Glossary

The following glossary of terms, provided in part by EPA's Plain English Guide to Clean Air Act, contains definitions for technical words used throughout this Web site.

Absorption - The uptake of water, other fluids, or dissolved chemicals by a cell or an organism (as tree roots absorb dissolved nutrients in soil.)

Acid Deposition - A complex chemical and atmospheric phenomenon that occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall to earth as rain, snow, or fog. The dry forms are acidic gases or particulates.

Acid Rain - Air pollution produced when acid chemicals are incorporated into rain, snow, fog or mist. The "acid" in acid rain comes from sulfur oxides and nitrogen oxides, products of burning coal and other fuels and from certain industrial processes. The sulfur oxides and nitrogen oxides are related to two strong acids sulfuric acid and nitric acid. When sulfur dioxide and nitrogen oxides are released from power plants and other sources, winds blow them far from their source. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to Earth in the rain, snow, fog or mist. In areas where the weather is dry, the acid chemicals may become incorporated into dusts or smokes. Acid rain can damage the environment, human health and property.

Adsorption - Removal of a pollutant from air or water by collecting the pollutant on the surface of a solid material; e.g., an advanced method of treating waste in which activated carbon removes organic matter from waste-water.

Aerosol - 1. Small droplets or particles suspended in the atmosphere, typically containing sulfur. They are usually emitted naturally (e.g. in volcanic eruptions) and as the result of anthropogenic (human) activities such as burning fossil fuels. 2. The pressurized gas used to propel substances out of a container.

Afterburner - In incinerator technology, a burner located so that the combustion gases are made to pass through its flame in order to remove smoke and odors. It may be attached to or be separated from the incinerator proper.

Agricultural Residue - Plant parts, primarily stalks and leaves, not removed from the fields with the primary food or fiber product. Examples include corn stover (stalks, leaves, husks, and cobs); wheat straw; and rice straw.

Air Pollutant - Any substance in air that could, in high enough concentration, harm man, other animals, vegetation, or material. Pollutants may include almost any natural or artificial composition of airborne matter capable of being airborne. They may be in the form of solid particles, liquid droplets, gases, or in combination thereof. Generally, they fall into two main groups (1) those emitted directly from identifiable sources and (2) those produced in the air by interaction between two or more primary pollutants, or by reaction with normal atmospheric constituents, with or without photoactivation. Exclusive of pollen, fog, and dust, which are of natural origin, about 100 contaminants have been identified. Air pollutants are often grouped in categories for ease in classification; some of he categories are solids, sulfur compounds, volatile organic chemicals, particulate matter, nitrogen compounds, oxygen compounds, halogen compounds, radioactive compound, and odors.

Air Pollution - The presence of contaminants or pollutant substances in the air that interfere with human health or welfare, or produce other harmful environmental effects.

Air Pollution Control Device - Mechanism or equipment that cleans emissions generated by a source (e.g. an incinerator, industrial smokestack, or an automobile exhaust system) by removing pollutants that would otherwise be released to the atmosphere.

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Air Quality Standards - The level of pollutants prescribed by regulations that are not being exceeded during a given time in a defined area.

Air Toxics - Any air pollutant for which a national ambient air quality standard (NAAQS) does not exist (i.e. excluding ozone, carbon monoxide, PM-10, sulfur dioxide, nitrogen oxide) that may reasonably be anticipated to cause cancer; respiratory, cardiovascular, or developmental effects; reproductive dysfunctions, neurological disorders, heritable gene mutations, or other serious or irreversible chronic or acute health effects in humans.

Airborne Particulates - Total suspended particulate matter found in the atmosphere as solid particles or liquid droplets. Chemical composition of particulates varies widely, depending on location and time of year. Sources of airborne particulates include dust, emissions from industrial processes, combustion products from the burning of wood and coal, combustion products associated with motor vehicle or non-road engine exhausts, and reactions to gases in the atmosphere.

Alternative Fuels - Fuels that can replace ordinary gasoline, such as compressed natural gas (CNG), alcohols, liquefied petroleum gas (LPG), and electricity. Alternative fuels may have particularly desirable energy efficiency and pollution reduction features. The 1990 Clean Air Act encourages development and sale of alternative fuels.

Ambient Air - Any unconfined portion of the atmosphere open air, surrounding air.

Ambient Air Quality Standards - (See Air Quality Standards.)

Anaerobic Decomposition - Decomposition of cellulose and proteins occurring in the absence of oxygen, such as in landfill waste, producing methane and carbon dioxide. Anaerobic bioreactors increase the rate of methane generation, which can then be collected and used for energy recovery.

