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a permit.
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Contact information
For information on air permits terms and definitions, contact:
Barbara Pavliscak
Air management engineer

Air permits glossary

This glossary provides plain English, non-technical definitions of terms frequently used in the DNR's Air Management Program related to air permits. Please refer to the Wisconsin Administrative Code [exit DNR] and Code of Federal Register [exit DNR] for the legal definitions of specific terms.

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Acid Rain
Air pollution produced when acid chemicals such as sulfur oxides and nitrogen oxides combine with moisture in the air and fall to the Earth as acidic rain, snow, fog, or mist. The main sources of these pollutants are vehicles and industrial and power-generating plants.
Actual Emissions
The emissions generated by a facility over a specified period of time, taking into account actual operating conditions including any reductions made by a control device or technique.
Ambient Air
The portion of the atmosphere external to buildings and which is breathed by the general public.
Refers to being made or generated by a human or caused by human activity. The term is used in the context of global climate change to refer to emissions that are the result of human activities, as well as other possible climate-altering activities such as deforestation or urbanization. Related to Biogenic.
Attainment area
An area considered to have air quality as good as, or better than, the acceptable level of a pollutant in the air set by the federal government(acceptable levels are called ambient air quality standards as defined in the Clean Air Act).


BACT - Best Available Control Technology
The most technologically sophisticated pollution control technology for a specific industry or process, which will result in the removal of the greatest amount of air pollutants. BACT requirements are intended for major sources in attainment areas and are determined case by case. BACT does take into account energy, environmental, and economic costs.
Basic Emissions Unit
The smallest collection of equipment which in combination emits or is capable of emitting any air contaminant.
Refers to being produced by biological processes of living organisms. Related to Anthropogenic.


CFCs (chlorofluorocarbons)
These and some other related chemicals have been used in great quantities in industry for refrigeration and air conditioning. When released into the air, they rise into the upper atmosphere and reduce the protective ozone layer.
Carbon dioxide equivalent (CO2E)
A carbon dioxide equivalent is the amount (usually in metric tons) of a greenhouse gas that would have the same global warming potential of one metric ton of carbon dioxide. In technical terms, the amount of a greenhouse gas by weight emitted into the atmosphere that would produce the same estimated radiative forcing as one metric ton of carbon dioxide. The carbon dioxide equivalent for a gas is derived by multiplying the tons of the gas by its global warming potential. Related to global warming potential.
Clean Air Act
The original Clean Air Act, signed into law by President Lyndon B. Johnson in 1963, was considered to be the first modern environmental law enacted by the United States Congress. The Clean Air Act of 1970, reviewed and amended by Congress in 1975, 1977, and 1990, has formed the basis of the currrent federal air pollution control program.
Control Technology
Equipment processes or actions used to reduce air pollution. The extent of pollution reduction varies among technologies and measures.
Criteria air pollutants
Six very common air pollutants regulated by the EPA on the basis of certain criteria (namely, information on public health and/or environmental effects of pollution). These pollutants are carbon monoxide (CO), lead (Pb), ozone (O3), particulate matter (PM), sulfur dioxide (SO2), and nitrogen oxides (NOx).


A release of pollutants into the air from a source. Emissions released from any point other than a flue or stack are called fugitive emissions.
Emission Unit
Any part of a stationary source which emits or is capable of emitting any air contaminant.
EOP - Elective Operation Permit
EPA Class Codes
Code Description
A Major Source
SM80 Synthetic Minor, PTE > 80% of the Major Source Threshold
SM Synthetic Minor, PTE < 80% of the Major Source Threshold
B Natural Minor
S2VP Natural Minor – Stage II Vapor Recovery


A business that operates one or more stationary or mobile air pollution emission sources. Examples of facilities include Fiberglass Boat Manufacturers, Pulp and Paper Mills, Dry Cleaners, Municipal Landfills, Foundries.
FESOP - Federally Enforceable State Operation Permit
Also called Synthetic Minor Permit/Source or Non-Part 70 Permit.
FOP - Federal Operation Permit
Also referred to as Part 70 Permit, Title V Permit/Source or Major Source.


