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Above the Clouds

Glossary of Air Quality Terms




Air Toxics - Sometimes referred to as toxic air pollutants or hazardous air pollutants (HAPS), air toxics are defined as pollutants that cause or may cause cancer or other serious health effects. 


Air Quality Complaint - Residents who see or smell air pollution are asked to file an air quality complaint. In Allegheny County, air quality complaints about issues such as smoke and foul odors are handled through the health department. Residents can make an air quality complaint by calling or emailing the Allegheny County Health Department, or by completing an online form on its website. Have an air quality complaint, but you live outside of Allegheny County? These complaints can be directed to the state Department of Environmental Protection. For more information on how to make an air quality complaint and who to make one to, check out our FAQ.


Air Quality Index - Also referred to as AQI, the Air Quality Index is a data tool facilitated by the U.S. Environmental Protection Agency (EPA) that reports daily air quality. The Air Quality Index provides information about how clean or polluted your air is, and what associated health effects might be a concern for you. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. Ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health in this country.


Acid Rain - Precipitation, as rain, snow, or sleet, containing relatively high concentrations of acid-forming chemicals, as the pollutants from coal smoke, chemical manufacturing, and smelting, that have been released into the atmosphere and combined with water vapor. Acid rain is harmful to the environment.


Allegheny Alerts - This service is operated by Allegheny County and provides time-sensitive notifications regarding important information for residents. Those who sign up for the free service can specify what types of Allegheny Alerts they wish to receive, such as information from the health department about air quality and other issues, heads up about road closures and more. Users can also specify where the Alerts are delivered - by phone, by email, or via text message. Signing up for Allegheny Alerts is easy and can be accomplished here.


Allegheny County Health Department (ACHD) - The Allegheny County Health Department is a part of Allegheny County government. Its mission is to protect, promote and preserve the health and well-being of all Allegheny County residents. Also referred to as ACHD, the nationally accredited department provides quality public health services to more than 1.2 million county residents. As part of its mission, ACHD regulates air quality issues in the county. Learn more about the ACHD Air Quality Program.


Ambient Air Quality - Ambient air quality criteria, or standards, are concentrations of pollutants in the air, and typically refer to outdoor air. The criteria are specified for a variety of reasons including for the protection of human health, buildings, crops, vegetation, ecosystems, as well as for planning and other purposes.


Article XXI - In Allegheny County, Article XXI is an ordinance that sets forth air pollution control regulations. 


Attainment Area - An attainment area is a politically or geographically defined region that is in compliance with the National Ambient Air Quality Standards, as set by the Environmental Protection Agency (EPA). A region can be an attainment area for one pollutant and a nonattainment area for another.




Benzene - Benzene is a colorless volatile liquid hydrocarbon present in coal tar and petroleum, and used in chemical synthesis. Its use as a solvent has been reduced because of its carcinogenic properties.


Best Available Control Technology - Best available control technology (BACT) is a pollution control standard mandated by the federal Clean Air Act. The EPA determines what air pollution control technology will be used to control a specific pollutant to a specified limit. When a BACT is determined, factors such as energy consumption, total source emission, regional environmental impact, and economic costs are taken into account. It is the current EPA standard for all polluting sources that fall under the New Source Review guidelines and is determined on a case-by-case basis. The BACT standard is considered somewhat stricter than the reasonably available control technology (RACT).



Carbon Dioxide (CO2) - a colorless, odorless gas produced by burning carbon and organic compounds and by respiration. It is naturally present in air and is absorbed by plants in photosynthesis. Carbon dioxide is the most significant long-lived greenhouse gas in Earth's atmosphere.


Carbon Monoxide (CO) - a colorless, odorless gas produced by incomplete carbon combustion. Carbon monoxide is considered a criteria pollutant by the EPA. CO can build up indoors and poison people and animals who breathe it.