Area Source - Any source of air pollution that is released over a relatively small area but which cannot be classified as a point source. Such sources may include vehicles and other small engines, small businesses and household activities, or biogenic sources such as a forest that releases hydrocarbons. May be referred to as nonpoint source.

Attainment Area - A geographic area in which levels of a criteria air pollutant meet the health-based primary standard (national ambient air quality standard, or NAAQS) for the pollutant. An area may have on acceptable level for one criteria air pollutant, but may have unacceptable levels for others. Thus, an area could be both attainment and nonattainment at the same time. Attainment areas are defined using federal pollutant limits set by regulatory agencies.

Baghouse Filter - Large fabric bag, usually made of glass fibers, used to eliminate intermediate and large (greater than PM20 in diameter) particles. This device operates like the bag of an electric vacuum cleaner, passing the air and smaller particles while entrapping the larger ones.

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Best Available Control Technology (BACT) - An emission limitation based on the maximum degree of emission reduction (considering energy, environmental, and economic impacts) achievable through application of production processes and available methods, systems, and techniques. BACT does not permit emissions in excess of those allowed under any applicable Clean Air Act provisions. Use of the BACT concept is allowable on a case-by-case basis for major new or modified emissions sources in attainment areas and applies to each regulated pollutant.

Best Available Retrofit Technology (BART) - Under NAAQS, EPA requires states to make a BART analysis to predict the effect of forcing all plants to install scrubbers. States must then either carry out the forced scrubbing program, or create a state cap and trade system.

Bioaccumulants - Substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized or excreted.

Capture Efficiency - The fraction of organic vapors generated by a process that are directed to an abatement or recovery device.

Carbon Canister Vapor Capture - An automotive filter in which activated carbon has been placed so that gas tank fuel vapors, which have accumulated when the vehicle is not running, are trapped in the filter. When the engine is running, hot air is forced into the filter and pushes out the vapors into the engine. In this way pollution is reduced and fuel conservation is maintained.

Carbon Monoxide (CO) - A colorless, odorless, poisonous gas, produced by incomplete burning of carbon-based fuels, including gasoline, oil and wood. Carbon monoxide is also produced from incomplete combustion of many natural and synthetic products. For instance, cigarette smoke contains carbon monoxide. When carbon monoxide gets into the body, the carbon monoxide combines with chemicals in the blood and prevents the blood from bringing oxygen to cells, tissues and organs. The body's parts need oxygen for energy, so high-level exposures to carbon monoxide can cause serious health effects, with death possible from massive exposures. Symptoms of exposure to carbon monoxide can include vision problems, reduced alertness, and general reduction in mental and physical functions. Carbon monoxide exposures are especially harmful to people with heart, lung and circulatory system diseases.

Cardiovascular Diseases - A group of diseases of the blood vessels that includes coronary heart disease, stroke, and hypertension.

Catalytic Converter - An air pollution abatement device that removes pollutants from motor vehicle exhaust, either by oxidizing them into carbon dioxide and water or reducing them to nitrogen.

Catalytic Incinerator - A control device that oxidizes volatile organic compounds (VOCs) by using a catalyst to promote the combustion process. Catalytic incinerators require lower temperatures than conventional thermal incinerators, thus saving fuel and other costs.

Centrifugal Collector - A mechanical system using centrifugal force to remove aerosols from a gas stream or to remove water from sludge.

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CFCs (chlorofluorocarbons) - Chemicals used in industry for refrigeration and air conditioning, and in consumer products. CFCs and their relatives, when released into the air, rise into the stratosphere, a layer of the atmosphere high above the Earth. In the stratosphere, CFCs and their relatives take part in chemical reactions which result in reduction of the stratospheric ozone layer, which protects the Earth's surface from harmful effects of radiation from the sun. The 1990 Clean Air Act includes provisions for reducing releases (emissions) and eliminating production and use of these ozone-destroying chemicals.

Class I Area - Under the Clean Air Act, an area in which visibility is protected more stringently than under the national ambient air quality standards; includes national parks, wilderness areas, monuments, and other areas of special national and cultural significance.

Clean Air Act - Originally passed in 1963, although the 1970 version of the law is the basis of today�s U.S. national air pollution program. The 1990 Clean Air Act Amendments are the most far-reaching revisions of the 1970 law, and are usually referred to as the 1990 Clean Air Act.

Clean Fuels - Low-pollution fuels that can replace ordinary gasoline. These are alternative fuels, including gasohol (gasoline-alcohol mixtures), natural gas and LPG (liquefied petroleum gas).

