There are many types of air pollutants. The exact composition and concentration of pollutants depend on the source, the type of fuel and/or chemicals involved and, in some cases, the meteorological conditions under which the pollutant is emitted. Air pollutants are responsible for a wide range of adverse health and environmental effects.
“Criteria” air pollutants are those pollutants for which the Clean Air Act directs the U.S. Environmental Protection Agency (EPA) to establish air quality criteria in the form of National Ambient Air Quality Standards (NAAQS). The six criteria air pollutants are ozone (commonly referred to as smog), particulate matter (commonly referred to as soot), sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide (NO2) and lead. There are two types of NAAQS: 1) “primary,” or health based, must be set at a level determined by EPA to be “requisite to protect the public health with an adequate margin of safety;” and 2) “secondary,” are set at a level “requisite” to protect public welfare from “any known or anticipated effects associated with the pollutant in the ambient air,” including effects on vegetation, crops, wildlife, buildings and national monuments and visibility. EPA is required by the Clean Air Act to review and, as necessary, revise NAAQS every five years.
In addition, there are 187 toxic or “hazardous” air pollutants (HAPs). These 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.
Reductions in air pollution can be achieved by a variety of methods, including control technologies and control measures and can be implemented through regulatory, market-based or voluntary programs. A control strategy may include a combination of different voluntary measures or mandatory controls, may focus on one or several pollutants or sources of air pollution and can be implemented on a local, regional, national or international scale. Air pollution control technologies have achieved impressive results in reducing emissions from the industrial and mobile source sectors by as much as 90 to 99 percent. Continuing advances in air pollution control technology should enable further emissions reductions to offset increased emissions caused by continued population growth and worldwide economic development.