What is lung cancer caused by in nonsmoker?

Possibly genetic,etc: Nonsmokers account for 15% of lung cancer cases, and these cases are often due to a combination of genetic factors, radon gas, asbestos, and air pollution including secondhand smoke. Asbestos can also cause mesothelioma, a cancer of the pleura or lining on the surface of the lungand diaphragm. The main type of lung cancer in nonsmokers is adenocarcinoma, particularly bronchoalveolar carcinoma.

Many things: >80% of lung cancer is caused by smoking. However never smokers (defined as <100 cigarettes in lifetime) can get smoking. Risk factors include radon exposure, second hand smoke, and genetic mutations such as egfr activating mutations.