Climate Change (also referred to as 'global climate change') - Sometimes used to refer to all forms of climatic inconsistency, but because the Earth's climate is never static, the term is more properly used to imply a significant change from one climatic condition to another. In some cases, 'climate change' has been used synonymously with the term, 'global warming'; scientists however, tend to use the term in the wider sense to also include natural changes in climate.

Combustion - Burning of fuels such as coal, oil, gas, and wood. Many important pollutants, such as sulfur dioxide, nitrogen oxides, and particulates (PM-10) are combustion products.

Computer Controlled Air/Fuel Management System - Maintains the air/fuel ratio in the correct operating range needed for complete combustion without an excess of either air or fuel, resulting in low engine-out emissions and maximized catalyst performance.

Concentration - The relative amount of a substance mixed with another substance. Examples are 5 ppm of carbon monoxide in air and 1 mg/l of iron in water.

Contaminant - Any physical, chemical, biological, or radiological substance or matter that has an adverse effect on air, water, or soil.

Continuous Emission Monitoring Systems (CEMS) - Machines that measure, on a continuous basis, pollutants released by a source. The 1990 Clean Air Act requires continuous emission monitoring systems for certain large sources.

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Control Technology; Control Measures - Equipment, processes or actions used to reduce air pollution. The extent of pollution reduction varies among technologies and measures. In general, control technologies and measures that do the best job of reducing pollution will be required in the areas with the worst pollution. For example, Best Available Control Technology/Best Available Control Measures (BACT/ BACM) are required in serious nonattainment areas for particulates, a criteria air pollutant. A similar high level of pollution reduction will be achieved with maximum achievable control technology (MACT) that will be required for sources releasing hazardous air pollutants.

Criteria Air Pollutants - The 1970 amendments to the Clean Air Act required EPA to set National Ambient Air Quality Standards for certain pollutants known to be hazardous to human health. EPA has identified and set standards to protect human health and welfare for six pollutants ozone, carbon monoxide, total suspended particulates, sulfur dioxide, lead, and nitrogen oxide. The term, "criteria pollutants" derives from the requirement that EPA must describe the characteristics and potential health and welfare effects of these pollutants. It is on the basis of these criteria that standards are set or revised.

Curtailment Programs - Restrictions on operation of fireplaces and woodstoves in areas where these home heat sources make major contributions to pollution.

Cyclone Collector - A device that uses centrifugal force to remove large particles from polluted air.

Data Quality Objectives (DQOs) - Qualitative and quantitative statements of the overall level of uncertainty that a decision-maker will accept in results or decisions based on environmental data. They provide the statistical framework for planning and managing environmental data operations consistent with user's needs.

Diluent - Any liquid or solid material used to dilute or carry an active ingredient.

Eight-Hour Ozone Standards - Since 1971, the U.S. Environmental Protection Agency (EPA) has established national air quality standards for ozone. Revised in 1997, the current national air quality standard for ozone is 0.08 parts per million (ppm), or 80 parts per billion (ppb), averaged over 8 hours. For a given geographic area to be in compliance, its fourth highest 8-hour concentration in a year, averaged over three years, must be equal to or less than that amount.

Electrostatic Precipitator (ESP) - A device that removes particles from a gas stream (smoke) after combustion occurs. The ESP imparts an electrical charge to the particles, causing them to adhere to metal plates inside the precipitator. Rapping on the plates causes the particles to fall into a hopper for disposal.

Emission - Release of pollutants into the air from a source. We say sources emit pollutants. Continuous emission monitoring systems (CEMS) are machines that some large sources are required to install, to make continuous measurements of pollutant release.

Emission Factor - The relationship between the amount of pollution produced and the amount of raw material processed. For example, an emission factor for a blast furnace making iron would be the number of pounds of particulates per ton of raw materials.

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Emission Inventory - A listing, by source, of the amount of air pollutants discharged into the atmosphere of a community; used to establish emission standards.

Enforcement - The legal methods used to make polluters comply with air quality regulations. Enforcement methods include citations of polluters for violations of the law (citations are much like traffic tickets), fines and even jail terms. EPA and the state and local governments are responsible for enforcement of the Clean Air Act, but if they don't enforce the law, members of the public can sue EPA or the states to get action. Citizens can also sue violating sources, apart from any action EPA or state or local governments have taken. Before the 1990 Clean Air Act, all enforcement actions had to be handled through the courts. The 1990 Clean Air Act gave EPA authority so that, in some cases, EPA can fine violators without going to court first. The purpose of this new authority is to speed up violating sources' compliance with the law and reduce court time and cost.

