Toxic air pollutants, also referred to as hazardous air pollutants, are substances that cause or may cause cancer or other serious health effects, such as reproductive, birth or developmental defects, and neurological, cardiovascular, and respiratory disease. They can be found in gaseous, aerosol, or particulate forms. Some toxic air pollutants (e.g., mercury) are persistent bioaccumulative toxics, which means they are stored indefinitely in the body and increase over time. These toxics can deposit onto soils or surface waters, where they are taken up by plants and are ingested by animals, with concentrations increasing as the toxics move up through the food chain to humans. Toxic air pollutants include, among others, formaldehyde; acrolein; benzene; naphthalene; arsenic and metals, such as cadmium, mercury, chromium and lead. Sources of hazardous air pollutants include stationary sources, such as power plants, factories, dry cleaners, and hospitals, as well as mobile sources such as cars, buses, and construction equipment. In the United States, the federal Clean Air Act identifies 187 hazardous air pollutants to be regulated. “National Emission Standards for Hazardous Air Pollutants” (NESHAPS) have been established for industries emitting the listed pollutants and generally require the use of “Maximum Achievable Control Technology” (MACT). Standards have been promulgated to apply to many kinds of sources, including hazardous and municipal waste incinerators, commercial/industrial boilers, chlor-alkali plants, cement kilns, dry cleaners, printing plants, degreasers and a host of others. Strategies for controlling toxic air pollutants include pollution prevention measures, such as product substitution, process modification, work-practice standards and materials separation; coal cleaning (relevant to mercury control); flue gas treatment technologies (such as "scrubbers"); and alternative strategies. Motor vehicles are significant sources of toxic air pollution, so programs to reduce emissions from cars, trucks and buses also decrease concentrations of toxic air pollutants. These programs include reformulated gasoline, the federal motor vehicle control program and gasoline sulfur control requirements, among others.