Over the past three decades, significant progress has been made in understanding the health effects of air pollutants. Advances in research have improved the understanding of the actions, exposure-response characteristics, and mechanisms of action of many common air pollutants. A multidisciplinary approach using epidemiology, animal toxicology, and controlled human exposure studies has contributed to the database. This review emphasizes studies of humans but also draws on findings from other disciplines. Air pollutants have been shown to cause responses ranging from reversible changes in respiratory symptoms and lung function to increased respiratory morbidity and mortality. Air pollution has been a problem since the beginning of civilization. Major air pollution events in the 20th century, such as the Meuse Valley, Donora, and London smog episodes, led to increased interest in the health effects of air pollutants. The Clean Air Act of 1963 and its amendments led to the implementation of National Ambient Air Quality standards for several major pollutants. The investigation of the effects of air pollution on human health has followed a multidisciplinary approach using animal toxicology, epidemiology, controlled human exposure studies, and molecular and cell biology. Ozone has received significant scientific attention, and much is now known about its effects on lung function and morphology. Ozone exposure can cause pulmonary function responses, changes in breathing patterns, and increased respiratory symptoms. Repeated exposure to ozone can lead to increased pulmonary function responses initially but then attenuated responses. Ozone exposure is associated with increased asthma attacks in asthmatics and changes in airway reactivity. Sulfur dioxide has been a major component of air pollution, particularly in the London fog episodes. Sulfur dioxide exposure can cause increased airway resistance, bronchoconstriction, and increased airway responsiveness. Asthmatics are more sensitive to sulfur dioxide than healthy subjects. Repeated exposure to sulfur dioxide can lead to diminished responsiveness in asthmatics. Nitrogen dioxide is a major component of oxidant air pollution and is associated with increased incidence of lower respiratory tract infections in children and increased airway responsiveness in asthmatics. Long-term exposure to nitrogen dioxide is associated with increased susceptibility to lower respiratory tract illness. Carbon monoxide is an odorless, colorless gas that can cause poisoning at high concentrations. Carbon monoxide exposure can impair exercise performance by reducing maximal oxygen uptake. Patients with cardiovascular disease are at increased risk from carbon monoxide exposure. Particulate matter and acidic pollutants are associated with increased mortality and morbidity. Particulate matter exposure is strongly associated with morbidity and has been linked to increased respiratory hospital admissions. Lead exposure is associated with neurological deficits, particularly in children, and modest elevations in blood pressure in adults. Ambient air and pollutant mixtures are complex and vary from location to location and day to day. Mixtures of pollutants tend to produce effects that are additive. The understanding of the human health effects of many toxic air pollutants will require reliance on animal-to-human extrapolation using animal toxicologyOver the past three decades, significant progress has been made in understanding the health effects of air pollutants. Advances in research have improved the understanding of the actions, exposure-response characteristics, and mechanisms of action of many common air pollutants. A multidisciplinary approach using epidemiology, animal toxicology, and controlled human exposure studies has contributed to the database. This review emphasizes studies of humans but also draws on findings from other disciplines. Air pollutants have been shown to cause responses ranging from reversible changes in respiratory symptoms and lung function to increased respiratory morbidity and mortality. Air pollution has been a problem since the beginning of civilization. Major air pollution events in the 20th century, such as the Meuse Valley, Donora, and London smog episodes, led to increased interest in the health effects of air pollutants. The Clean Air Act of 1963 and its amendments led to the implementation of National Ambient Air Quality standards for several major pollutants. The investigation of the effects of air pollution on human health has followed a multidisciplinary approach using animal toxicology, epidemiology, controlled human exposure studies, and molecular and cell biology. Ozone has received significant scientific attention, and much is now known about its effects on lung function and morphology. Ozone exposure can cause pulmonary function responses, changes in breathing patterns, and increased respiratory symptoms. Repeated exposure to ozone can lead to increased pulmonary function responses initially but then attenuated responses. Ozone exposure is associated with increased asthma attacks in asthmatics and changes in airway reactivity. Sulfur dioxide has been a major component of air pollution, particularly in the London fog episodes. Sulfur dioxide exposure can cause increased airway resistance, bronchoconstriction, and increased airway responsiveness. Asthmatics are more sensitive to sulfur dioxide than healthy subjects. Repeated exposure to sulfur dioxide can lead to diminished responsiveness in asthmatics. Nitrogen dioxide is a major component of oxidant air pollution and is associated with increased incidence of lower respiratory tract infections in children and increased airway responsiveness in asthmatics. Long-term exposure to nitrogen dioxide is associated with increased susceptibility to lower respiratory tract illness. Carbon monoxide is an odorless, colorless gas that can cause poisoning at high concentrations. Carbon monoxide exposure can impair exercise performance by reducing maximal oxygen uptake. Patients with cardiovascular disease are at increased risk from carbon monoxide exposure. Particulate matter and acidic pollutants are associated with increased mortality and morbidity. Particulate matter exposure is strongly associated with morbidity and has been linked to increased respiratory hospital admissions. Lead exposure is associated with neurological deficits, particularly in children, and modest elevations in blood pressure in adults. Ambient air and pollutant mixtures are complex and vary from location to location and day to day. Mixtures of pollutants tend to produce effects that are additive. The understanding of the human health effects of many toxic air pollutants will require reliance on animal-to-human extrapolation using animal toxicology