Enteric Fermentation - The natural digestive process in ruminant animals (e.g. cattle and sheep) that produces methane as a by-product.

Eutrophication - The slow aging process during which a lake, estuary, or bay evolves into a bog or marsh and eventually disappears. During the later stages of eutrophication the water body is choked by abundant plant life due to higher levels of nutritive compounds such as nitrogen and phosphorus. Human activities can accelerate the process.

Exhaust Gas Recirculation System (EGR) - The controlled diversion of some of the combustion gases back into the combustion chamber, lowering the combustion temperature and reducing nitrogen oxides in the engine. This is a very effective process, because oxides of nitrogen tend to rise disproportionately with increased combustion temperatures. There are two methods of exhaust gas recirculation internally through overlap of valve opening times and externally with recirculation valves and manifolds.

Exposure - The amount of radiation or pollutant present in a given environment that represents a potential health threat to living organisms.

Fabric Filter - A cloth device that catches dust particles from industrial emissions.

Flow Rate - The rate, expressed in gallons -or liters-per-hour, at which a fluid escapes from a hole or fissure in a tank. Such measurements are also made of liquid waste, effluent, and surface water movement.

Flue Gas - The air coming out of a chimney after combustion in the burner it is venting. It can include nitrogen oxides, carbon oxides, water vapor, sulfur oxides, particles and many chemical pollutants.

Flue Gas Desulfurization (FGD) - A technology that employs a sorbent, usually lime or limestone, to remove sulfur dioxide from the gases produced by burning fossil fuels. Flue gas desulfurization is current state-of-the art technology for major SO2 emitters, like power plants.

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Fossil Fuel - Fuel derived from ancient organic remains; e.g. peat, coal, crude oil, and natural gas.

Fuel Cell - An electrochemical engine (no moving parts) that converts the chemical energy of a fuel, such as hydrogen, and an oxidant, such as oxygen, directly to electricity. The principal components of a fuel cell are catalytically activated electrodes for the fuel (anode) and the oxidant (cathode) and an electrolyte to conduct ions between the two electrodes.

Fugitive Emissions - Emissions not caught by a capture system.

Global Warming - An increase in the near surface temperature of the Earth. Global warming has occurred in the distant past as the result of natural influences, but the term is most often used to refer to the warming predicted to occur as a result of increased emissions of greenhouse gases. Scientists generally agree that the Earth's surface has warmed by about 1 degree Fahrenheit in the past 140 years. The Intergovernmental Panel on Climate Change (IPCC) recently concluded that increased concentrations of greenhouse gases are causing an increase in the Earth's surface temperature and that increased concentrations of sulfate aerosols have led to relative cooling in some regions, generally over and downwind of heavily industrialized areas. (See climate change)

Grain Loading - The rate at which particles are emitted from a pollution source (i.e., number of grains per cubic foot of gas emitted).

Greenhouse Effect - The warming of the Earth's atmosphere attributed to a buildup of carbon dioxide or other gases. Some scientists think that this build-up allows the sun's rays to heat the Earth, while making the infra-red radiation atmosphere opaque to infra-red radiation, thereby preventing a counterbalancing loss of heat.

Greenhouse Gas - A gas, such as carbon dioxide or methane, which contributes to potential climate change.

Hazardous Air Pollutants (HAPs) - Chemicals that cause serious health and environmental effects. Health effects include cancer, birth defects, nervous system problems and death due to massive accidental releases such as occurred at the pesticide plant in Bhopal, India. HAPs are released by sources such as chemical plants, dry cleaners, printing plants, and motor vehicles (cars, trucks, buses, etc.)

Hydrocarbons (HC) - Chemical compounds that consist entirely of carbon and hydrogen.

Hydrogen Sulfide (H2S) - Gas emitted during organic decomposition. Also a by-product of oil refining and burning. Smells like rotten eggs and, in heavy concentration, can kill or cause illness.

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Incineration - A treatment technology involving destruction of waste by controlled burning at high temperatures; e.g., burning sludge to remove the water and reduce the remaining residues to a safe, non-burnable ash that can be disposed of safely on land, in some waters, or in underground locations.

Inhalable Particles - All dust capable of entering the human respiratory tract.

Inspection and Maintenance Program (I/M program) - Auto inspection programs that are required for some polluted areas. These periodic inspections, usually done once a year or once every two years, check whether a car is being maintained to keep pollution down and whether emission control systems are working properly. Vehicles that do not pass inspection must be repaired. Enhanced I/M programs, using special machines to check for such things as how much pollution a car produces during actual driving conditions, are required for some severely polluted areas.

