By the time the smog lifted five days later, twenty people had died and thousands were ill. Sooty, hazy air had long been considered a nuisance in many cities, soiling laundry and buildings, but after the dark days of the Depression, smoking factories and mills were seen as a sign of economic recovery. The growth of manufacturing and transportation during the economic boom that followed World War II, however, led to seriously deteriorated air quality in many urban areas. The smog emergency in Donora raised national awareness of the potential health hazard of poor air quality. In 1950 President Truman convened the first national air pollution conference and in 1963 the first federal Clean Air Act was passed.
Air pollutants originate from natural and human sources.
Natural sources include volcanic eruptions, sea spray, forest and grass fires, and wind-blown dust. Human sources include cars and trucks, power plants and factories, and agriculture. Pollutants can be gases, such as ozone, sulfur dioxide, carbon monoxide, and nitrogen oxides. They can also be aerosols, or particles that are dispersed in the atmosphere. Aerosols include dust, smoke particles from incomplete combustion, bacteria, pollen, and sea spray.