Glossary of Terms

The reduction in degree, intensity, or elimination of pollution.
Acid deposition
A term for the conversion of sulfur oxide and nitrogen oxide emissions into acidic compounds, which precipitate in rain, snow, fog, or dry particles.
Occurring over a short period of time; used to describe brief exposures and effects that appear promptly after exposure.
Adverse health effects
Health effects from exposure to air contaminants that may range from relatively mild temporary conditions, such as minor eye or throat irritation, shortness of breath, or headaches, to permanent and serious conditions such as birth defects, cancer, or damage to lungs, nerves, liver, heart, or other organs.
A particle of solid or liquid matter that can remain suspended in the air because of its small size (generally less than 1 micron).
An air pollution abatement device that removes undesirable organic gases through incineration.
So-called "pure" air is a mixture of gases containing about 78% nitrogen; 21% oxygen; less than 1% of carbon dioxide, argon, and other inert gases; and varying amounts of water vapor.
Air basin
An area defined by geographic or administrative boundaries; used for air pollution control programs.
Air monitoring
Sampling for and measuring of pollutants present in the atmosphere.
Air pollution
The presence of polluting gases and suspended particles in the atmosphere in excess of air quality standards.
Air quality criteria
The varying amounts of pollution and lengths of exposure at which specific adverse effects to health and comfort take place.
Air Quality Management District (AQMD)
Local agency charged with controlling air pollution and attaining air quality standards. The Air District is the regional AQMD that includes all of seven counties (Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo and Santa Clara) and the southern halves of Solano and Sonoma counties.
Air quality plan (AQP)
A plan developed to attain and maintain an air quality standard.
Air Quality Standard (AQS)
The prescribed level of a pollutant in the outside air that should not be exceeded during a specific time period to protect public health. Established by both federal and state governments.
A term denoting a geographical area that, because of topography, meteorology, and climate, shares the same air (see Air Basins).
Ambient air
Outside air; any portion of the atmosphere not confined by walls and a roof.
A hydrocarbon that consists of 1 or more benzenoid rings (i.e., benzene).
A mineral fiber that can pollute air or water and cause cancer or asbestosis when inhaled. EPA has banned or severely restricted its use in manufacturing and construction.
Association of Bay Area Governments (ABAG)
A voluntary joint-power comprehensive regional planning agency for the cities and counties of the Bay Area.
A medical condition characterized by abnormal restriction of breathing, especially in response to allergens or air contaminants.
The layer of life-supporting gases (air) that surrounds the earth.
A designation used when an area meets an air quality standard.
Attrition fire
A fire whose fuel is comprised of dead or pruned tree or bush branches, limbs, and cuttings.
Authority to Construct (A/C)
A pre-construction permit issued by the Air District.
An air pollution abatement device that traps particulates (dust) by forcing gas streams through large permeable bags usually made of glass fibers.
A provision in Air District permit regulations that allows a facility to obtain credits for reducing emissions beyond regulatory limits and use those credits at a later date, similar to how a savings account works.
Bay Area Clean Air Plan
The planning document produced by the Air District identifying all feasible measures for the reduction of ground-level ozone in the Bay Area as mandated by the California Clean Air Act.
Best Available Control Technology (BACT)
An emissions limitation based on using the most up-to-date methods, systems, techniques, and production processes available to achieve the greatest feasible emission reductions. These are the most stringent requirements for new or modified sources and are determined on a case-by-case basis as part of New Source Review.
Best Available Retrofit Control Technology (BARCT)
An emissions limitation based on the maximum degree of reduction achievable for existing sources taking into account environmental, energy, and economic impact.
Bi-fuel vehicle
A vehicle that has the ability to operate on gasoline or diesel as their primary fuel. These types of vehicles are NOT eligible for Air District funding.
Biodiesel is a cleaner-burning diesel replacement fuel made from natural, renewable sources such as new and used vegetable oils and animal fats. Like petroleum diesel, biodiesel operates in compression-ignition engines. Blends of up to 20% biodiesel (mixed with petroleum diesel fuels) can be used in nearly all diesel equipment and are compatible with most storage and distribution equipment. These low level blends (20% and less) don't require any engine modifications and can provide the same payload capacity as diesel. Using biodiesel in a conventional diesel engine substantially reduces emissions.
British Thermal Unit (BTU)
A unit of heat used to describe the capacity of boilers and furnaces. One BTU equals the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit at sea level.
The state agency created in 1991 to oversee the various state environmental agencies.
California Air Resources Board (CARB)
The state agency responsible for air pollution control in California.
