The vast majority (90%) of all cases of lung cancer are related to cigarette smoking with the highest risk for moderate-to-heavy long-term smokers. Breathing in other people’s cigarette smoke (passive smoking) can also increase the risk of lung cancer, although the risk is still much less than if you smoke yourself. Although people who smoke pipes and cigars have a lower risk of developing lung cancer than cigarette smokers, they are still at a much greater risk than non-smokers.

A number of other factors have been linked to the development of lung cancer: 

• People who have been exposed to asbestos have a higher risk of developing lung cancer, particularly if they also smoke, as asbestos and tobacco act together to increase the risk. Many people have been exposed to asbestos during their working lives. Low-level exposure increases the risk of lung cancer only slightly while heavy exposure may result in a much higher risk of developing lung cancer.
  
  
• People who have had tuberculosis or other lung diseases such as emphysema are more likely to develop lung cancer. Lung cancer tends to develop in areas of the lung that are scarred or damaged from tuberculosis.
  
  
• Environmental or occupational (eg miners) exposure to radon , a radioactive gas that occurs naturally in soil and rock, can cause damage to the lungs that may lead to lung cancer. Smoking increases the risk of lung cancer even more for people who are already at risk because they have been exposed to radon.
  
  
• Occupational, medical or environmental exposure to other forms of radiation and certain chemicals (eg uranium, chromium and nickel) can also cause lung cancer. 
  
  
• Researchers have suggested that exposure to certain air pollutants such as the fumes from the combustion of diesel and other fossil fuels may increase the risk of lung cancer. 
  
  
• Although there is no evidence that lung cancer can be inherited, lung cancer occurs more commonly in smokers in some families than in smokers in the general population.  It may be that these people have a 'faulty' gene that makes them more sensitive to the harmful effects of cigarette smoke. 
  
  
• Age – lung cancer occurs more often in people over the age of 65 years