Clean Air Act - The Clean Air Act (CAA) is the comprehensive federal law that regulates air emissions from stationary and mobile sources. One of the goals of the Act was to set and achieve National Ambient Air Quality Standards (NAAQS) in every state by 1975 in order to address the public health and welfare risks posed by certain widespread air pollutants. Learn more.


Clean Construction - when a construction project utilizes the best available retrofit technology on all on-road diesel vehicles and off-road construction equipment being utilized at a construction site. By doing this contractors and developers help to minimize the diesel pollution being emitted at that construction site.  Here in Pittsburgh, GASP was one of the main organizations pushing for the city to implement a clean construction program. The first “clean construction” project began in Duck Hollow in 2019.


Coke - a solid fuel made by heating coal in the absence of air so that the volatile components are driven off.  The largest coke-manufacturing facility in the United States is US Steel’s Clairton Coke Works, located in Clairton, Pennsylvania. 


Coke Battery - A coke battery is made up of multiple ovens. Coal is crushed and blended prior to being charged in a coke oven.  The Clairton Coke Works has ten coke oven batteries and produces approximately 4.3 million tons of coke annually.


Combustion Stack - A combustion stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air.


Compliance Report - The term compliance report refers to those created by companies in order to comply with rules, standards, laws, and regulations set by regulatory bodies and government agencies. Failure to comply means businesses are subject to regulatory penalties, including fines and imprisonment.


Consent Decree - A consent decree is defined as a legal agreement entered into by the United States through the Environmental Protection Agency or the Department of Justice. Consent decrees are lodged with a court. 


Continuous Emissions Monitor - A continuous emission monitoring system, also referred to as CEMS is the total equipment necessary for the determination of a gas or particulate matter concentration or emission rate using pollutant analyzer measurements and a conversion equation, graph, or computer program to produce results in units of the applicable emission limitation or standard. CEMS are required under some of the EPA regulations for either continual compliance determinations or determination of exceedances of the standards. The individual subparts of the EPA rules specify the reference methods that are used to substantiate the accuracy and precision of the CEMS.


Criteria Air Pollutant - The criteria pollutants are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Criteria pollutants are the only air pollutants with national air quality standards, set by the federal Environmental Protection Agency, that define allowable concentrations of these substances in ambient air.




Department of Environmental Protection - The Pennsylvania Department of Environmental Protection (or DEP) is the agency in Pennsylvania responsible for protecting and preserving the land, air, water, and public health through enforcement of the state's environmental laws.




Emergency episode monitoring station - Emergency Episode Monitoring Stations, also referred to as EEMS, are ambient air monitors designated for use in emergency pollution episodes.  This designation is mostly of historical interest.


Emissions - Emissions are defined as the production and discharge of something, especially gas or radiation.


Enforcement Order - Generally, an enforcement order is one issued by a regulatory agency or court to force a person or organization to comply with a regulation or law.


Environmental Protection Agency (EPA) - The Environmental Protection Agency (or EPA) is an agency of the United States federal government whose mission is to protect human and environmental health. It was established in December 1970 via an executive order from then-President Richard Nixon. 


Environmental Hearing Board - The Pennsylvania Environmental Hearing Board (or EHB) hears appeals from actions taken by the Pennsylvania Department of Environmental Protection. The EHB is composed of five judges who are appointed by the governor and confirmed by the Senate. The Environmental Hearing Board holds hearings and issues adjudications, opinions, and orders. Hearings before the EHB are similar to non-jury civil trials being adjudicated in the Common Pleas Courts or Federal District Courts. Appeals from EHB decisions are taken to the Commonwealth Court of Pennsylvania. 




Flaring - Gas flaring is defined as a combustion device to burn associated, unwanted or excess gases and liquids released during normal or unplanned over-pressuring operation in many industrial processes, such as oil-gas extraction, refineries, chemical plants, coal industry, and landfills. Gas flaring is a significant source of greenhouse gas emissions. 