California Clean Air Act (CCAA)
State legislation enacted in 1988, and amended in 1992 and 1996, mandating a planning process to attain state ambient air quality standards.
Carbon dioxide (CO2)
A colorless, odorless, non-poisonous gas that results from fossil fuel combustion and is a normal constituent of ambient air.
Carbon monoxide
A colorless, odorless, toxic gas produced by the incomplete combustion of carbon-containing substances. One of the major air pollutants, it is emitted in large quantities by exhaust from gasoline-powered vehicles.
Any substance that can cause or contribute to the production of cancer.
Catalytic converter
An air pollution abatement device used primarily on motor vehicles and other sources. It removes organic contaminants by oxidizing them into carbon dioxide and water through chemical reaction. May convert nitrogen dioxide to nitrogen and oxygen or promote other similar reactions.
A family of inert, nontoxic, and easily liquefied chemicals used in refrigeration, air conditioning, packaging, insulation, or as solvents and aerosol propellants. Because CFCs are not destroyed in the lower atmosphere they drift into the upper atmosphere where their chlorine components destroy the ozone layer.
Marked by long duration or frequent recurrence, as with a chronic disease.
Clean Air Act (CAA)
Long-standing federal legislation that is the legal basis for the national clean air programs, last amended in 1990.
Clean Air Vehicle
A vehicle that does not use gasoline or diesel as its primary fuel and is certified by CARB to meet very stringent tailpipe emission standards.
Coefficient of Haze (COH)
A measurement of the quantity of dust and smoke in the atmosphere in a theoretical 1,000 linear feet of air. A COH of less than 1 is considered clean air and more than 3 is considered dirty air.
Burning, that is, the production of heat and light energy through chemical change, such as the oxidation of hydrocarbon fuel.
Continuous Emission Monitor (CEM)
A type of emissions monitoring device installed to operate continuously inside of a smoke stack or other emissions source.
Criteria pollutants
As required by the Clean Air Act, the EPA identifies and set standards to protect human health and welfare for six pollutants: ozone, carbon monoxide, particulate matter, sulfur dioxide, lead, and nitrogen oxide. The term "criteria pollutants" derives from the requirement that the EPA must describe the characteristics and potential health and welfare effects of these pollutants. EPA periodically reviews new scientific data and may propose revisions to the standards as a result.
An air pollution abatement device that removes heavy particles from an air stream through centrifugal force.
Equipment that removes grease, dirt, or unwanted materials from any part or product. Degreasers typically use solvents, as liquid baths or condensing vapors, to remove such material.
Dew point
The temperature at which droplets of water condense from air (dependent on the prevailing humidity).
Diesel engine
A type of internal-combustion engine that uses low-volatility petroleum fuel and fuel injectors, and initiates combustion using compression ignition (as opposed to spark ignition, which is used with gasoline engines).
Engines that operate on a combination of natural gas and diesel fuel.
Solid particulate matter that can become airborne.
The interrelationship of organisms and their environment, and the science that is concerned with that relationship.
Electrostatic Precipitator
An air pollution abatement device that removes particulate matter from a gas stream by imparting an electrical charge to the particles for mechanical collection on an electrode.
Emission factor
The relationship between the amount of pollution produced and the amount of raw material processed or burned. For example, the emission factor for oxides of nitrogen from fuel oil combustion in an industrial boiler would be the number of pounds of oxides of nitrogen emitted per 1000 gallons of fuel oil burned. By using the emission factor of a pollutant and specific data regarding quantities of material used by a given source, it is possible to compute emissions for the source. This approach is used in preparing an emissions inventory.
Emission inventory
A list of air pollutants emitted into a community's atmosphere in amounts (commonly tons) per day or year, by type of source.
Emission standard
The maximum amount of pollution that is allowed to be discharged from a polluting source. For example, the number of pounds of dust that may be emitted per hour from an industrial process.
Emulsified diesel
Diesel that is mixed with a small percentage of water and an agent that keeps the water and diesel mixed. By adding the water to the diesel, a smaller amount emissions are created when the fuel is burned.
The aggregate of all the external conditions and influences affecting the life, development, and ultimately the survival of an organism. More commonly, the earth's crust, water resources, life forms, and atmosphere.
Environmental Protection Agency (EPA)
The federal agency responsible for control of air and water pollution, toxic substances, solid waste, and cleanup of contaminated sites.
Equivalent opacity
The application of the Ringelmann system to the evaluation of the density of other than black smoke (see Ringelmann).
