Reducing Air Pollution
Although most of the world's cities and industrial regions have been plagued with air pollution for centuries, there was little organized effort to combat the problem until the mid-20th century. In the United States, many cities and all of the states began to set legal standards regarding the amount and types of pollutants that were permitted. The federal government in 1970 established the Environmental Protection Agency (EPA), whose duties include administering the Clean Air Act. This act provides for financial assistance to state pollution-control agencies and sets strict standards for automotive emissions. The EPA sets general standards for air quality and operates an air monitoring network. Pollution levels are reported to the public through the media by means of the Pollution Standards Index. The index indicates pollution levels as numbers on a scale of 0 to 500; values above 100 represent levels known to cause adverse health effects.
Many devices and systems have been developed to reduce or prevent industrial air pollution. Electrostatic precipitators, for example, remove pollutant particles by ionizing them (charging them electrically) and collecting them on electrodes that are oppositely charged. Cyclone separators rotate impure air with a force that hurls particles against the side walls of the separator. In scrubbers, air is passed through water sprays that remove impurities.
Federal regulations for restricting automotive air pollution require the installation of various antipollution devices on automobiles and trucks at the time of manufacture. The most common of these is the catalytic converter.
In the home, most air-conditioning systems contain mechanical filters or electrostatic precipitators to remove dust, smoke, dander, and other particles from indoor air. Mechanical filters and electrostatic precipitators are also used in forced-air furnaces and in portable air cleaners, or air purifiers, which are designed to remove such particles from a relatively small area.