Flow Monitor - Flow measures Particulate Matter (PM1, PM2.5, PM10), Volatile Organic Compounds (VOCs) and is the only personal sensor to measure nitrous oxides (NO2) caused by exhaust gases. Flow and its companion app give you the minute-by-minute breakdown of the pollutants you were exposed to throughout your day by measuring real-time concentrations of NO2, VOC, PM1, PM2.5 and PM10. With the help of your phone’s GPS, Flow builds out the air quality map of all the places it’s been by your side. 


Fugitive Leaks - Fugitive leaks are defined as emissions of gases or vapors from pressurized equipment due to leaks and other unintended or irregular releases of gases, mostly from industrial activities.




Greenhouse Gases - The term greenhouse gases refers to gases that contribute to the greenhouse effect by absorbing infrared radiation such as carbon dioxide and chlorofluorocarbons.


Ground-level Ozone (O3) - Ground-level ozone is a colorless and highly irritating gas that forms just above the earth's surface. It is called a "secondary" pollutant because it is produced when two primary pollutants react in sunlight and stagnant air. These two primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ozone can cause the muscles in the airways to constrict, trapping air in the alveoli. This leads to wheezing and shortness of breath.




Hazardous Air Pollutants (HAPS) - Hazardous air pollutants, also known as toxic air pollutants or air toxics, are those pollutants that cause or may cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental and ecological effects. EPA is required to control 187 hazardous air pollutants.  Some examples of HAPS include: Asbestos, Benzene, Formaldehyde, and Styrene.  


Hydrocarbon - a compound of hydrogen and carbon, such as any of those which are the chief components of petroleum and natural gas.


Hydrogen Sulfide - Also known as H2S, hydrogen sulfide is a colorless poisonous gas with a smell of rotten eggs.  H2S can come from natural sources like volcanoes and hot springs and industrial sources like coke-making.  Pennsylvania has required H2S in the ambient air to be below .005 parts per million (ppm), averaged over 24 hours.



Inversion - Inversions occur most frequently or severely during the winter months when normal atmospheric conditions (cool air above, warm air below) become inverted. Inversions trap a dense layer of cold air under a layer of warm air. The warm layer acts much like a lid, trapping pollutants in the cold air near the valley floor.








Lead - Lead is an elemental heavy metal found naturally in the environment as well as in manufactured products. Lead can be released directly into the air, as suspended particles. Historic major sources of lead air emissions were motor vehicles and industrial sources. The EPA classifies lead as a criteria pollutant.




Major Source - Major sources are defined as sources that emit 10 tons per year of any of the listed toxic air pollutants, or 25 tons per year of a mixture of air toxics. You can track the major sources of air pollution in your community by using GASP’s Air Quality Permits Clearinghouse.


Mercury - Mercury is a naturally-occurring chemical element found in rock in the earth's crust, including in deposits of coal. Elemental or metallic mercury is a shiny, silver-white metal and is liquid at room temperature. It is used in older thermometers, fluorescent light bulbs and some electrical switches. At room temperature, exposed elemental mercury can evaporate to become an invisible, odorless toxic vapor. If heated, it is a colorless, odorless gas.  


Methane - Methane is a colorless, odorless substance that is the primary component of natural gas. It is emitted into the atmosphere during the production, processing, storage, and distribution of natural gas and crude oil. Methane accounts for about 10 percent of man-made greenhouse gas emissions.


Method 9 - Many stationary sources of air pollution discharge visible emissions into the atmosphere - usually in the shape of a plume. Method 9 involves the determination of plume opacity by trained and certified observers. 


Mobile Source - Mobile source air pollution includes any air pollution emitted by motor vehicles, airplanes, locomotives, and other engines and equipment that can be moved from one location to another. 




National Ambient Air Quality Standards (NAAQS) - National Ambient Air Quality Standards are standards set by the Environmental Protection Agency for harmful pollutants under the authority of the Clean Air Act. NAAQS is applied for outdoor air throughout the country.