International Air Pollution - Pollution that moves across national borders. The 1990 Clean Air Act includes provisions for cooperative efforts to reduce pollution that originates in one North American country and affects another.

Interstate Air Pollution - Pollution that moves across state borders. Because often, air pollution moves out of the state in which it is produced into another state, the 1990 Clean Air Act includes many provisions, such as interstate compacts, to help states work together to protect the air they share.

Irritant - A substance that causes irritation of the skin, eyes, or respiratory system. Effects may be acute from a single high-level exposure, or chronic from repeated low-level exposures to such compounds as chlorine, nitrogen dioxide, and nitric acid.

Isoconcentration - More than one sample point exhibiting the same isolated concentration.

Kyoto Protocol - An international agreement adopted in December 1997 in Kyoto, Japan.� The Protocol sets binding emission targets for developed countries that would reduce their emissions on average 5.2 percent below 1990 levels.

Lead (Pb) - 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.

Low NOx Burners - One of several combustion technologies used to reduce emissions of nitrogen oxides.

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Lowest Achievable Emission Rate (LAER) - Under the Clean Air Act, the most stringent emission limitation derived from either (1) the most stringent emission limitation in the implementation plan of any state for such source or category of source; or (2) the most stringent emission limitation achieved in practice by such class or category of sources.

Material Safety Data Sheets (MSDS) - Product safety information sheets prepared by manufacturers and marketers of products containing toxic chemicals. These sheets can be obtained by requesting them from the manufacturer or marketer. Some stores, such as hardware stores, may have material safety data sheets on hand for products they sell.

Maximum Available Control Technology (MACT) - The emission standard for sources of air pollution requiring the maximum reduction of hazardous emissions, taking cost and feasibility into account. Under the Clean Air Act Amendments of 1990, the MACT must not be less than the average emission level achieved by controls on the best performing 12 percent of existing sources, by category of industrial and utility sources.

Media - Specific environments--air, water, soil--which are the subject of regulatory concern and activities.

Mercury (Hg) - Heavy metal that can accumulate in the environment and is highly toxic if breathed or swallowed.

Methane - A colorless, nonpoisonous, flammable gas created by anaerobic decomposition of organic compounds. A major component of natural gas used in the home.

Mobile Sources - Moving objects that release pollution; mobile sources include cars, trucks, buses, planes, trains, motorcycles and gasoline-powered lawn mowers. Mobile sources are divided into two groups on-road vehicles, which include cars, trucks and buses, and nonroad vehicles, which includes trains, planes, lawn mowers, and some portable equipment.

Monitoring (monitor) - Measurement of the types and amounts of air pollution. The 1990 Clean Air Act requires states to monitor community air in polluted areas to check on whether the areas are being cleaned up according to schedules set out in the law. Also, the 1990 Clean Air Act requires certain large polluters to perform enhanced monitoring to provide an accurate picture of their pollutant releases such as keeping detailed records, participating in periodic inspections, and installing Continuous Emissions Monitoring Systems (CEMS).

National Emissions Standards for Hazardous Air Pollutants (NESHAPS) - Emissions standards set by EPA for an air pollutant not covered by NAAQS that may cause an increase in fatalities or in serious, irreversible, or incapacitating illness. Primary standards are designed to protect human health, secondary standards to protect public welfare (e.g. building facades, visibility, crops, and domestic animals).

National Low Emissions Vehicle Program (NLEV) - A program that creates voluntary requirements that U.S. automakers can adopt in lieu of compliance with other vehicle emission control measures. The program applies to the manufacture of new light-duty vehicles and new light-duty trucks up to 6,000 lb gross vehicle weight rating (GVWR).

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National Pollutant Release Inventory (NPRI) - A Canadian program, created in 1992, to provide information on pollutants released to the environment and transferred for disposal. Reporting of NPRI is mandated under the Canadian Environmental Protection Act (CEPA) and is due by June 1 of the following year. Each year, Environment Canada updates the NPRI substance list and revises reporting criteria after consultation with stakeholders.

Neurological Disorders - Disorders of the central nervous system (brain, brainstem and cerebellum), the peripheral nervous system (including cranial nerves), and the autonomic nervous system (parts of which are located in both central and peripheral nervous system). Major conditions include, headache; dementia, including Alzheimer�s disease; seizures and epilepsy; sleep disorders; infections; movement disorders such as Parkinson�s disease; and spinal cord disorders.

