Most of the occupational diseases are related to air pollution at the work place. The distribution of gases and vapours and their human absorption are briefly described. The deposition of particles is a function of their physical properties, of the pattern of the respiratory airways and of pulmonary ventilation parameters. Several defence mechanisms also play a role. Illustrative examples are given of occupational diseases caused by solvents, carbon monoxide, metals, dusts (silicosis, asbestosis, pneumoconiosis due to dust of hard metals), allergens and carcinogens. The importance of prevention is emphasized.