Nitrogen Oxides - Nitrogen oxides are a group of seven gases and compounds composed of nitrogen and oxygen. Nitrogen oxides react with sunlight and other chemicals to form smog. Nitrogen oxides and sulfur dioxide react with substances in the atmosphere to form acid rain. Nitrogen dioxide is used to produce rocket fuels and explosives. 


Nonattainment - In United States environmental law, a non-attainment area is an area considered to have air quality worse than the National Ambient Air Quality Standards as defined in the Clean Air Act Amendments of 1970 . Non-attainment areas must have and implement a State Implementation Plan (SIP) to meet the standard, or risk losing some forms of federal financial assistance. An area may be a non-attainment area for one pollutant and an "attainment area" for others.


Notice of Violation Letter (NOV) - A notice of violation letter is issued when a business or entity fails to comply with the terms of a permit. The Allegheny County Health Department, Pennsylvania Department of Environmental Protection (DEP), and the U.S. Environmental Protection Agency (EPA), issue notices of violation letters regarding noncompliance with environmental regulations.




Opacity - Opacity is a measurement of how dense the dust particles are in the air and takes into account how much light is obscured by the rising dust when looking at a solid, colored background. Opacity is measured in percentages from 0 to 100 percent, and measurement is actually performed by visible observation.




Particulate Matter - Particulate matter, sometimes referred to as PM and particle pollution, is defined as a mixture of solid particles and liquid droplets found in the air. Some particles (i.e.: dirt, dust, soot, and smoke) are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope. There are two types of particulate matter: PM10, which are inhalable particles with diameters that are generally 10 micrometers and smaller; and PM2.5, which are fine inhalable particles with diameters that are generally 2.5 micrometers and smaller. To put that into perspective, PM2.5 particles are about 30 times smaller than the width of a single human hair. Particulate matter is one of the criteria air pollutants.  


Parts Per Million - A weight-to-weight ratio used to describe concentrations. Parts per million (ppm) is the number of units of mass of a contaminant per million units of total mass.


Parts Per Billion - A weight-to-weight ratio used to describe concentrations. Parts per billion (ppb) is the number of units of mass of a contaminant per billion units of total mass.


Photochemical Assessment Monitoring Station - Photochemical Assessment Monitoring Stations, also called PAMS, are ambient air monitoring sites mandated by the 1990 Clean Air Act amendments. Placed in areas of high ozone, they monitor volatile organic compounds, nitrogen oxides, ozone and meteorological parameters.  The ACHD Air Quality Program operates PAMS sensors.  


Potential to Emit - The Environmental Protection Agency (EPA), in its current regulations, defines a source’s potential to emit air pollutants as follows: “Potential to emit” is the maximum capacity of a stationary source to emit under its physical and operational design. Any physical or operational limitation on the source to emit an air pollutant, including air pollution control equipment and restrictions on hours of operation, or on the type or amount of material combusted, stored, or processed, shall be treated as part of its design if the limitation is enforceable by the EPA.


PurpleAir Monitor - The PurpleAir is one of many low-cost devices that use laser optical particle counters to estimate particulate matter mass concentrations in air for PM1.0, PM2. 5 and PM10.




Quench Tower - A Quench Tower or Condenser is often the first step in conditioning a high temperature, polluted air stream so that particulate, acid gases, metals, and other emissions can be adequately removed.



Readily Achievable Control Technologies (RACT) - pollution control technology that is reasonably available and both technologically and economically feasible.



Smell PGH - Smell PGH is a mobile phone app designed by the CREATE Lab at Carnegie Mellon University to engage Pittsburgh residents in tracking pollution odors across our region. This allows residents to track where odors are frequently concentrated, and link those smell events to poor air quality in, or upwind from, those areas. These complaints are sent in real time to the Allegheny County Health Department, although it is not widely used by inspectors.


