Glossary of Terms

Glossary of Terms

For your convenience, assembled here is a glossary of relevant terms that appear throughout the Montrose Environmental Group website.

New visitors may benefit from reviewing the following and/or tracing the definitions to their source site for additional context.

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The estimation of the human hazard potential of a substance by determining its systemic toxicity in a test system following an acute exposure (e.g., oral, dermal, inhalation). Its assessment has traditionally been based on the median lethal dose (LD50) value. For a substance to have systemic toxic effects it must be absorbed by the body and distributed by the circulation to sites in the body where it exerts toxic effects.


An AQIA is the process of predicting the dispersion of project pollutants based upon site characteristics and local / reginal meteorological conditions. Dispersion results are then overlaid upon existing local / regional ambient pollutant concentrations to predict whether the project could cause or significantly add to an exceedance or air quality standards.

Ambient air monitoring is the systematic, long-term assessment of pollutant levels by measuring the quantity and types of certain pollutants in the surrounding, outdoor air.


“Area” sources are those sources that emit less than 10 tons annually of a single hazardous air pollutant or less than 25 tons annually of a combination of hazardous air pollutants.


The most stringent emission limitation or control technology must be met for new emission sources to obtain permits from an air quality permitting agency. BACT must have been achieved in practice, included in any state implementation plan (SIP) approved by USEPA, and cost effective as compared to measures as listed in a SIP or regulations as adopted by regulatory agencies.

A continuous emission monitoring system (CEMS) is the total equipment necessary for the determination of a gas or particulate matter concentration or emission rate using pollutant analyzer measurements and a conversion equation, graph, or computer program to produce results in units of the applicable emission limitation or standard. CEMS are required, under some EPA regulations, for either continual compliance determinations or determination of exceedances of the standards. (

Combined cycle units utilize the high temperature exhaust gases from a combustion turbine to produce steam, which is then supplied to a steam turbine to generate additional electric power without requiring additional fuel. Combined Cycle Units are nearly twice as efficient as simple cycle plants, but take more time to get to full load.

A combustion turbine draws air into the engine, compresses it, which then flows into the combustors. Fuel (natural gas) is mixed with the compressed air and ignited. The hot exhaust gas then flows into a turbine that is mechanically connected to and drives the compressor. The same shaft also turns the electrical generator.

This is the same technology as used for the engines on a jet airplane. Unlike an airplane, since combustion turbine generator is on the ground where weight, size, and shape are less of a concern, the combustion process and exhaust gases are treated to produce far cleaner engine exhaust.

A system for measuring and reporting on a real-time continuous basis the exhaust emissions to the South Coast Air Quality Management District, the regulatory body that reviews the project and issues the air permit.

The development of adverse effects as the result of long-term exposure to a toxicant or other stressor. It can manifest as direct lethality, but more commonly refers to sublethal endpoints such as decreased growth, reduced reproduction, or behavioral changes such as impacted swimming performance.

The criteria pollutants are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Criteria pollutants are the only air pollutants with national air quality standards that define allowable concentrations of these substances in ambient air.


An approach that routinely examines whole-body chemical concentrations of an exposed organism that is associated with an adverse biological response.

Data acquisition (DAQ) is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable software. Compared to traditional measurement systems, PC-based DAQ systems exploit the processing power, productivity, display, and connectivity capabilities of industry-standard computers providing a more powerful, flexible, and cost-effective measurement solution. (

Dioxins are termed “persistent organic pollutants” (POPs), i.e., they take a long time to break down once in the environment. Dioxins are highly toxic and can cause cancer, reproductive and developmental problems, damage to the immune system, and can interfere with hormones. Dioxins are found throughout the world in the environment and they accumulate in the food chain, primarily in the fatty tissue of animals. More than 90% of human exposure is through food, predominantly meat and dairy products, fish, and shellfish.(

Emission Reduction Credit (ERC) represents eligible emission reductions and may be used to offset an emissions increase in accordance with federal or state New Source Review (NSR) regulations. ERC eligibility is based upon an emission reduction being real, quantifiable and permanent.