Global warming potential (GWP)
An index used to compare the relative heat-trapping ability of different greenhouse gases to that of carbon dioxide (because it is the most common greenhouse gas). In technical terms, GWP is the cumulative radiative forcing effects of a greenhouse gas over a specified time horizon (usually 100 years) resulting from the emission of one kilogram of that greenhouse gas relative to one kilogram of carbon dioxide. Related to carbon dioxide equivalent.
GOP - General Operation Permit
Greenhouse gases (GHG)
Gases that are transparent to solar radiation, allowing sunlight to enter the Earth's atmosphere, while preventing radiant energy from leaving the atmosphere. Gases include water vapor, carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6).


Hazardous Air Pollutant
Chemicals that cause serious health and environmental effects. These are separate from criteria air pollutants. The U.S. EPA has listed 188 hazardous air pollutants [exit DNR]. Examples include benzene (found in gasoline), perchlorethlyene (used in some dry cleaning facilities), and methylene chloride.


Indirect Source
A new source or facility which does not directly emit air pollutants. For example, a facility with a new parking lot attracts air contaminants from the exhaust of motor vehicles but does not directly emit the emissions from the surface of the parking lot.
Inspection and Maintenance Program (I/M program)
An auto inspection program that may be required for some non-attainment areas. These inspections, usually done once a year or once every two years, check whether a car is being maintained to keep emissions down and to make sure emission control systems are working properly.


LAER - Lowest Achievable Emission Rate
The lowest pollution control limit that is technically achievable in practice for a particular industry or process. These stringent measures are for controlling air emissions from major sources in non-attainment areas. Economic costs cannot be considered in determining LAER.


MACT - Maximum Achievable Control Technology
The maximum degree of reduction possible for federally regulated hazardous air pollutants. MACT is established by the federal EPA and applies to new and existing sources.
Major Source
Defined in terms of attainment and non-attainment areas for air quality. In attainment areas, a facility or stationary source which emits or is able to emit 100 tons per year (tpy) or more of any criteria air pollutant, or which emits or is able to emit 10 tpy or more of any individual federal hazardous air pollutant, or which emits or is able to emit 25 tpy or more of combined federal hazardous air pollutants. In non-attainment areas, a major source is defined using lower emissions thresholds than would be applied in attainment areas.
Metric ton
A unit of weight equal to 2,204.6 pounds. In the context of global climate change, the metric ton is the unit of choice for measuring emissions. Emission amounts are often expressed in terms of million-metric-tons (MMT).
A unit of length in the metric system where one micron equals one millionth of a meter. To protect public health, air quality regulations refer to various sizes of particle pollution using this measure. For comparison, the average human hair is 70 microns in diameter, whereas fine particles are 2.5 microns or less.
Minor Source
Also defined in terms of attainment and non-attainment areas for air quality. In attainment areas, any stationary source facility that emits less than 100 tons per year (tpy) of any criteria air pollutant, less than 10 tpy of any individual federal hazardous air pollutant, or less than 25 tpy of all federal hazardous air pollutants combined. In non-attainment areas, a minor source is defined using lower emissions thresholds than would be applied in attainment areas.
Mobile Source
Moving sources that release pollution. These consist of on-road sources such as cars, trucks, and buses; and non-road sources such as trains, planes, and boats.
MOP - Mandatory Operation Permit (no longer used)


National Ambient Air Quality Standards (NAAQS)
A level of air quality set by the EPA intended to protect human health and public welfare. Standards have been set for six "criteria" pollutants: carbon monoxide, lead, nitrogen dioxide, particulate matter, ozone, and sulfur oxides.
New Source Review
A federal program ensuring that air quality is not significantly degraded from the addition of new and modified factories, industrial boilers, and power plants.
Non-attainment area
An area classified either by how severely it violates air quality standards or by the type of standard it exceeds. It is possible for an area to be in attainment for one pollutant, but non-attainment for a different pollutant.


