Human inputs of outdoor air pollutants occur mostly in industrialized and urban areas where people, cars, and factories are concentrated. Most of these pollutants are generated by the burning of fossil fuels in power and industrial plants (stationary sources, Figure 1-11, p. 17) and in motor vehicles (mobile sources). However, human inputs began long before the industrial era, and have steadily increased. Air pollution is a very old problem (see Case Study that follows).