New Source Performance Standards (NSPS) - U.S. federal standards promulgated for major and minor sources on a category-category basis. NSPS are national emission standards that are progressively tightened over time to achieve a steady rate of air quality improvement without unreasonable economic disruption. The NSPS imposes uniform requirements on new and modified sources through the nation. These standards are based on the best demonstrated technology (BDT).

New Source Review (NSR) - A program used in development of permits for new or modified industrial facilities which are in a nonattainment area, and which emit criteria air pollutants. The two major requirements of NSR are Best Available Control Technology (BACT) and emission offset.

Nitric Oxide (NO) - A gas formed by combustion under high temperature and high pressure in an internal combustion engine. NO is converted by sunlight and photochemical processes in ambient air to nitrogen oxide. NO is a precursor of ground-level ozone pollution, or smog.

Nitrogen Dioxide (NO2) - The result of nitric oxide combining with oxygen in the atmosphere; major component of photochemical smog.

Nitrogen Oxides (NOx) - A criteria air pollutant. Nitrogen oxides are produced from burning fuels, including gasoline and coal. Nitrogen oxides are smog formers, which react with volatile organic compounds to form smog. Nitrogen oxides are also major components of acid rain.

Nonattainment Area - A geographic area in which the level of a criteria air pollutant is higher than the level allowed by the federal standards. A single geographic area may have acceptable levels of one criteria air pollutant but unacceptable levels of one or more other criteria air pollutants; thus, an area can be both attainment and nonattainment at the same time. It has been estimated that 60% of Americans live in nonattainment areas.

Non-Methane Hydrocarbon (NMHC) - The sum of all hydrocarbon air pollutants, excluding methane; significant precursors to ozone formation.

Non-Methane Organic Gases (NMOG) - The sum of all organic air pollutants, excluding methane. NMOG account for aldehydes, ketones, alcohols, and other pollutants that are not hydrocarbons but are precursors of ozone.

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Non-Point Source - Diffuse pollution source (i.e. without a single point of origin or not introduced into a receiving stream from a specific outlet). The pollutants are generally carried off the land by storm water. Common nonpoint sources are agriculture, forestry, urban, mining, construction, dams, channels, land disposal, saltwater intrusion, and city streets. May be referred to as area source.

Non-Road Emissions - Pollutants emitted by combustion engines on farm and construction equipment, gasoline-powered lawn and garden equipment, powerboats, outboard motors, and some portable equipment.

Offset - A method used in the 1990 Clean Air Act to give companies which own or operate large (major) source in a nonattainment area flexibility in meeting overall pollution reduction requirements when changing production processes. If the owner or operator of the source wishes to increase release of a criteria air pollutant, an offset (reduction of a somewhat greater amount of the same pollutant) must be obtained either at the same plant or by purchasing offsets from another company.

Opacity - The amount of light obscured by particulate pollution in the air; clear window glass has zero opacity, a brick wall is 100 percent opaque. Opacity is an indicator of changes in performance of particulate control systems.

Organic Chemicals/Compounds - Naturally occurring (animal or plant-produced or synthetic) substances containing mainly carbon, hydrogen, nitrogen, and oxygen.

Oxidant - A collective term for some of the primary constituents of photochemical smog.

Oxygenated Fuel (oxyfuel) - A special type of gasoline, which burns more completely than regular gasoline in cold start conditions; more complete burning results in reduced production of carbon monoxide, a criteria air pollutant. In some parts of the country, where carbon monoxide release from cars starting up in cold weather makes a major contribution to pollution, gasoline refiners must market oxygenated fuels, which contain higher oxygen content than regular gasoline.

Ozone - A gas that is a variety of oxygen. The oxygen gas found in the air consists of two oxygen atoms stuck together; this is molecular oxygen. Ozone consists of three oxygen atoms stuck together (O3) into an ozone molecule. Ozone occurs in nature; it produces the sharp smell you notice near a lightning strike. High concentrations of ozone gas are found in a layer of the atmosphere -- the stratosphere -- high above the Earth. Stratospheric ozone shields the Earth against harmful rays from the sun, particularly ultraviolet B. Smog's main component is ozone; this ground-level ozone is a product of reactions among chemicals produced by burning coal, gasoline and other fuels, and chemicals found in products including solvents, paints, hairsprays, etc.

