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. One of the most serious urban air quality problems is smog, which is formed when emissions from fuel combustion react with sunlight or atmospheric compounds. Industrial or gray smog is a winter phenomenon that occurs when coal or oil with a high sulfur content is burned. Sulfur dioxide is produced in the combustion process, which reacts with water vapor in the atmosphere, resulting in airborne sulfuric acid.