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Air Permits Glossary

This glossary provides plain English, nontechnical 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). See the definition for nonattainment below.
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. The BACT requirements are intended for major sources in attainment areas and are determined case by case. The 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. For example, a paint spray gun or degreasing unit.
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 current 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.
An acronym for elective operation permit. A facility that is otherwise exempt from the requirement to obtain an operation permit may apply for an elective operation permit.
EPA class codes
The EPA and DNR use class codes to establish compliance and enforcement priorities. The DNR assigns codes based on permits issued to the facility and emission rates reported on the facility's most recent emissions inventory. The DNR annual emissions billing rates are based on the class code assigned to a source.
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
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 and foundries.
An acronym for federally enforceable state operation permit. Also called synthetic minor permit or non-part 70 permit.
An acronym for federal operation permit. Also referred to as part 70 permit, title V permit or major source permit.
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.
An acronym for general operation permit. A permit that is issued to a specific industry group with similar applicable requirements. The permit is issued and individual sources that meet eligibility criteria may apply for coverage under the GOP.
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 187 hazardous air pollutants [exit DNR]. Examples include benzene (found in gasoline), perchlorethlyene (used in some dry cleaning facilities) and methylene chloride.
Inspection and Maintenance Program (I/M program)
An auto inspection program that may be required for some nonattainment 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.
An acronym for 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 nonattainment areas. Economic costs cannot be considered in determining LAER.
An acronym for maximum achievable control technology. The maximum degree of reduction possible for federally regulated hazardous air pollutants. MACT is established by the U.S. EPA and applies to new and existing sources.
Major source
In attainment areas, for new source review purposes, it is a facility or stationary source which emits or is able to emit 250 tons per year (tpy) or more of any criteria air pollutant, unless the facility fits within one of 27 source categories listed in ch. NR 405, Wis. Adm. Code, for which the threshold is 100 tpy. In attainment areas, for existing sources requiring an operation permit, it is a facility or stationary source which emits or is able to emit 100 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 nonattainment areas, whether for new or existing sources, a major source may be defined using lower emissions thresholds than would be applied in attainment areas, depending on the nonattainment designation of the area. Refer to the definitions of attainment and nonattainment on this page.
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
A minor source is a facility that emits or is able to emit less than the major source thresholds for the area, depending on whether the area is designated as attainment or nonattainment. See the definition of major source for the thresholds.
Mobile source
Moving sources that release pollution. These consist of on-road sources such as cars, trucks and buses; and nonroad 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 permit program ensuring that air quality is not significantly degraded from the addition of new and modified sources of air pollution, such as: factories, industrial boilers and power plants. Issued to a source for new construction, reconstruction or modification of a source. A permit issued under NSR may also be referred to as a construction permit.
Nonattainment 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 nonattainment 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. It differs from normal oxygen (O2) in having three atoms in its molecule (O3). Depending on where it occurs, ozone can be beneficial (as in the upper atmosphere where it protects the Earth from the sun's ultraviolet rays), or harmful (at ground level). Unhealthy ozone is the main component of smog and is created by burning fossil fuels, such as gasoline and coal, by the use of solvents in products, such as paints and cleaning liquids, and by industrial processes that produce ozone precursors (like NOx, CO and volatile organic compounds (VOCs). 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
Solid or liquid particles suspended in the air. Smaller particles are able to more deeply 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 PM10 (less than 10 micrometers in diameter) and PM2.5 (less than 2.5 micrometers in diameter).
A document that resembles a license or contract, 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, NSR permit, PSD 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. A permit issued under PSD may also be called a construction permit.
Prevention of significant deterioration (PSD) 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 Clean Air Act Amendments).
Potential to emit (PTE)
The maximum amount of any air contaminant that a stationary source may emit, when considering its physical and operational design limitations.
An acronym for reasonably available control technology. The RACT is a pollution control standard defined by the EPA to establish air pollution control technology that is considered reasonably available, considering technological and economic feasibility, for an industry sector to control a specific pollutant to a specified limit. The RACT becomes an emission limit or control requirement set by a state air program for new and existing facilities in nonattainment areas.
Each major source operation permit will need to be renewed, generally before the five-year term expires. A timely renewal application can extend the term of the permit until the department issues a renewed permit. A renewed operation permit will have a term of five years. Minor and synthetic minor operation permits do not have an expiration date and therefore are not required to be renewed, although the department may require, or the facility may request, an expiration date for cause.
Revisions are changes to a permit that do not meet the definition of construction, reconstruction or modification. Any permits defined on this page 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.
An acronym for registration operation permit. An ROP is a permit issued to sources with low actual or potential emissions. There are multiple ROPs available at different ranges of emissions thresholds. The permit is issued and individual sources that meet eligibility criteria may apply for coverage under an ROP.
An acronym for reactive organic gas. See definition of volatile organic compounds. The EPA formerly defined the regulated organic compounds in outdoor air as reactive organic gases (ROG) to clarify its meaning as being limited to reactive chemicals. The EPA later changed that terminology to volatile organic compounds (VOC); however, references to ROG remain in older state and federal rules. For the purposes of state air pollution regulations, ROGs can be considered equivalent to VOCs.
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.
An acronym for state operation permit. Also called natural minor permit 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.
Synthetic minor
A source with the potential to emit above major source thresholds but accepts permit limitations to restrict emissions below those thresholds in order to be issued a minor source permit. May also be called FESOP. Synthetic minor sources can be categorized as either SM or SM80. An SM80 source has emissions restricted at a rate of at least 80% of the major source threshold, whereas SM source emissions are restricted below 80% of the major source threshold.
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, especially 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. May also be called FOP or major source.
An acronym for tons per year. This is the typical measure used to describe emissions from a source.
An acronym for 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. The 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.

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