Ethyl alcohol, a volatile alcohol containing two carbons (CH3CH2OH). For fuel use, it would be produced by fermentation of corn or other plant products.
The physical transformation of a liquid to a gas at any temperature below its boiling point.
A measured level of an air pollutant higher than the national or state ambient air quality standard.
Flexible Fuel Vehicle
Vehicles that can use either alcohol fuels (methanol or ethanol) or a combination of alcohol fuel and unleaded gasoline.
An organic compound that contains fluorine. Some of these compounds may affect health, but they are non-reactive and therefore do not form smog.
Fossil fuels
Coal, oil, and natural gas; called fossil fuels because they are the remains of ancient plant and animal life.
Solid particles under 1 micron in diameter, formed as vapors condense or as chemical reactions take place.
A combustion chamber; an enclosed structure in which fuel is burned to heat air or material.
Greenhouse effect
The warming of the earth's atmosphere caused by a buildup of carbon dioxide and other trace gases. This buildup allows light from the sun's rays to heat the earth but prevents a counterbalancing loss of heat.
Ground-level monitor (GLM)
A type of air pollution monitoring device located around major industrial facilities to measure ambient levels of certain pollutants.
A family of chemical elements that includes fluorine, chlorine, bromine, and iodine.
Halogenated organic compounds
Organic compounds containing one or more atoms of a halogen. These compounds tend to be stable and non-reactive, and therefore have low smog-producing potential.
Health risk
The probability that exposure to a given set of toxic air contaminants will result in an adverse health effect. The health risk is affected by several factors: the amount and toxicity of emissions; the weather; how far sources are from people; the distance between sources; and the age, health and lifestyle of the people living and working at the receptor location. The term "risk" usually refers to the increased chance of contracting cancer as a result of an exposure and is expressed as a probability, e.g., chances-in-a-million.
Health risk assessment
A document that identifies the risks and quantities of possible adverse health effects that may result from exposure to emissions of toxic air contaminants. A health risk assessment cannot predict specific health effects; it only describes the increased possibility of adverse health effects based on the best scientific information available.
Hot Spot
A location where emissions from specific sources may expose individuals and population groups to elevated risks of adverse health effects (including, but not limited to, cancer) and contribute to the cumulative health risks of emissions f...
Any of a vast number of compounds containing carbon and hydrogen in various combinations; found especially in fossil fuels. Some of the hydrocarbon compounds are major air pollutants; they may be active participants in the photochemical process or affect health.
Hydrogen sulfide (H2S)
A gas characterized by a "rotten egg" smell that is often produced by and found in the vicinity of oil refineries, chemical plants, and sewage treatment plants.
The burning of household or industrial waste in a combustion chamber.
Inert gas
A gas such as helium, neon, or argon that does not react with other substances under ordinary conditions.
Inorganic gaseous pollutant
A gaseous pollutant that is not an organic compound. Examples are sulfur dioxide, hydrogen sulfide, and nitrogen oxides.
Internal Combustion Engine (ICE)
An engine in which both the heat energy and the ensuing mechanical energy are produced inside the engine.
The phenomenon of a layer of warm air pressing down on cooler air below it. Inversions are a special problem because they prevent the natural dispersion and dilution of air contaminants.
Lowest Achievable Emission Rate (LAER)
Under the Clean Air Act, the rate of emissions that reflects (a) the most stringent emissions limitation in the state implementation plan identified for a source unless the owner or operator demonstrates such limitations are not achievable or (b) the most stringent emissions limitation achieved in practice, whichever is more stringent.
Major source
A source that emits or has the potential to emit more than 100 tons of any pollutant regulated under the federal Clean Air Act, more than 10 tons of any hazardous air pollutants, or 25 tons of all hazardous air pollutants.
Manual of Procedures (MOP)
A manual of Air District enforcement, permitting, source testing, laboratory, and monitoring procedures used by staff and industry to determine whether industries are meeting Air District regulations. Also contains guidelines for environmental processes under the California Environmental Quality Act (CEQA), and procedures for calculating and generating mobile source emission credits.
Maximum Achievable Control Technology (MACT)
EPA standards mandated by the 1990 amendments to the federal Clean Air Act for the control of toxic emissions from various industries. Industries range from dry cleaners to petroleum refineries.
A single carbon alcohol, generally produced from natural gas (methane).
Metropolitan Transportation Commission (MTC)
The regional agency that provides comprehensive regional transportation planning for the Bay Area and distributes federal and state transportation assistance funds.
A prefix meaning 1/1,000,000. Abbreviated by the Greek letter µ.
