The air we breathe contains certain gases and particles, some of which may be a threat to man. The respiratory tract is their first target because it comes into direct contact with them, and to protect the body itself it mobilizes defense mechanisms of high complexity. To understand these it is necessary to analyze the toxic modalities of these air contaminants: composition, deposition and transit through the respiratory tract. As far as the particles are concerned, the aerodynamic diameter is an important factor in determining the manner of deposition. Also of importance are shape, density, degree of humidity and radioactivity. Some particles are infectious, some are antigenic, and some are toxic due to their physical structure or chemical composition. Emphasis is placed on the role of aerosols, particularly tobacco smoke, as pollution factors or co-factors with other pollutants. The characteristics of the exposed subject capable of defining a risk profile varying from person to person are dwelt on. This approach presupposes a separation, within the cleaning and defense mechanisms, of the mechanical means which bring physical phenomena into play on the basis of fixed structures in the respiratory tract (cilium, mucous cells, surfactant). This is followed by study of the immunologic means of defense essentially involving cellular elements residing temporarily in the lung (T or B lymphocytes, plurinuclears, alveolar histiocytes in particular). Many enzyme systems are also involved in this pulmonary defense and in keeping homeostasis working. Particular attention is focused on some of them, such as the antiproteases and cytochrome P450.(ABSTRACT TRUNCATED AT 250 WORDS)