Cigarette smoking is responsible for approximately 90% of cases of lung cancer.
Some 30% of heavy smokers develop lung cancer (when compared with only 1% of nonsmokers).
Genetic factors also influence susceptibility to the effects of smoking, and consequently, the risk of developing lung cancer.
Exposure to environmental tobacco smoke (“passive smoking” or “second-hand smoking”) increases the risk of lung cancer in nonsmokers.
Other agents (“carcinogens”) can increase the risk of lung cancer by themselves or together with smoking (i.e. asbestos, radiation and various toxic chemicals). The number of cases they cause is small compared to lung cancer due to smoking