Air Quality Glossary

Search our glossary for definitions of important air quality terms.

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AAQS (Ambient Air Quality Standards)

Health- and welfare-based standards for clean outdoor air that identify the maximum acceptable average concentrations of air pollutants during a specified period of time.

Acute Health Effect

An adverse health effect that occurs over a relatively short period of time (e.g., minutes or hours).


Particles of solid or liquid matter that can remain suspended in air for long periods of time because of extremely small size and light weight.

Air Pollutants

Amounts of foreign and/or natural substances occurring in the atmosphere that may result in adverse effects on humans, animals, vegetation and/or materials.

Air Quality Simulation Model

A mathematical relationship between emissions and air quality that simulates the transport, dispersion and transformation of compounds emitted into the air.

Air Toxics

A generic term referring to a harmful chemical or group of chemicals in the air. Typically, substances that are especially harmful to health, such as those considered under the EPA's hazardous air pollutant program or California's AB 1807 toxic air contaminant program, 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.

Alternative Fuels

Fuels such as methanol, ethanol, natural gas and liquid propane gas that are cleaner burning and help to meet ARB's mobile and stationary emission standards.

Ambient Air

The air at a particular time and place outside of structures. Often used interchangeably with outdoor air.

APCD (Air Pollution Control District)

A county agency with authority to regulate stationary, indirect and area sources of air pollution (e.g., power plants, highway construction and housing developments) within a given county, and governed by a district air pollution control board composed of the elected county supervisors. (Compare AQMD).

AQMD (Air Quality Management District)

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 composed mostly of elected officials from within the region. (Compare APCD).

AQMP (Air Quality Management Plan)

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).

ARB (California Air Resources Board)

The state's lead air quality agency, consisting of a nine-member governor-appointed board. It is
responsible for attainment and maintenance of the state and federal air quality standards, and is fully responsible for motor vehicle pollution control. It oversees county and regional air pollution management programs.

Area-wide Sources (also known as "area" sources)

Stationary sources of pollution (e.g., water heaters, natural gas furnaces, fireplaces and wood stoves) that are typically associated with homes and non-industrial sources. The CCAA requires districts to include area sources in the development and implementation of the AQMPs.

Attainment Area

A geographic area that is in compliance with the National and/or California Ambient Air Quality Standards (NAAQS or CAAQS).

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BACT (Best Available Control Technology)

The most up-to-date methods, systems, techniques and production processes available to achieve the greatest feasible emission reductions for given regulated air pollutants and processes. BACT is a requirement of NSR (New Source Review) and PSD (Prevention of Significant Deterioration).

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CAAQS (California Ambient Air Quality Standard)

A legal limit that specifies the maximum level and time of exposure in the outside air for a given air pollutant and which is protective of human health and public welfare (Health and Safety Code 39606b). CAAQSs are recommended by the California Office of Environmental Health Hazard Assessment and adopted into regulation by the Air Resources Board. CAAQSs are the standards which must be met per the requirements of the California Clean Air Act.

CCAA (California Clean Air Act)

A California law passed in 1988 that provides the basis for air quality planning and regulation,
independent of federal regulations. A major element of the act is the requirement that local APCDs/AQMDs in violation of the CAAQS must prepare attainment plans that identify air quality problems, causes, trends and actions to be taken to attain and maintain California's air quality standards by the earliest practicable date.

CEQA (California Environmental Quality Act)

A California law that sets forth a process for public agencies to make informed decisions on discretionary project approvals. The process aids decision makers in determining whether any environmental impacts are associated with a proposed project. It requires environmental impacts associated with a proposed project to be eliminated or reduced, and that air quality mitigation measures have been implemented.

CFCs (Chlorofluorocarbons)

Any of a number of substances consisting of chlorine, fluorine and carbon. CFCs are used for refrigeration, foam packaging, solvents and propellants. They are proven to cause depletion of the atmosphere's ozone layer.

Chronic Health Effect

An adverse health effect that occurs over a relatively long period of time (e.g., months or years).

CO (Carbon Monoxide)

A colorless, odorless gas resulting from the incomplete combustion of fossil fuels. More than 80 percent of the CO emitted in urban areas is contributed by motor vehicles. CO interferes with the blood's ability to carry oxygen to the body's tissues and results in numerous adverse health effects. CO is a criteria air pollutant.

CO2(Carbon Dioxide)

A colorless, odorless gas that occurs naturally in the earth's atmosphere. Significant quantities are also emitted into the air by fossil fuel combustion. Emissions of CO2 have been implicated with increasing the greenhouse effect.

Criteria Air Pollutant

An air pollutant for which acceptable levels of exposure can be determined, and for which an ambient air quality standard has been set. Examples include: ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide and PM-10 (see individual pollutant definitions).

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Emission Inventory

An estimate of the amount of pollutants emitted from mobile and stationary sources into the atmosphere over a specific period, such as a day or a year.

Emission Offset (also known as an emission trade-off)

A rule-making concept whereby approval of a new or modified stationary source of air pollution is conditional on the reduction of emissions from other existing stationary sources of air pollution. These reductions are required in addition to reductions required by BACT.

