Publisher Summary This chapter provides an overview of air pollution. The air over towns has been polluted for many centuries by products of combustion—complete and incomplete—of fuels. To the coal smoke, which was the predominant pollutant in former times, industry and the motor vehicle have added many more substances of proven or potential toxicity. The modern mixture is extremely complex, and the need to determine its chemical and physical nature and the variations in quantity and quality has posed formidable problems in sampling and analytical techniques. The use of the electron microscope has lead to a much greater knowledge of the structure and composition of the myriads of particles present in town air. Knowledge of the nature of particulate and gaseous pollutants has been essential to the proper study of their clinical effects. Polluted air can kill the old and ill, can make established disease worse, and can cause disease. Carbon monoxide, when inhaled, interferes with transport of oxygen by the blood and, when present in high enough concentrations, can impair consciousness and performance. It pollutes the air in traffic-laden streets and many measurements have been made of the concentrations commonly found in busy streets. Large numbers of measurements show that the levels of saturation of the blood in those exposed to much traffic are well below those found in most smokers.