California Air Resources Board

A numerical index used for reporting severity of air pollution levels to the public. It replaces the formerly used Pollutant Standards Index (PSI). Like the PSI, the AQI incorporates five criteria pollutants -- ozone, particulate matter, carbon monoxide, sulfur dioxide and nitrogen dioxide -- into a single index. The new index also incorporates the 8-hour ozone standard and the 24-hour PM2.5 standard into the index calculation. AQI levels range from 0 (Good air quality) to 500 (Hazardous air quality). The higher the index, the higher the level of pollutants and the greater the likelihood of health effects. The AQI incorporates an additional index category -- unhealthy for sensitive groups -- that ranges from 101 to 150. In addition, the AQI comes with more detailed cautions. For more information, see our air quality index page.

A group of counties or portions of counties, or an individual county specified in law with authority to regulate stationary, indirect and area sources of air pollution within the region and governed by a regional air pollution control board comprised mostly of elected officials from within the region. (See also air pollution control district). For more information, please see our local air district directory.

A plan prepared by an APCD/AQMD, for a county or region designated as a non-attainment area, for the purpose of bringing the area into compliance with the requirements of the national and/or California ambient air quality standards. AQMPs are incorporated into the State Implementation Plan (SIP).

An individual employed by the local, state, or federal government to manage air quality.

A mathematical relationship between emissions and air quality which simulates on a computer the transport, dispersion and transformation of compounds emitted into the air. For more information, please see our software webpage.

The prescribed level of a pollutant in the outside air that should not be exceeded during a specific time period to protect public health. Established by both federal and state governments. (See also ambient air quality standards.) For more information please see our ambient air quality standards.

Advisory groups that provide forums for communication, cooperation and coordination in the development and implementation of air quality control measures. They may be comprised of representatives from the ARB, citizen groups, environmental groups, industry, local air districts and the U.S. EPA.

(See California Air Resources Board.)

A generic term referring to a harmful chemical or group of chemicals in the air. Substances that are especially harmful to health, such as those considered under U.S. EPA's hazardous air pollutant programor California's AB 1807 and/or AB 2588 air toxics programs, are considered to be air toxics. Technically, any compound that is in the air and has the potential to produce adverse health effects is an air toxic. For more information, visit our toxics website.

A control measure adopted by the ARB (Health and Safety Code Section 39666 et seq.), that reduces emissions of toxic air contaminants. For more information, see our ATCM webpage.

A subset of air basin, the term denotes a geographical area that shares the same air because of topography, meteorology and climate.

An authorization to emit, during a specified year, up to one ton of carbon dioxide equivalent.

Pursuant to Assembly Bill 1811, ARB with the California Energy Commission, developed a joint plan to spend $25 million for the purposes of incentivizing biofuels and high-efficiency, low-emitting vehicle technology. The funds were for developing specific measures to reduce air pollution and greenhouse gas emissions from fuels and mobile sources. AB 1811 required the funds to be encumbered by June 30, 2007, and expended by June 30, 2009.

Fuels such as methanol, ethanol, natural gas and liquid petroleum gas that are cleaner burning and help to meet ARB's mobile and stationary emission standards. These fuels may be used in place of less clean fuels for powering motor vehicles. For more information, visit our alternative fuels website.

The air occurring at a particular time and place outside of structures. Often used interchangeably with "outdoor air." (See also air.)

Health- and welfare-based standards for outdoor air which identify the maximum acceptable average concentrations of air pollutants during a specified period of time. (See also CAAQS and NAAQS and Criteria Air Pollutant.) For more information, visit our ambient air quality standards website.

A non-profit organization that provides a forum for producers, consumers and representatives of government and industry to write laboratory test standards for materials, products, systems and services. ASTM publishes standard test methods, specifications, practices, guides, classifications and terminology. For more information, visit our ASTM website.

A pungent colorless gaseous compound of nitrogen and hydrogen that is very soluble in water and can easily be condensed into a liquid by cold and pressure. Ammonia reacts with NOx to form ammonium nitrate -- a major PM2.5 component in the western United States.

A biochemical process in which bacteria break down biodegradable organic material, such as manure, in an oxygen-free environment. Temperature, moisture, nutrient content and pH, can be controlled through the use of an airtight chamber (digester). The break-down of the organic material results in biogas, a mixture of methane (CH₄), carbon dioxide (CO₂) and trace amounts of other gases.

Those sources for which a methodology is used to estimate emissions. This can include area-wide, mobile and natural sources and also groups of stationary sources (such as dry cleaners and gas stations). The California Clean Air Act requires air districts to include area sources in the development and implementation of the AQMP. In the California emission inventory all sources that are not reported as individual point sources are included as area sources. The federal air toxics program defines a source that emits less than 10 tons-per-year of a single hazardous air pollutant (HAP) or 25 tons-per-year of all HAPs as an area source. For more information, visit our area-wide source methodologies website.

Sources of pollution where the emissions are spread over a wide area, such as consumer products, fireplaces, road dust and farming operations. Area-wide sources do not include mobile sources or stationary sources.

A type of hydrocarbon, such as benzene or toluene. Some aromatics are toxic.

A mineral fiber that can pollute air or water and cause cancer or asbestosis when inhaled. The U.S. EPA has banned or severely restricted its use in manufacturing and construction and the ARB has imposed limits on the amount of asbestos in serpentine rock that is used for surfacing applications.

A chronic inflammatory disorder of the lungs characterized by wheezing, breathlessness, chest tightness and cough.

The gaseous mass or envelope of air surrounding the Earth. From ground-level up, the atmosphere is further subdivided into the troposphere, stratosphere, mesosphere and the thermosphere.

A geographical area identified to have air quality as good as, or better than, the national and/or California ambient air quality standards (NAAQS/CAAQS). An area may be an attainment area for one pollutant and a nonattainment area for others.

A pre-construction permit issued by an air district.