Studies in animals have shown that a wide range of airborne particulates including cigarette smoke, acid aerosols, metals, organic compounds, and combustion products can interfere with the normal defense processes of the lung to enhance susceptibility to respiratory infection or exacerbate allergic diseases. Such detrimental effects are less easy to quantify in humans because of the difficulties in obtaining comprehensive exposure history and health status in large populations and because of the inherent dangers of inducing disease in clinical studies. In this article we describe examples of how air pollutants affect lung disease in experimental animal systems. This information can be used to predict the health risk of simple and complex exposures and to lend insight into the mechanisms of air pollution toxicity.