The cause of every type of cancer is still not fully understood.

However, it is increasingly thought that changes in the genes that program cell functions become faulty. This results in abnormal cell growth and/or multiplication.

It now appears that in many types of cancers, external influences – such as diet, lifestyle, chemicals and environment – may trigger these faulty genes into action. 

In the case of lung cancer, several external influences (or risk factors) have been identified as having a strong link to the development of tumour growth in the lungs. These risk factors include:

• Tobacco smoke
• Chemical substances: found within cigarettes and other smoking substances, such as pipes and cigars. Some of these chemicals are also found in cannabis (marijuana).
• Passive smoking: long term exposure to breathing in the air of smokers in your household/workplace.
• Environmental factors: exposure to air pollution, such as chemical and radiation leaks. Asbestos is a recognised cause of lung cancer, or specifically a condition called mesothelioma, which is not strictly a lung cancer, but rather a cancer of the membranes which surround the lungs.

Approximately 90% of people who have lung cancer are past or present smokers. However, it is not possible to predict when a smoker may develop lung cancer. The risk of developing lung cancer decreases in people who stop smoking, but they are still at higher risk than people who have never smoked.