Although lung cancer has been reported in asbestos workers who do not smoke, most people exposed to asbestos who develop cancer are smokers. The association of cigarette smoking and exposure to asbestos in the induction of lung cancer has received much attention, and studies have shown a synergism of action, resulting in a more than a simple additive effect. Smoking appears to have a multiplicative effect on the risks related to asbestos exposure. This conclusion, drawn from various cohort studies, is also supported by experimental evidence which suggests that asbestos particle clearance in smokers is considerably lower than in non-smokers. Some researchers believe that damage to ciliated cells and macrophages amongst smokers may impair the clearance of asbestos fibres from the lung.