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.

Ozone – the chemical compound O3 – is found both at ground level (i.e., ambient ozone) and in the upper portions of the Earth’s atmosphere (i.e., stratospheric ozone). Ozone in the ambient air - commonly referred to as smog – is not emitted directly by any source. Instead it is formed through a reaction of precursor pollutants in the presence of sunlight.

Particle pollution – known as particulate matter (PM) – is a mixture of solid particles and liquid droplets found in the air. Some particles, like soot, are large or dark enough to be visible to the naked eye, while others are so tiny they can only be seen with an electron microscope.

Nitrogen oxides (NOx) consist of a variety of reactive gases composed of different levels of nitrogen and oxygen and are most typically formed from the combustion of certain fuels (oil, coal, gasoline, diesel) at high temperatures.

Sulfur dioxide (SO2 ) is one of a collection of highly reactive gases known as oxides of sulfur (SOx). Over 90 percent of SO2  emissions come from fossil fuel combustion at power plants and other industrial facilities.

Regional haze is the result of a number of sources and activities located across a broad geographic area emitting fine particle pollution, known as PM2.5, and its precursors, including sulfur dioxide and oxides of nitrogen. Sources of fine particles and their precursors can be natural or manmade.

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.