A number of environmental risk factors have been identified, most of which relates to occupational exposures such as asbestos, tar, soot, and a number of metals such as arsenic, chromium, and nickel.
Air pollution has also been linked to increased risk of lung cancer.
Indoor radon-222, a radioactive gas that percolates up soil and becomes concentrated inside buildings, have been posed as a significant risk factor for lung cancer.
Health Phys. 2004 Jul;87(1):68-74.
Smoking potentiates the effect of a number of occupational lung carcinogens (e.g. asbestos), such that risk is multiplicative instead of additive.