A unit of length equal to one thousandth of a millimeter, or about 1/25,000 of an inch.
A prefix meaning 1/1,000.
Liquid particles up to 100 microns in diameter.
Mixing depth
The expanse in which air rises from the earth and mixes with the air above it until it meets air of equal or warmer temperature.
Mobile source
A moving source of air pollution; includes cars, trucks, motorcycles, and airplanes.
National Ambient Air Quality Standards (NAAQS)
Health-based pollutant concentration limits established by EPA that apply to outside air (see Criteria Pollutants).
National Emissions Standards for Hazardous Air Pollutants (NESHAPS)
Emissions standards set by EPA for air pollutants not covered by NAAQS that may cause an increase in deaths or in serious, irreversible, or incapacitating illness; includes toxic emissions such as benzene.
Natural gas
Natural gas is a mixture of hydrocarbons (mainly methane [CH4]) and is produced either from gas wells or in conjunction with crude oil production. Because of the gaseous nature of this fuel, it must be stored onboard a vehicle in either a compressed gaseous state (CNG) or in a liquefied state (LNG).
New Source Performance Standards (NSPS)
Pollutant emission limits for newly constructed sources; defined in the Regulation 10.
New Source Review (NSR)
A permitting procedure for new or modified stationary sources found in Regulation 2. NSR applies if the emissions from the new source are above a trigger level.
Nitric oxide (NO)
Precursor of ozone, nitrogen dioxide (NO2), and nitrate; usually emitted from combustion processes. Converted to NO2 in the atmosphere, it then becomes involved in the photochemical process and/or particulate formation.
Nitrogen oxides (NOx)
Gases formed in great part from atmospheric nitrogen and oxygen when combustion takes place under conditions of high temperature and high pressure; considered a major air pollutant and precursor of ozone.
Nonattainment area
Defined geographic area that does not meet one or more of the federal air quality standards for the criteria pollutants.
Open burning
The uncontrolled burning of waste materials in the open, in outdoor incinerators, or in an open dump, either intentionally or accidentally. Open burning is regulated in the Bay Area.
Organic compounds
A large group of chemical compounds that contain carbon. All living organisms are made up of organic compounds. Some types of organic gases, including olefins, substituted aromatics and aldehydes, are highly reactive -- i.e., have high ozone-producing potential. Standards to control organic compounds are found in the BAAQMD's Regulation 8.
An air pollutant containing oxygen that can react chemically with other substances. Ozone and nitrogen compounds are examples of oxidants.
Ozone (O3)
A pungent, colorless, toxic gas. Close to the earth's surface it is produced photochemically from hydrocarbons, oxides of nitrogen, and sunlight, and is a major component of smog. At very high altitudes, it protects the earth from harmful ultraviolet radiation.
Ozone depletion
Destruction of the stratospheric ozone layer, which shields the earth from ultraviolet radiation. This destruction is caused by the breakdown of certain chlorine and/or bromine-containing compounds (chlorofluorocarbons or halons) that catalytically destroy ozone molecules in the stratosphere.
A particle of solid or liquid matter; soot, dust, aerosols, fumes, and mists.
Parts per million (PPM)
The number of parts of a given pollutant in a million parts of air.
Permit to Operate (P/O)
An operational permit issued annually by the Air District to sources that meet regulation requirements.
Photochemical process
the process by which sunlight acts upon various compounds, causing a chemical reaction to occur.
Photochemical smog
Produced when hydrocarbons and oxides of nitrogen combine in the presence of sunlight to form ozone.
A visible or measurable discharge of a contaminant from a given point of origin that can be measured according to the Ringelmann scale.
PM10 (particulate matter less than 10 microns)
Tiny solid or liquid particles of soot, dust, smoke, fumes, and aerosols. The size of the particles (10 microns or smaller, about 0.0004 inches or less) allows them to easily enter the air sacs in the lungs where they may be deposited, resulting in adverse health effects. PM10 also causes visibility reduction and is a criteria air pollutant.
PM2.5 (particulate matter less than 2.5 microns)
Tiny solid or liquid particles, generally soot and aerosols. The size of the particles (2.5 microns or smaller, about 0.0001 inches or less) allows them to easily enter the air sacs deep in the lungs where they may cause adverse health effects; PM2.5 also causes visibility reduction.
Pollutant Standards Index (PSI)
A system developed by the federal government for reporting air pollution concentrations to the public as numerical values between 0 and 500.
Any number of devices using mechanical, electrical, or chemical means to collect particulates. Used to measure, analyze, or control particulates.