The adjective applied to the first facility-wide operation permit issued to a facility. Also the first construction permit issued for a given project.
A gas which is a variety of oxygen. Depending on where it occurs, ozone can be beneficial (as in the upper atmosphere where it protects the Earth from the Sun's ultra-violet rays), or harmful at ground level. Unhealthy ozone is the main component of smog and is created by burning fuels such as gasoline and coal, by the use pf solvents in products such as paints and cleaning liquids, and by industrial processes that produce ozone precursors. Ground level ozone is unhealthy to breathe and can damages trees, crops, corrode masonry, and cause paint to darken.
Ozone Precursor
Any gas in the atmosphere that reacts to form ozone as a byproduct of the reaction. For our purposes, nitrogen oxide compounds (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO) are the gases of most concern.


Particulate Matter
The solid or liquid particles suspended in the air. The smaller the particles the deeper they penetrate portions of the lungs. Particle pollution may affect sensitive people such as children and people with respiratory diseases. There are several classifications of particulate matter based on the diameter of the particle, ranging from - 10 (less than 10 micrometers in diameter) and - 2.5 (less than 2.5 micrometers in diameter).
A document that resembles a license, required by the Clean Air Act for regulated sources of air pollution. Permits are required for both the operation of existing plants (operating permit) and for the construction of new plants or modifications to existing plant (construction permit).
Prevention of Significant Deterioration (PSD)
A federal permit program for facilities defined as major sources under the New Source Review program. The intent of the program is to prevent the air quality in an attainment area from getting worse.
Prevention of Significant Deterioration Increment
The maximum allowable increase in concentration that is allowed to occur above a baseline concentration for a pollutant. The baseline concentration is defined for each pollutant (and relevant averaging time) and, in general, is the ambient concentration existing at the time that the first complete PSD permit application affecting the area is submitted." (USEPA, 1990).
Potential to Emit (PTE)
The maximum amount of any air contaminant that a stationary source may emit, within its physical and operational design.


RACT - Reasonably Available Control Technology
Emission limits set by a state air program on existing facilities in non-attainment areas.
At prescribed intervals, each operation permit will need to be renewed. The renewed operation permit will last another five years.
Any of above can be revised to a significant extent, to a minor extent or for administrative corrections. Revisions retain the same expiration date as the permit they are revising.
ROP - Registration Operation Permit


A mixture of pollutants produced by chemical reactions in the air. A major portion of smog-forming chemicals comes from the burning of petroleum-based fuels such as gasoline.
SOP - State Operation Permit
Also called Natural Minor Permit/Source or Non-Part 70 Permit.
Any device or opening designed or used to emit air contaminants to the ambient air.
Start of Construction
Means to engage in a program of on-site construction, including site clearing, grading, dredging, landfilling, changing equipment, substituting equipment, or even moving the location of equipment specifically designed for a stationary source in preparation for the fabrication, erection or installation of the building components of the stationary source.
State Implementation Plan (SIP)
A detailed description of the programs a state uses to carry out its responsibilities under the Clean Air Act.
Stationary Source
Air pollution sources that remain in one place and cannot relocate to a different area.


Temperature Inversion
One of the weather conditions that are often associated with serious smog episodes in some portions of the country. During a temperature inversion, air doesn't rise because it is trapped near the ground by a layer of warmer air above it. Pollutants, espeically smog and smog-forming chemicals, including volatile organic compounds (VOCs) cannot escape.
Title V
Title V (five) of the 1990 Clean Air Act Amendments requires all major sources and some minor sources of air pollution to obtain an operating permit. Title V operating permits may apply to minor sources if the source emits federally regulated hazardous air pollutants or is subject to some other federal air pollution standard.
An acronym for tons per year. This is the typical measure used to describe emissions from a source.
Total Suspended Particulates refers to particles, typically less than 100 microns in diameter, that can remain suspended in the atmosphere.


Volatile organic compounds (VOCs)
Organic chemical compounds that have high enough vapor pressures under normal conditions to significantly vaporize and enter the atmosphere. VOCs contribute significantly to photochemical smog production and certain health problems. Examples of VOCs include gasoline, industrial chemicals such as benzene, solvents such as toluene and xylene, and tetrachloroethylene (perchloroethylene, the principal dry cleaning solvent). Benzene, another VOC that is also a hazardous air pollutant, causes cancer.
Last revised: Monday April 04 2016