Smelting - Smelting is a process of applying heat to ore in order to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals.


Smog - Smog is defined as fog or haze combined with smoke and other atmospheric pollutants.


Smoke Reading - A type of visual opacity monitoring that can help determine noncompliance with air quality permit standards. Also referred to as Method 9 readings.


State and Local Air Monitoring Stations (SLAMS) - State and Local Air Monitoring Stations, also called SLAMS, are ambient monitoring stations designed and operated by a state or local air pollution control agency to meet its SIP requirements. Only population-oriented SLAMS acquire data for determining compliance with standards. All SLAMS must meet monitor siting criteria and be operated under quality-assurance requirements specified by the US EPA.


Stationary Source - A stationary source in air quality terminology is any fixed emitter of air pollutants, such as fossil fuel burning power plants, petroleum refineries, petrochemical plants, food processing plants and other heavy industrial sources.


State Implementation Plan (SIP) - A State Implementation Plan (SIP) is a collection of regulations and documents used by a state, territory, or local air district to reduce air pollution in areas that do not meet National Ambient Air Quality Standards, or NAAQS.


Stratospheric ozone -  a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation. This is typically regarded as 'good' ozone since it reduces the harmful effects of ultraviolet (UV-B) radiation.


Sulfur Dioxide (SO2) - a heavy pungent toxic gas that is easily condensed to a colorless liquid, is used especially in making sulfuric acid, in bleaching, as a preservative, and as a refrigerant, and is a major air pollutant especially in industrial areas. The EPA classifies sulfur dioxide as a criteria pollutant.


Sulfur Oxides - Sulfur oxides are a group of ambient air pollutants, which consist of both gaseous and particulate chemical species, including sulfur monoxide, sulfur dioxide, sulfur trioxide, and disulfur monoxide.


Supplemental Environmental Project (SEP) - Most federal actions against businesses or individuals for failure to comply with environmental laws are resolved through settlement agreements. As part of a settlement, an alleged violator may propose to undertake a project to provide tangible environmental or public health benefits to the affected community or environment, that is closely related to the violation being resolved, but goes beyond what is required under federal, state or local laws. 


Synthetic Minor Facilities - Synthetic Minor facilities are ones whose potential to emit - without taking limitations and/or restrictions - exceed the Title V applicability threshold. By taking a restriction in hours of operation or an emission limitation, or employing other approved methods, a major facility by definition becomes a minor facility and escapes the Title V Operating Permit program.




Title V Operating Permit - Title V of the Clean Air Act requires major stationary sources of air pollutants, and certain other sources, to: obtain an operating permit and once issued, must operate in compliance with that permit. Title V operating permits are verified at least annually their compliance with permit requirements.


Toxic Air Pollutant - Also known as Hazardous Air Pollutants, air toxics are those pollutants that cause or may cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental and ecological effects.




Urban Airshed Model - The Urban Airshed Model (UAM) is an urban scale, three-dimensional, grid type, numerical simulation model. The model incorporates a condensed photochemical kinetics mechanism for urban atmospheres.




Visual Emissions  - A visible emission is the visible particulate matter that occurs as the result of a process. Smoke exiting a stack at a power plant is a visible emission. Dust produced from driving down a gravel road is a visible emission. Smoke from the chimney of your fireplace is a visible emission.


Volatile Organic Compounds - Volatile organic compounds (VOCs) are organic chemicals that have a high vapor pressure at ordinary room temperature. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air. VOCs include human-made and naturally occurring chemical compounds. Harmful VOCs typically are not acutely toxic, but have compounding long-term health effects. Because the concentrations are usually low and the symptoms slow to develop, research into VOCs and their effects is difficult.










Zero Air - Zero Air is air which has had hydrocarbons removed via a process of oxidative catalysis to ensure it only contains less than 0.1 parts per million (PPM) of total hydrocarbons. 

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