Compounds that change chemically or physically after being emitted into the air and eventually produce air pollutants. For example, organic compounds are precursors for ozone.
Prevention of Significant Deterioration (PSD)
An EPA program in which state and/or federal permits are required to restrict emissions in areas that meet federal standards for criteria pollutants.
A colorless, naturally occurring, radioactive, inert gaseous element formed by the radioactive decay of radium atoms in soil or rocks.
Reactive Organic Gases (ROG)
Classes of organic compounds, especially olefins, substituted aromatics, and aldehydes, that react more rapidly in the atmosphere to form photochemical smog or ozone.
Reasonable further progress (RFP)
A specified rate of progress toward meeting an air quality standard, as set forth in law or in a plan.
Reasonably Available Control Technology (RACT)
Air pollution abatement equipment that is both technologically feasible and cost effective.
Ringelmann Chart
A series of charts, numbered 0 to 5, that simulate various smoke densities by presenting different percentages of black. A Ringelmann No. 1 is equivalent to 20% black; a Ringelmann No. 5 is 100% black. They are used for measuring the opacity or equivalent obscuration of smoke arising from stacks and other sources by matching the actual effluent with the various numbers, or densities, indicated by the charts.
Saturated hydrocarbon
An organic compound consisting of only carbon and hydrogen atoms with no double or triple bonds. Examples are ethane, methane, and propane. They are relatively unreactive, (i.e., do not form photochemical smog as rapidity as other organics).
A device that uses a high energy liquid spray to remove aerosol and gaseous pollutants from an air stream. The gases are removed either by absorption or chemical reaction.
A term used to describe many air pollution problems. Smog is a contraction of smoke and fog; in California, it describes the irritating, stagnant haze resulting from the sun's effect on pollutants in the air.
Very fine carbon particles that appear black when visible.
State Implementation Plan (SIP)
EPA-approved state plans for attaining and maintaining national ambient air quality standards.
Stationary source
A fixed, non-mobile producer of pollution, usually at industrial or commercial facilities.
Storage tank
Any stationary container, reservoir, or tank used for the storage of liquids. District regulations usually only apply to the storage of organic liquids.
The portion of the atmosphere that is 10 to 25 miles above the earth's surface.
Sulfur oxides
Pungent, colorless gases formed primarily by the combustion of sulfur-containing fossil fuels, especially coal and oil. Considered major air pollutants, sulfur oxides may impact human health and damage vegetation.
Title III
A section of the 1990 amendments to the federal Clean Air Act that deals with the control of toxic air emissions.
Title V
A section of the 1990 modifications to the federal Clean Air Act that requires a federally enforceable operating permit for major sources of air pollution.
The configuration of a surface, especially the earth's surface, including its relief and the position of its natural and man-made features.
Total organic gases (TOG)
Gaseous organic compounds, including reactive organic gases and relatively unreactive organic gases, such as methane.
Total suspended particulates (TSP)
Particles of solid or liquid matter (such as soot, dust, aerosols, fumes and mist) up to approximately 30 microns in size.
Toxic air pollutants
Air pollutants that may cause or contribute to an increase in mortality or in serious illness, or which may pose a present or potential hazard to human health.
Toxic Best Available Control Technology (TBACT)
Similar to BACT standards, except it applies to sources of toxic emissions. In many cases, it is the same as BACT. The standards are based on using the most up-to-date methods, systems, techniques, and production processes available to achieve the greatest feasible emission reductions. These are the most stringent requirements for new or modified sources and are determined on a case-by-case basis.
Transportation control measures (TCMs)
Strategies to reduce vehicle trips, vehicle use, vehicle miles traveled, vehicle idling, or traffic congestion for the purpose of reducing motor vehicle emissions.
Transportation Fund for Clean Air (TFCA)
Air District grants to public agencies for eligible transportation projects that reduce emissions from motor vehicles.
The layer of the atmosphere nearest the earth's surface. The troposphere extends outward about 5 miles at the poles and 10 miles at the equator.
Underground Storage Tank
A tank located completely or partially underground that is designed to hold gasoline or other petroleum products or chemical solutions.
Permission granted for a limited time under stated conditions for a person or company to operate outside the limits prescribed in a regulation.
Volatile organic compound (VOC)
An organic compound that evaporates readily at atmospheric temperatures. A major precursor of ozone.
Wood-burning pollution
Air pollution caused by emissions of particulate matter, carbon monoxide, and odorous and toxic substances from wood-burning stoves and fireplaces.

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Last Updated: 3/24/2021