Glossary of Terms

Glossary Alphabet Filter

Air Quality Index (AQI)

A nationally calculated index for reporting and forecasting daily air quality presented as a color-coded scale that ranges from Green or Good (0 to 50) through to Maroon or Hazardous (301 to 500). As the AQI value increases, the percentage of the population increases of those who are likely to experience increasingly severe adverse health effects. The AQI is based on the four most common air pollutants regulated under the Clean Air Act: ground-level ozone (O3), particle pollution (PM10 and PM2.5), carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen dioxide (NO2).

At-Risk Individuals

include people with chronic conditions (heart or lung disease, diabetes, etc.), especially the very young, the elderly and women of childbearing age.

Attainment Area

A geographic area where the measured concentrations of the criteria pollutants are within the National Ambient Air Quality Standards (NAAQS).

Black Carbon (BC)

or soot, is formed during high-temperature fuel combustion. Diesel vehicles are the major source of BC in the U.S. In Pittsburgh, BC concentrations are found to be elevated near busy roadways and industrial facilities such as coke ovens, steel mills and in the river valley.

Carbon Dioxide (CO2)

Is the most significant, persistent greenhouse gas in the Earth’s atmosphere capable of lasting 1,000 years. It is produced by the combustion of carbon-based fuels such as wood and other organic materials and fossil fuels such as coal, peat, petroleum and natural gas. It is also a byproduct in many large-scale oxidation processes.

Carbon Dioxide Equivalent (CO2e)

The amount of carbon dioxide that would produce a similar greenhouse gas effect as a different amount of a different chemical. The most prevalent GHGs that make up this measurement are CO2, methane (CH4) and nitrous oxide (N2O).

Carbon Monoxide (CO)

A colorless, odorless gas that can be harmful when inhaled in large amounts. CO is released when something is burned and when the supply of oxygen is limited. The greatest sources of CO in outdoor air are cars, trucks and other vehicles or machinery that burn fossil fuels.


A chemical or physical agent capable of causing cancer.


The nickname for a petrochemical plant, which takes ethane, a component of different types of gases found in shales, and processes it by breaking chemical bonds using large quantities of energy and high temperature—or ‘cracks’ it—into ethylene to make plastic pellets (called nerdles).

Criteria Pollutants

The EPA has established national ambient air quality standards (NAAQS) that define limits on concentrations of six of the most common air pollutants in the air we breathe. They are: ozone (O3), particulate matter, carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxides (SOx), and lead (Pb).

Environment Facility Application Compliance Tracking System (eFACTS)

is an online database run by the Pennsylvania Department of Environmental Protection (DEP). It allows the public to search for records on permits and inspections for facilities regulated by oil and gas, air quality and other agency divisions.

Excess Emissions

Any emissions in excess of what is permitted other than fugitive emissions. Typically occurs during startup, shutdown or when process or pollution control equipment malfunctions or breaks down.


The burning of gas and chemical by-products in order to prevent disruptions in operations, relieve pressure within the system and adjust product quality. Elevated flares are located above the facility and are designed to burn larger volumes of gas than ground flares. Ground flares are located at ground level and can either be enclosed or open.


short for hydraulic fracturing, is a gas well extraction technique that fractures shale rock layers using a pressurized liquid. This process injects high pressure ‘fracking fluid’ (a mixture of water and hazardous chemicals) into a wellbore to create cracks in the deep-rock formations that allow the release of natural gas or petroleum. Each well takes about 1 – 3 million gallons of fracking fluid. After the fracking process is completed, the well returns about 1/3 of the fluid immediately as flowback water, a briny mixture of water, fracking fluid and minerals dissolved from the shale. The well also returns about 500 barrels of producer fluid, a supersaturated mixture of water, fracking fluid and dissolved minerals from the well.

Fugitive Emissions

Air pollution emissions due to process equipment leaks that cannot reasonably pass through a stack chimney or vent.

Greenhouse Gases (GHG)

act like an insulating blanket, trapping heat in the atmosphere and warming the Earth. Four common GHG are carbon dioxide, methane, nitrous oxide and fluorinated gases, which remain in the atmosphere for differing amounts of time, ranging from a few years to thousands of years.

Ground-Level Ozone (O3)

is formed in the atmosphere through reactions with other pollutants. Ozone often forms in the summer months when nitrogen oxides (NOx) and volatile organic compounds (VOC) combine and react in the presence of sunlight and warm temperatures. Ozone irritates the eyes and upper respiratory system, hampers breathing and causes asthma.

