The main cause of lung cancer is smoking. There are substances present is tobacco that are known to be carcinogenic. These cause damage to the cells of the lungs and these damaged cells can turn into cancerous ones after some time.

Some other factors that increase the risk of lung cancer are the age at which the person started smoking, the period for which the person has been smoking and how many cigarettes he smoked. Passive smoking can also cause cancer or at least increases the risk considerably. The other causes of lung cancer include exposure to radiation and cancer causing substances.