talk about urban smoke / pollution, lung cancer was rare until cigs were introduced despite the cities being horribly smoky. Radon in the home has minimal impact on non-smokers. Uranium miners got lots of lung cancer but almost all were smokers. I suspect hexavalent chromium in industry is a risk.

Inhaling smoke: Tricyclic aromatic hydrocarbons produced by combustion of tar or any other organic chemical is known to be carcinogenic. Halogenated aromatic hydrocarbons, benzpyrenes, to include some. Other chemicals may not be carcinogenic by themselves but act as co carcinogens when combined with cigarette smoke, like asbestos particles. High levels of radon gas increases risk of lung cancer. Radiation overd.