Emission Standard

The maximum amount of a pollutant that is allowed to be discharged from a polluting source, such as an automobile or smoke stack.

EPA (Environmental Protection Agency)

The United States agency charged with setting policy and guidelines, and carrying out legal mandates for the protection of national interests in environmental resources.

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FCAA (Federal Clean Air Act)

A federal law passed in 1970 and amended in 1977 and 1990 that forms the basis for the national air pollution control effort. Basic elements of the act include national ambient air quality standards for major air pollutants, air toxics standards, acid rain control measures and enforcement provisions.

FIP (Federal Implementation Plan)

In the absence of an approved State Implementation Plan (SIP), a plan prepared by the EPA which provides measures that non-attainment areas must take to meet the requirements of the Federal Clean Air Act.

Fugitive Dust

Dust particles that are introduced into the air through certain activities such as soil cultivation, off-road vehicles or any vehicles operating on open fields or dirt roadways.

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Greenhouse Effect

The warming effect of the earth's atmosphere on the earth. Light energy from the sun passes through the earth's atmosphere and is absorbed by the earth's surface and re-radiated into the atmosphere as heat energy. The heat energy is then trapped by the atmosphere, creating a situation similar to that which occurs in a greenhouse or a car with its windows rolled up. Many scientists believe that the emission of CO and other gases into the atmosphere may increase the greenhouse effect and contribute to global warming.

Growth Management Plan

A plan for a given geographical region containing demographic projections (i.e., housing units, employment and population) through some specified point in time, and which provides recommendations for local governments to better manage growth and reduce projected environmental impacts.

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Hazardous Air Pollutant (HAP)

An air pollutant considered by the EPA to be particularly hazardous to health. Emission sources of hazardous air pollutants are identified by the EPA, and emission standards are set accordingly.


Any of a large number of compounds containing various combinations of hydrogen and carbon atoms. They may be emitted into the air as a result of fossil fuel combustion, fuel volatilization and solvent use, and are a major contributor to smog. (Also see ROG).

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Indirect Source

Any facility, building, structure or installation, or combination thereof, that generates or attracts mobile
source activity that results in emissions of any pollutant (or precursor) for which there is a state ambient air quality standard. Examples of indirect sources include employment sites, shopping centers, sports facilities, housing developments, airports, commercial and industrial development, and parking lots and garages.

Indirect Source Control Program

Rules, regulations, local ordinances and land use controls, and other regulatory strategies of air pollution control districts or local governments used to control or reduce emissions associated with new and existing indirect sources. Indirect source control programs include regulatory strategies such as transportation control measures (e.g., South Coast's Regulation XV for employer-based trip reduction); parking charges; land use controls that reduce the need for vehicle travel and increase transit, bicycle and pedestrian access; and source-specific regulations such as truck idling and travel schedule requirements.

Indirect Source Review

A major component of an indirect source control program which applies to new and modified indirect sources. Strategies for indirect source review include permit programs, review and comment on new and modified indirect source projects through the California Environmental Quality Act (CEQA) process, and coordination of air quality, transportation and land use policies through local government general plans. Indirect source review reduces emissions from new and modified sources through best available mitigation measures and additional offsite mitigation such as offsets and mitigation fee.


A layer of warm air in the atmosphere that lies over a layer of cooler air, trapping pollutants.

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LAER: Lowest Achievable Emission Rate.

The most up-to-date methods, systems, techniques and production processes available to achieve the greatest feasible emission reductions for given regulated air pollutants and processes. In SCAQMD, BACT is a requirement of NSR (New Source Review) and PSD (Prevention of Significant Deterioration). The term BACT, as used in state law, means an emission limitation that will achieve the lowest achievable emission rates, which means the most stringent of either the most stringent emission limits contained in the SIP for the class or category of source, (unless it is demonstrated that one limitation is not achievable) or the most stringent emission limit achieved in practice by that class in category of source. BACT, under state law, is more stringent than federal BACT, and is equivalent to federal LAER, which applies to NSR permit actions. Federal LAER determination does not consider cost effectiveness.

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Mobile Sources

Sources of air pollution such as automobiles, motorcycles, trucks, off-road vehicles, boats and airplanes. (Contrast with stationary sources).

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NAAQS (National Ambient Air Quality Standards)

Standards set by the federal EPA for the maximum levels of air pollutants that can exist in the outdoor air without unacceptable effects on human health or the public welfare.

NSR (New Source Review)

A program used in development of permits for new or modified industrial facilities that are in a non-attainment area, and which emit non-attainment criteria air pollutants. The two major requirements of NSR are Best Available Control Technology and Emission Offset.

Non-Attainment Area

A geographic area identified by the EPA and/or ARB as not meeting either NAAQS or CAAQS standards for a given pollutant.

Nitrogen Oxides (Oxides of Nitrogen, NO2)

A general term pertaining to compounds of nitric oxide (NO), nitrogen dioxide (NO2), and other oxides of nitrogen. Nitrogen oxides are typically created during combustion processes, and are major contributors to smog formation and acid deposition. NO is a criteria air pollutant, and may result in numerous adverse health effects.