Hazardous Air Pollutants (HAPS)

Pollutants that contribute to mortality, serious irreversible illness or incapacitating reversible illness. The Clean Air Act lists 197 HAPS. Common HAPs include benzene, ethylbenzene, toluene and xylene, typically emitted by industrial plants and petroleum refineries and coke ovens.

HEPA (High-Efficiency Particulate Air) Filter 

A pleated, mechanical air filter that traps dust and dirt that would negatively impact indoor air quality. HEPA filters can remove 99.97% of dust, pollen, mold, bacteria and airborne particles that are 0.3 microns (µm) in diameter or larger.


is an atmospheric condition where a layer of cooler air becomes trapped near the earth’s surface by a layer of warmer air above. When the air cannot rise, pollution on the ground becomes trapped and can accumulate, leading to higher concentrations of ozone and particle pollution. The river valleys of Southwestern Pennsylvania are frequently subject to inversions.


Much of our exposure to lead comes from human activities including the use of fossil fuels, past use of leaded gasoline, some industrial facilities and the past use of lead-based paint. Lead can affect almost every organ and system in the body; Children and babies are particularly at risk. Even low levels can cause behavior and learning problems, lower IQ, slowed growth, hearing problems and anemia.

Micrograms per Cubic Meter of Air, or ug/m3

A microgram is equivalent to a millionth of a gram or a thousandth of a milligram.

Million square cubic feet of gas per day, (MMscfd)

is a standard measurement of gas that is used to indicate the capacity of compression and processing facilities.

Mixing Height

Used to describe the vertical extent that the air in the atmosphere is mixing through convection — surface heat causing air to rise, and cooler air sinking.  So pollution at the surface (or sometime that is in a layer well above the surface) will mix between the surface and aloft.



Air modeling is a way to mathematically simulate atmospheric conditions and behavior. It is usually performed using computer programs. For example, air pollution modeling can help estimate how much of a specific air pollutant will be present at different distances from the source.

National Ambient Air Quality Standards, (NAAQS)

are the limits on pollution volumes that the EPA deems necessary to protect air quality and health. NAAQS have been established for the six criteria pollutants under the Clean Air Act.

Nitrogen Oxides (NOx)

are major components of smog.

Non-Point Source Pollution

Emissions in the air from a multitude of sources such as diesel emissions from vehicles and from wood burning.

Nonattainment Area

A geographic area where the measured concentrations of any of the six criteria pollutants exceed the National Ambient Air Quality Standards.

Particle Pollution

Also called soot, particulate pollution is a mixture of solid particles and liquid droplets. Fine particles are a subset of this group, the smallest of which measures 2.5 microns or less in diameter –1/30th the width of a human hair.


Particulate matter in air, comprised of solid particles and liquid droplets, that has an aerodynamic diameter less than 10 micrometers. These inhalable particles are generally 10 micrometers in size and smaller.


These fine, microscopic particles with a diameter less than 2.5 micrometers threaten public health because they can be inhaled deeply into the lungs, enter the bloodstream and cause adverse health effects. The average human hair is 30 times larger than PM2.5.

Point Source Pollution

Deliberate environmental releases from fixed points such as smokestacks from industry. Point source pollution stems from gas and coal-fired power plants, manufacturing facilities, chemical plants, coke production facilities and steel mills.


Rapid cooling of a material or chemical. In an ethane cracking facility cracked gases are quenched with water to cool gas and reduce its volume before compression. Quenching also can absorb some acidic components of the cracked gases.

Sensitive Groups (At-Risk Populations)

A term used for people who have an increased risk of experiencing adverse health effects related to air pollution exposures. These increased risks are often due to biological factors, external or non-biological factors, higher exposures and/or increased dose at a given concentration. The severity of the health effects that these groups experience may be much greater than in the general population.


When cracker furnaces, compressors and other equipment are turned off. Different levels and types of air pollution may be released during shutdown in comparison to normal, everyday operations.

Sulfur Dioxide

A criteria pollutant harmful to human health and the environment. The largest sources of SO2 emissions are fossil fuel combustion at power plants and other industrial facilities.

Title V

The Clean Air Act requires that major point sources of pollution that emit or have the potential to emit more than 100 tons per year of a single pollutant obtain and operate in compliance with an operating permit. Sources with Title V permits are required by the Act to certify compliance with the applicable requirements of their permits at least annually. In Allegheny County, the Allegheny Health Dept. handles Title V permitting.


The U.S. Toxics Release Inventory. Under federal law, manufacturing facilities are required to report the amounts of approximately 650 toxic chemicals that they release into the environment or produce as waste.

Wood Burning

Pollution from wood burning is especially dangerous for those with existing health conditions, children, and the elderly, it is hazardous even to young, healthy people. Wood burning causes inflammation of the lungs and decreases lung volume.