Interaction of gaseous and particulate pollutants in the respiratory tract: mechanisms and modulators.

Human contact with air pollution usually involves exposure to more than one chemical, and biological responses to the inhalation of polluted atmospheres likely depend upon the interplay between individual materials. Thus, characterizing effects from exposures to mixtures of air pollutants is necessary for adequate quantitation of health risks. Exposure to gas/particle mixtures may result in respiratory tract responses which are additive, or reflect synergistic or antagonistic interactions. The occurrence and type of interaction depends upon numerous factors, including the biological endpoint being examined and the specific exposure conditions, such as concentration, duration, and the physicochemical characteristics of the exposure atmosphere. It is, therefore, not always possible to predict solely from the presence of certain pollutants in a complex atmosphere exactly whether there will be an interaction and, if so, what type it will be. This complicates attempts to relate responses observed in laboratory studies of mixtures to those which may occur under ambient patterns of exposure, an extrapolation needed for human risk assessment.