Ozone Hole - Thin place in the ozone layer located in the stratosphere high above the Earth. Stratospheric ozone thinning has been linked to destruction of stratospheric ozone by CFCs and related chemicals. The 1990 Clean Air Act has provisions to reduce and eliminate ozone destroying chemicals' production and use. Ozone holes have been found above Antarctica and above Canada and northern parts of the United States, as well as above northern Europe.

Particulates; Particulate Matter (PM-10) - A criteria air pollutant. Particulate matter includes dust, soot and other tiny bits of solid materials that are released into and move around in the air. Particulates are produced by many sources, including burning of diesel fuels by trucks and buses, incineration of garbage, mixing and application of fertilizers and pesticides, road construction, industrial processes such as steel making, mining operations, agricultural burning (field and slash burning), and operation of fireplaces and woodstoves. Particulate pollution can cause eye, nose and throat irritation and other health problems.

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Parts Per Billion (ppb)/Parts Per Million (ppm) - Units commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water, land, or air.

Permit - A document that resembles a license, required by the Clean Air Act for big (major) sources of air pollution, such as power plants, chemical factories and, in some cases, smaller polluters. Usually permits will be given out by states, but if EPA has disap

Permit Fees - Fees paid by businesses required to have a permit. Permit fees are like the fees drivers pay to register their cars. The money from permit fees will help pay for state air pollution control activities.

Photochemical Oxidants or Smog - Air pollutants formed by the action of sunlight on oxides of nitrogen and hydrocarbons.

PM10/PM2.5 - PM10 is measure of particles in the atmosphere with a diameter of less than 10 or equal to a nominal 10 micrometers. PM2.5 is a measure of smaller particles in the air.

Point Source - A stationary location or fixed facility from which pollutants are discharged; any single identifiable source of pollution; e.g. a pipe, ditch, ship, ore pit, factory smokestack.

Pollutant Release and Transfer Register (PRTR) - A Mexican program, created in 2001, requiring mandatory reporting by industrial facilities of potentially hazardous polluted materials emitted or passed into the environment from various media. Similar to the Toxic Release Inventory (TRI) in U.S. and the National Pollutant Release Inventory (NPRI) in Canada.

Precipitator - Pollution control device that collects particles from an air stream.

Precursor - In photochemistry, a compound antecedent to a pollutant. For example, volatile organic compounds (VOCs) and nitrogen oxides react in sunlight to form ozone or other photochemical oxidants. As such, VOCs and oxides of nitrogen are precursors.

Prevention of Significant Deterioration (PSD) - EPA program in which state and/or federal permits are required in order to restrict emissions from new or modified sources in places where air quality already meets or exceeds primary and secondary ambient air quality standards.

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Primary Standard - A pollution limit based on health effects. Primary standards are set for criteria air pollutants.

Quality Assurance/Quality Control (QA/QC) - A system of procedures, checks, audits, and corrective actions to ensure that all research design and performance, environmental monitoring and sampling, and other technical and reporting activities achieve the program�s desired data quality objectives (DQOs).

Reasonably Available Control Technology (RACT) - Control technology that is reasonably available, and both technologically and economically feasible. Usually applied to existing sources in nonattainment areas; in most cases is less stringent than New Source Performance Standards (NSPS).

Reformulated Gasoline - Specially refined gasoline with low levels of smog-forming volatile organic compounds (VOCs) and hazardous air pollutants. The 1990 Clean Air Act requires sale of reformulated gasoline in the nine smoggiest areas in the U.S.

Regional Greenhouse Gas Initiative (RGGI) - A cooperative effort by Northeastern and Mid-Atlantic states to reduce carbon dioxide emissions. The RGGI states will develop a regional strategy by April 2005 for controlling emissions. This strategy will more effectively control interstate transport and international transport of greenhouse gases, and will require electric power generators in participating states to reduce carbon dioxide emissions.

Regional Haze - The haze produced by a multitude of sources and activities, which emit fine particles and their precursors across a broad geographic area. The U.S. regulations require states to develop plans to reduce the regional haze that impairs visibility in Class I areas.

Remote Sensing - The collection and interpretation of information about an object without physical contact with the object; e.g., satellite imaging, aerial photography, and open path measurements.

Respiratory Disease - A disease affecting the respiratory system.

Running Losses - Evaporation of motor vehicle fuel from the fuel tank while the vehicle is in use.

Scrubber - An air pollution device that uses a spray of water or reactant or a dry process to trap pollutants in emissions.

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Secondary Standard - A pollution limit based on environmental effects such as damage to property, plants, visibility, etc. Secondary standards are set for criteria air pollutants.

