The mucociliary clearance system is a first line of defense against inhaled agents, and so its compromise can adversely affect health. The purpose of this paper is to provide a review of data on the effect of in vivo air pollutant exposures on the clearance of test particles from airways. Data from both animals and humans are compared whenever possible, so that estimates of human health effects may be made. Mechanisms of action are also discussed, presenting the view that for low level exposures, changes in secretions are probably responsible for most observed changes in clearance. The pollutants pertinent to this review are those that are common in the environment and most likely to have impacts on large numbers of people: sulfur oxides, sulfuric acid mist, O3, NO2, particulates, diesel exhaust, and cigarette smoke.