To foster innovation and advance new technologies, EPA also reviews, tests, and approves other methods, called Federal Equivalent Methods (FEMs), which are based on different sampling and/or analyzing technologies than FRMs, but are required to provide the same decision-making quality when making NAAQS attainment determinations.


To support monitoring efforts, EPA scientists develop and evaluate methods for accurately and reliably measuring these pollutants in outdoor air. These methods — called Federal Reference Methods (FRMs) — are used by states and other monitoring organizations to assess implementation actions needed to attain the federal Clean Air Act’s National Ambient Air Quality Standards (NAAQS).


Performance audit refers to an independent examination of a program, function, operation or the management systems and procedures of a governmental or non-profit entity to assess whether the entity is achieving economy, efficiency and effectiveness in the employment of available resources. The examination is objective and systematic, generally using structured and professionally adopted methodologies.


Fourier Transform Infrared Spectroscopy (FTIR) identifies chemical bonds in a molecule by producing an infrared absorption spectrum. The spectra produce a profile of the sample, a distinctive molecular fingerprint that can be used to scan samples for different components. FTIR is effective for detecting functional groups and characterizing covalent bonding information.

Furan is a clear, colorless, flammable liquid cyclic ether with an ethereal odor. It is used as an intermediate in the production of tetrahydrofuran, pyrrole, and thiophene. Exposure to furan, which occurs through inhalation, causes eye and skin irritation and central nervous system depression. Furan is mutagenic in animals and is anticipated to be a human carcinogen based on evidence of carcinogenicity in animal studies. (

Chromatography is a technique for separating chemical substances that relies on differences in partitioning behavior between a flowing mobile phase and a stationary phase to separate the components in a mixture.(

Also known as toxic air pollutants or air toxics, are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. (

HAP testing is the determination of the concentration and mass output of specific hazardous air pollutants (HAPs) emitted from a source. This source may be a process vent but is most likely to be the chemically complex emissions from a combustion device.

An analysis conducted to predict the theoretical exposure of a local population to increased acute and chronic health risk, as well as cancer risk. An HRA entails using local meteorological data and project characteristics to predict the dispersion and resulting ambient concentrations of project pollutants.

Is an energy recovery heat exchanger that recovers heat from the hot gas stream of the combustion turbine and produces steam that can then be used to drive a steam turbine

GC-MS combines the features of gas chromatography and mass spectrometry to identify different substances within a sample. Typical applications include drug detection, forensic substance identification and environmental analysis etc.


HPLC is a form of column chromatography that pumps a sample mixture or analyte in a solvent (known as the mobile phase) at high pressure through a column with chromatographic packing material (stationary phase). The sample is carried by a moving carrier gas stream of helium or nitrogen. HPLC has the ability to separate and identify compounds that are present in any sample that can be dissolved in a liquid in trace concentrations as low as parts per trillion.

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The “resolution” is a value that represents the instrument’s ability to distinguish two particles of different masses. The greater the GC instrument’s resolution, the greater its usefulness for analysis. A GC instrument provides more accurate results for larger molecules when the instrument has a high resolution. A high-resolution GC instrument is advisable for analyzing materials with high molecular masses. A low-resolution GC instrument may not sufficiently characterize a large mass substance. (

MS analysis is commonly used in engine exhaust analysis and petroleum product analysis. MS identifies substances by electrically charging the specimen molecules, accelerating them through a magnetic field, breaking the molecules into charged fragments and detecting the different charges.  A spectral plot displays the mass of each fragment.  A technician can use a compound’s mass spectrum for qualitative identification.  The technician uses these fragment masses as puzzle pieces to piece together the mass of the original molecule, the “parent mass.” (

LORS refers to applicable laws, ordinances, regulations and standards. The discussion of LORS is used in environmental impact reports (EIR) to provide the regulatory setting for the impact analysis.

