Despite current regulations, which limit the levels of certain air pollutants, there are still a number of adverse health effects that result from exposure to these agents. Numerous epidemiological studies have noted an association between the levels of air pollution and hospital admissions for a variety of different health reasons, including a number of respiratory diseases, as well as increased morbidity and mortality associated with various respiratory conditions and diseases. Because of the large impact respiratory virus infections have on morbidity and even mortality, it is important to understand whether and how exposure to common air pollutants could exacerbate the susceptibility to and severity of respiratory virus infections. This review focuses on current epidemiological and experimental studies, which have examined the association between and effect of air pollutants and respiratory viral infections, as well as potential mechanisms associated with these effects. Examined in this review are U.S. Environmental Protection Agency (EPA) "criteria" pollutants nitrogen dioxide (NO(2)), ozone (O(3)), and particulate matter (PM), as well as indoor pollutants such as environmental tobacco smoke (ETS) and combustion products of biomass fuels. Although a number of studies indicate associations between exposure to air pollutants and increased risk for respiratory virus infections, potential mechanisms mediating these effects are largely unexplored. Therefore, additional studies, both epidemiologic and mechanistic, are necessary to increase our understanding of how exposure to air pollutants could affect respiratory virus infections, especially in populations already at risk of developing significant morbidity/mortality after infections with respiratory viruses.