There are four main categories of air contaminants which are of public health concern: Criteria Air Pollutants, Toxic Air Contaminants, Hazardous Air Pollutants, and Greenhouse Gases.
Criteria Air Pollutants
Criteria Air Pollutants are a group of compounds that are regulated in California and at the national level. They are air pollutants for which an acceptable level of exposure can be determined and an ambient (outdoor) air quality standard has been set.
The term 'criteria air pollutants' comes from the requirement that the U.S. EPA must describe the characteristics and potential health and welfare effects of these pollutants. The U.S. EPA and California Air Resources Board (ARB) periodically review new scientific data and may propose revisions to the standards as a result. Contaminants listed by both U.S. EPA and California ARB as Criteria Air Pollutants are: Ozone, Particulate Matter (PM10 and PM2.5), Carbon monoxide (CO), Sulfur dioxide (SO2), Nitrogen dioxide (NO2), and Lead.
Toxic Air Contaminants
Toxic Air Contaminants (TACs) are air contaminants not included in the California Ambient Air Quality Standards (CAAQS) but that are considered hazardous to human health. They are designated at the state level in California. TACs are defined by the California Air Resources Board (ARB) as those pollutants that may cause or contribute to an increase in deaths or in serious illness, or which may pose a present or potential hazard to human health.
TACs are considered to have no safe level of exposure. ARB has the authority to categorize pollutants as TACs, and currently, there are over 250 compounds categorized as TACs.
Hazardous Air Pollutants
Hazardous Air Pollutants (HAPs) are air contaminants that are not included as Criteria Air Pollutants but that are considered hazardous to human health by the US Environmental Protection Agency. They are designated at the federal level. Similar to the California-defined Toxic Air Contaminants (TACs), compounds are designated as HAPs because of specific health concerns based on clinical or animal studies.
Greenhouse gases are another category of air contaminants. They are grouped specifically because of their potential roles in global warming. Greenhouse gases trap the heat (radiation) coming from the earth's surface in the atmosphere and contribute to global warming. The main greenhouse gases are: Water vapor, Carbon dioxide (CO2), Methane, Nitrous oxide, Fluorocarbons, and Ozone. Besides any direct health effects the compounds may have, the concern about greenhouse gases is based their potential to promote climate change, which itself has many potential health and other effects.