Any stationary source or group of stationary sources located within a contiguous area and under common control that emits or has the potential to emit considering controls, in the aggregate, 10 tons per year or more of any hazardous air pollutant or 25 tons per year or more of any combination of hazardous air pollutants. (

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.

The chemical concentration that is expected to kill 50% of a group of organisms.

The chemical concentration that is expected to have one or more specified effects in 50% of a group of organisms.

A common component of ambient air quality monitoring programs. Data may be used for modeling applications and correlating pollutant measurements with prevailing meteorological conditions and pollutant transport.

This method is applicable for the determination of VOC leaks from process equipment. Sources include but are not limited to valves, flanges, and other connections, pumps and compressors, pressure relief devices, process drains, open-ended valves, pump and compressor seal system degassing vents, accumulator vessel vents, agitator seals, and access door seals.

The Mine Safety and Health Administration (MSHA) is an agency of the United States Department of Labor which administers the provisions of the Federal Mine Safety and Health Act of 1977 (Mine Act) to enforce compliance with mandatory safety and health standards as a means to eliminate fatal accidents, to reduce the frequency and severity of nonfatal accidents, to minimize health hazards, and to promote improved safety and health conditions in the nation’s mines. (

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for pollutants that are common in outdoor air, considered harmful to public health and the environment, and that come from numerous and diverse sources. (

New Source Review (NSR) is a permitting program established by the Congress as part of the 1977 Clean Air Act Amendments.  NSR program protects air quality when emissions sources are newly built or modified.

Cameras for detecting methane, sulfur hexafluoride, and hundreds of other industrial gases quickly, accurately, and safely–without shutting down systems. With FLIR OGI cameras, it is possible to scan broad sections of equipment rapidly and survey areas that are hard to reach with traditional contact measurement tools. OGI cameras can also detect leaks from a safe distance, displaying these invisible gases as clouds of smoke.

Particulate matter (PM), also called particle pollution, is a general term for extremely small particles and liquid droplets in the atmosphere PM2.5 (fine particles): d ≤ 2.5 m (

Particulate matter (PM), also called article pollution, is a general term for extremely small particles and liquid droplets in the atmosphere. PM10 (coarse articles): d ≤ 10 m (

PCBs are a group of man-made organic chemicals consisting of carbon, hydrogen, and chlorine atoms. The number of chlorine atoms and their location in a PCB molecule determine many of its physical and chemical properties. PCBs have no known taste or smell, and range in consistency from an oil to a waxy solid. (

Polycyclic aromatic hydrocarbons (PAHs) are a class of chemicals that occur naturally in coal, crude oil, and gasoline. They also are produced when coal, oil, gas, wood, garbage, and tobacco are burned. PAHs generated from these sources can bind to or form small particles in the air. High-temperature cooking will form PAHs in meat and in other foods. Naphthalene is a PAH that is produced commercially in the United States to make other chemicals and mothballs. Cigarette smoke contains many PAHs. (

Prevention of Significant Deterioration (PSD) is applicable to new major emission sources or major modification at existing sources where the area in which the source is located is in attainment or unclassifiable with the National Ambient Air Quality Standards (NAAQS).  PSD requirements include BACT and an ambient air quality impact analysis.

is required on existing sources in areas that are not meeting national ambient air quality standards (i.e., non-attainment areas). BACT, or Best Available Control Technology, is required on major new or modified sources in clean areas (i.e., attainment areas). (

In SNCR systems, a reagent is injected into the flue gas in the furnace within an appropriate temperature window. Emissions of NOx can be reduced by 30% to 50%. The NOx and reagent (ammonia or urea) react to form nitrogen and water. A typical SNCR system consists of reagent storage, multi-level reagent-injection equipment, and associated control instrumentation. The SNCR reagent storage and handling systems are similar to those for SCR systems. However, because of higher stoichiometric ratios, both ammonia and urea SNCR processes require three or four times as much reagent as SCR systems to achieve similar NOx reductions.


SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The reducing agent reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of carbon dioxide (CO2). The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid, which can be rapidly hydrolyzed to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions in excess of 90%. (

A power plant that uses a combustion turbine to drive a generator to produce electrical power.  Similar to the engines seen under the wing of an airplane, instead of producing thrust to push an airplane through the air, the power is used to drive an electrical generator.  Like the engines on an airplane, the combustion turbine can start quickly and go to full power, several times a day if needed, to quickly provide energy to the electric system.

Steam Turbine Generator is a device that uses the high temperature, high pressure steam from the Heat Recover Steam Generator and expands it through the turbine section that then rotates a shaft driving an electrical generator and producing electricity.  The ‘spent’ steam in then condensed back into water that is then re-used in the Heat Recovery Steam Generator.

A source that otherwise has the potential to emit regulated NSR pollutants in amounts that are at or above the thresholds for major sources in 40 CFR 49.167, 40 CFR 52.21 or 40 CFR 71.2, as applicable, but has taken a restriction so that its PTE is less than such amounts for major sources. Such restrictions must be enforceable as a practical matter (as defined in 40 CFR 49.152). (

A collection of techniques to provide information regarding how much a source (usually a generalized category) contributes to the overall pollutant concentration at receptor (usually a monitoring site). Can be both qualitative and quantitative and can be used for various pollutants. (

Spectrophotometry is a method of measuring how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution. The basic principle is that each compound absorbs or transmits light over a certain range of wavelength. This measurement can also be used to measure the amount of a known chemical substance. Spectrophotometry is one of the most useful methods of quantitative analysis in various fields such as chemistry, physics, biochemistry, material, and chemical engineering and clinical applications. (

The Surface Mining Control and Reclamation Act of 1977 (SMCRA) is the primary federal law that regulates the environmental effects of coal mining in the United States. SMCRA created two programs: one for regulating active coal mines and a second for reclaiming abandoned mine lands. SMCRA also created the Office of Surface Mining, an agency within the Department of the Interior, to promulgate regulations, to fund state regulatory and reclamation efforts, and to ensure consistency among state regulatory programs. (

The Test Methods for Evaluating Solid Waste: Physical/Chemical Methods Compendium, also known as SW-846 or the Compendium, is EPA’s official collection of methods for use in complying with the Resource Conservation and Recovery Act (RCRA) regulations. SW-846 is organized into chapters providing guidance on how to use the methods and groups of methods, called “series”, which are organized by topic. (

The process of auditing and spot checking to verify secure operation of a system and its support software. If irregularities are discovered, the audit process includes analysis and identification of the problem, performing corrective actions necessary to resolve the situation, tracking open items actively, and briefing management on identified security deficiencies. (

Total suspended particles (TSP) is a regulatory measure of the mass concentration of particulate matter (PM) in community air. It was defined by the (unintended) size-selectivity of the inlet to the filter that collected the particles. Unfortunately, the size cut varied with wind speed and direction and was from 20 µm to 50 µm in aerodynamic diameter. Under windy conditions the mass tended to be dominated by large wind-blown soil particles of relatively low toxicity. (

TRI is a resource for learning about toxic chemical releases and pollution prevention activities reported by industrial and federal facilities. TRI data support informed decision-making by communities, government agencies, companies, and others. (

Ultra-Low Emission (ULE) standards were proposed in 2014 to save energy and protect the environment. ULE emission standards for PM are 10mg/Nm3 compared to the prior limit of 30mg/Nm3. In order to decrease emissions this dramatically, adjustments in control devices at power plants must be made.

A vapor recovery unit is a specialized compression package designed specifically to capture low pressure, wet gas streams from oil and condensate storage tanks (