A Cheat Sheet | Air Quality Definitions

Air Quality Definitions: A Cheat Sheet for Common Terms and Acronyms

What does that air quality acronym stand for? And actually mean? We’ve rounded up air quality definitions for 20+ of the industry’s most common terms.

By Cory Sander

There are many terms associated with air quality and many even have two or three letter abbreviations. It’s easy to find yourself thinking, “What does that stand for again? Or what does that air quality term mean?”

We’ve compiled some of the most common terminology along with brief air quality definitions. This list spans everything from air pollutants to regulatory and treatment terms. Think of it as a quick cheat sheet for environmental health professionals.

Air Toxics

Air pollutants that are regulated through an air toxic model in most states. It’s a catch-all category for pollutants without specific regulations of their own.

Attainment Area

A geographic area (usually defined by county) that meets all the minimum air quality standards.

BACT: Best Available Control Technology

Refers to the best technology to control pollution from a major emissions source. A major new or modified stationary emissions source must install best available control technology.

CO: Carbon Monoxide

A colorless, odorless gas that’s poisonous. It can play a role in urban pollution and ground-level ozone formation. High level, lethal concentrations can also easily build up indoors.

CAAA: Clean Air Act Amendment

The Clean Air Act is a federal law that dates back to 1970 (under EPA’s administration) and regulates the emission of air pollutants. There were major amendments to the law in 1977 and 1990 that updated emission standards.

CEMS: Continuous Emissions Monitoring System

Equipment installed at an air pollution source to measure or monitor compliance. Examples include a pressure drop system or temperature gauge.

De Minimis

A stationary pollution source deemed relatively small. It’s usually exempt from typical permitting, but these sources may need to track emissions or operational parameters to prove emissions are minimal.

GHG: Greenhouse Gas

There are many greenhouse gases, and all of them contribute to trapping heat in the Earth’s atmosphere and making the planet warmer. Three primary contributors are carbon dioxide, methane, and nitrous oxide. These air pollutants are regulated as a group.

HAPs: Hazardous Air Pollutants

Air pollutants that cause—or are suspected to cause—serious health or environmental effects. Examples include benzene, perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds.

LAER: Lowest Achievable Emission Rate

This represents the most stringent control technology achieved in practice regardless of cost. LAER is used to determine emission limits for the New Source Review (NSR) program.

Nonattainment Area

A geographic area (usually defined by counties) in which the level of a criteria air pollutant is higher than the allowable level. A single geographic area may have acceptable levels of one criteria air pollutant but unacceptable levels for one or more other criteria air pollutants.

MACT: Maximum Achievable Control Technology

Generally, the best available control technology, taking into account cost and technical feasibility. MACT stems from the 1990 amendment to the Clean Air Act for the control of toxic emissions from various industries.


A heavy metal that is highly toxic if breathed or swallowed. It is a regulated air pollutant. The largest human-generated source of mercury emissions in the United States is the burning of coal. Other sources include the combustion of waste and industrial processes that use mercury.

Mobile Source

A moving source of air pollution. These include motor vehicles, trains and sources that aren’t permanent or site specific (for example, a mobile power generator).

NAAQS: National Ambient Air Quality Standard

Standards established by EPA for maximum allowable concentrations of six criteria pollutants in outdoor air. The six criteria air pollutants include carbon monoxide, lead, nitrogen dioxide, ozone, particle pollution, and sulfur dioxide.

NESHAP: National Emission Standards for Hazardous Air Pollutants

Standards that regulate the emission of Hazardous Air Pollutants from specific industrial sources.

NO2: Nitrogen Dioxide

A gas that contributes to acid rain. It’s primarily released into the air when fuel is burned by cars, trucks and buses.

O3: Ozone

A gas that’s a main ingredient of smog in cities. It can be harmful near the Earth’s surface but at higher levels, it provides the Earth protection from the sun. It’s created by a chemical reaction involving other pollutants, specifically nitrogen and volatile organic compounds.


The amount of light obscured by particulate pollution in the air. Opacity is an indicator of changes in performance of particulate control systems. Certain emission sources will typically have opacity requirements (e.g., how dark a cloud of smoke can appear).

Pb: Lead

A heavy metal that is hazardous to health if breathed or swallowed. Its use in gasoline, paints, and plumbing compounds has been sharply restricted or eliminated by federal laws and regulations.

PPB: Parts Per Billion

A unit of concentration for pollutants in the air. It’s the equivalent of one microgram per 1 cubic meter of air.

PPM: Parts Per Million

A unit of concentration for pollutants in the air. It’s the equivalent of 1 milligram per 1 cubic meter of air.


Particulate matter that measure 10 microns and less in diameter and small enough to travel deep into the lungs. These include liquid and solid particles found in the air and may originate from fires, motor vehicles, industrial plants and more.

PM 2.5

Particulate matter that measures 2.5 microns and less in diameter and small enough to travel deep into the lungs. These include liquid and solid particles found in the air and may originate from fires, motor vehicles, industrial plants and more.  

PTE: Potential to Emit

The maximum amount an emission source is capable of emitting if it ran 24/7, 365 days a year.

RTO: Regenerative Thermal Oxidizer

A treatment system for exhaust air that absorbs heat from the exhaust then uses that same heat to destroy pollutants within the exhaust.

SIP: State Implementation Plan

Detailed descriptions of the programs a state will use to carry out its responsibilities under the Clean Air Act. State Implementation Plans are collections of the regulations used by a state to reduce air pollution. The Clean Air Act requires that EPA approve each State Implementation Plan.

SO2: Sulfur Dioxide

A pungent, colorless, gaseous pollutant formed primarily by the combustion of fossil fuels. It is one of the six criteria pollutants for which EPA has set national ambient air quality standards.

Title V

An operating permit program established as part of the 1990 amendment to the Clean Air Act. It requires large air pollution sources, such as a major factory or chemical plant, to operate under stringent guidelines, including emissions limits and monitoring, record keeping, and reporting requirements. It also requires that sources report compliance status with respect to permit conditions to the permitting authority.

TPY: Tons Per Year

It’s a measurement of the tons (i.e., 2,000 pounds) of pollutants emitted in a year and may be used as a limit in an air permit.

VOC: Volatile Organic Compound

Any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity. A VOC is one of a group of carbon-containing compounds that evaporate readily at room temperature. Examples of VOCs include trichloroethane; trichloroethylene; and BTEX. These contaminants typically are generated from metal degreasing, printed circuit board cleaning, gasoline, and wood preserving processes.

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