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OEHHA (Office of Environmental Health Hazard Assessment)

A department within the California Environmental Protection Agency that is responsible for evaluating chemicals for adverse health impacts and establishing safe exposure levels. OEHHA also assists in performing health risk assessments and developing risk assessment procedures for air quality management purposes.


A strong smelling, pale blue, reactive toxic chemical gas consisting of three oxygen atoms. It is a product of the photochemical process involving the sun's energy. Ozone exists in the upper atmosphere ozone layer as well as at the earth's surface. Ozone at the earth's surface causes numerous adverse health effects and is a criteria air pollutant. It is a major component of smog.

Ozone Layer

A layer of ozone 12 to 15 miles above the earth's surface which helps to filter out harmful ultraviolet rays from the sun. It may be contrasted with ground-level ozone, which exists at the earth's surface and is a harmful component of photochemical smog. A primary concern is that compounds such as chlorofluorocarbons (CFCs), used in air conditioning systems, are depleting the ozone layer. Stringent federal requirements have phased out production of chlorofluorocarbons in the U.S.

Ozone Precursors

Chemicals such as hydrocarbons and oxides of nitrogen, occurring either naturally or as a result of human activities, which contribute to the formation of ozone, a major component of smog.

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Written authorization from a government agency (e.g., an air quality management district) that allows for the construction and/or operation of an emissions-generating facility or its equipment within certain specified limits.

PM-10 (Particulate Matter)

A major air pollutant consisting of tiny solid or liquid particles of soot, dust, smoke, fumes and mists. The size of the particles (10 microns or smaller, about 0.0004 inches or less) allows them to easily enter the air sacs deep in the lungs where they may be deposited to result in adverse health effects. PM-10 also causes visibility reduction and is a criteria air pollutant.

Public Workshop

A workshop held by a public agency for the purpose of informing the public and obtaining its input on the development of a regulatory action or control measure by that agency.

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Risk Assessment

An evaluation of risk that estimates the relationship between exposure to a harmful substance and the likelihood that harm will result from that exposure. Risk assessments are generally expressed as the estimated chance per million that a person, exposed over some period of time (e.g., a 70-year lifetime) and some specified concentration of exposure, will experience a certain effect.

ROG (Reactive Organic Gas)

A reactive chemical gas, composed of hydrocarbons that may contribute to the formation of smog. Also sometimes referred to as Non-Methane Organic Compounds (NMOCs).

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Sensitive Groups

Identifiable subsets of the general population that are at greater risk than the general population to the toxic effects of a specific air pollutant (e.g., infants, asthmatics, elderly).

State Implementation Plan (SIP)

A document prepared by each state describing existing air quality conditions and measures which will be taken to attain and maintain national ambient air quality standards (see AQMP).


A combination of smoke, ozone, hydrocarbons, nitrogen oxides and other chemically reactive compounds which, under certain conditions of weather and sunlight, may result in a murky brown haze that causes adverse health effects. The primary source of smog in California is motor vehicles.

SO2(Sulfur Dioxide)

A strong smelling, colorless gas that is formed by the combustion of fossil fuels. Power plants, which may use coal or oil high in sulfur content, can be major sources of SO2. SO2 and other sulfur oxides contribute to the problem of acid deposition. SO2 is a criteria pollutant.

Stationary Sources

Non-mobile sources such as power plants, refineries and manufacturing facilities that emit air pollutants. (Contrast with mobile sources).

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TAC (Toxic Air Contaminant)

An air pollutant, identified in regulation by the ARB, which 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 under a different regulatory process (California Health and Safety Code Section 39650 et seq.) than pollutants subject to CAAQSs. Health effects to TACs may occur at extremely low levels, and it is typically difficult to identify levels of exposure that do not produce adverse health effects.

Toxic Hot Spot

An area where the concentration of air toxics is at a level at which individuals may be exposed to an elevated risk of adverse health effects. Air toxics hot spots may include sources such as landfills, sewage treatment plants and metal plating operations.

Transportation Control Measure (TCM)

Any control measure to reduce vehicle trips, vehicle use, vehicle miles traveled, vehicle idling or traffic congestion for the purpose of reducing motor vehicle emissions. TCMs can include encouraging the use of carpools and mass transit.

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Vapor Recovery Systems

Mechanical systems that collect and recover chemical vapors resulting from transfer of gasoline from operations, such as tank-to-truck systems at refineries, tanker-to-pipeline systems at offshore oil operations and pump-to-vehicle systems at gasoline stations.


The distance that atmospheric conditions allow a person to see at a given time and location. Visibility reduction from air pollution is often due to the presence of sulfur and nitrogen oxides, as well as particulate matter.

VOCs (Volatile Organic Compounds)

Hydrocarbon compounds that exist in the ambient air. VOCs contribute to the formation of smog and/or may themselves be toxic. VOCs often have an odor, and some examples include gasoline, alcohol and the solvents used in paints.

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