Smoking is the clinical marker in 80 to 90% of patients with lung cancer.

The risk of developing a bronchial form of cancer increases in smokers in relation to 2 elements:
- the number of cigars or cigarettes smoked per day
- the number of years tobacco has been consumed.
Inhaled smoke spreads into the bronchial organs. As the number of cigarettes and years increases, the smoke deteriorates the lungs’ protection system, allowing cancer to develop more easily.
Passive inhalation of tobacco smoke is also the cause of a large number of cases of lung cancer. In non-smokers, it is one of the main risk factors for lung cancer.


Prolonged exposure to gases or toxic substances can result in the onset of lung cancer. The risk factors are specifically indoor air pollution (certain homes) and atmospheric pollution (certain polluted areas).
Radon is another major risk factor for bronchial cancer in non-smokers. Radon becomes toxic when it contaminates the air inside a building. Houses or closed shelters built on cracked ground or dirt floors may be polluted with radon. This colourless, odourless gas is released naturally from the earth and rock.
With prolonged exposure, asbestos, arsenic, beryllium, cadmium, cobalt, and roofing tar for example can cause the onset of lung cancer.

Studies have shown that prolonged exposure to atmospheric pollution is the cause of many cases of cancer in the trachea, bronchi and lungs. Outdoor air pollution comes from industrial emissions and fine particle emissions from diesel engines.