Smog - A mixture of pollutants, principally ground-level ozone, produced by chemical reactions in the air involving smog-forming chemicals. A major portion of smog-formers comes from burning of petroleum-based fuels such as gasoline. Other smog-formers, volatile organic compounds, are found in products such as paints and solvents. Smog can harm health, damage the environment and cause poor visibility. Major smog occurrences are often linked to heavy motor vehicle traffic, sunshine, high temperatures and calm winds or temperature inversion (weather condition in which warm air is trapped close to the ground instead of rising). Smog is often worse away from the source of the smog-forming chemicals, since the chemical reactions that result in smog occur in the sky while the reacting chemicals are being blown away from their sources by winds.

Source - Any place or object from which pollutants are released. A source can be a power plant, factory, dry cleaning business, gas station or farm. Cars, trucks and other motor vehicles are sources, and consumer products and machines used in industry can be sources too. Sources that stay in one place are referred to as stationary sources; sources that move around, such as cars or planes, are called mobile sources.

Stack - A chimney, smokestack, or vertical pipe that discharges used air.

Stage I Controls - Systems placed on fuel storage tanks to control and capture gasoline vapors during loading of the tanks by delivery trucks.

Stage II Controls - Systems placed on service station gasoline pumps to control and capture gasoline vapors during refueling, including vapor recovery nozzles.

State Implementation Plan (SIP) - A detailed description 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 in nonattainment areas. The Clean Air Act requires that EPA approve each state implementation plan. Members of the public are given opportunities to participate in review and approval of state implementation plans.

Stationary Source - A place or object from which pollutants are released and which does not move around. Stationary sources include power plants, gas stations, incinerators, houses etc.

Stratosphere - Part of the atmosphere, the gases that encircle the Earth. The stratosphere is a layer of the atmosphere 9 to 31 miles above the Earth. Ozone in the stratosphere filters out harmful sun rays, including a type of sunlight called ultraviolet B, which has been linked to health and environmental damage.

Sulfur Dioxide - A criteria air pollutant and gas produced by burning coal, most notably in power plants. Some industrial processes, such as production of paper and smelting of metals, produce sulfur dioxide. Sulfur dioxide is closely related to sulfuric acid, a strong acid. Sulfur dioxide plays an important role in the production of acid rain.

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Temperature Inversion - One of the weather conditions that are often associated with serious smog episodes in some portions of the country. In a temperature inversion, air does not 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, are trapped close to the ground. As people continue driving, and sources other than motor vehicles continue to release smog-forming pollutants into the air, the smog level keeps getting worse.

Toxic Release Inventory (TRI) - Database of toxic releases in the U. S. compiled from SARA Title III Section 313 reports.

Transboundary Pollutants - Air pollution that travels from one jurisdiction to another, often crossing state or international boundaries. Also applies to water pollution.

Troposphere - The layer of the atmosphere closest to the earth's surface.

Ultraviolet B (UVB) - A type of sunlight. The ozone in the stratosphere, high above the Earth, filters out ultraviolet B rays and keeps them from reaching the Earth. Ultraviolet B exposure has been associated with skin cancer, eye cataracts and damage to the environment. Thinning of the ozone layer in the stratosphere results in increased amounts of ultraviolet B reaching the Earth.

United Nations Framework Convention on Climate Change (UNFCCC) - A treaty signed at the 1992 Earth Summit in Rio de Janeiro that calls for the "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."� The treaty includes a non-binding call for developed countries to return their emissions to 1990 levels by the year 2000.� The treaty took effect in March 1994 upon ratification by more than 50 countries.�

Vapor Recovery Nozzles - A special gas pump nozzles that will reduce release of gasoline vapor into the air when people put gas in their cars. There are several types of vapor recovery nozzles, so nozzles may look different at different gas stations. The 1990 Clean Air Act requires installation of vapor recovery nozzles at gas stations in smoggy areas.

Volatile - Any substance that evaporates readily.

Volatile Organic Compounds (VOCs) - Chemicals that produce vapors readily. At room temperature and normal atmospheric pressure, vapors escape easily from volatile liquid chemicals. Volatile organic chemicals include gasoline, industrial chemicals such as benzene, solvents such as toluene and xylene, and tetrachloroethylene (perchloroethylene, the principal dry cleaning solvent). Many volatile organic chemicals are also hazardous air pollutants; for example, benzene causes cancer.

Water Vapor - Water substance in vapor (gaseous) form; one of the most important of all constituents of the atmosphere.

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Did You Know?

EPA estimates that mobile sources of air toxins account for as much as half of all cancers attributed to outdoor sources of air toxins.