AERA guide | Definitions

For terms used in the AERA guide.

Accidental release: non-routine release to air due to various process upsets such as: start-ups, shutdowns, malfunctions of emission units or air pollution control systems

Acute exposure: Exposure to one or more doses of a contaminant within a short period of time. Acute exposure is evaluated using the maximum ambient air concentration of a contaminant that occurs during one hour.

Agency: Minnesota Pollution Control Agency

Air toxics: pollutants that are known or suspected to cause cancer or other adverse health effects, and may include criteria pollutants that have an inhalation health benchmark in addition to the air quality standard. The MPCA defines air toxics as any pollutant that has a health benchmark from the AERA hierarchy of toxicity information.

AP-42: Air pollutant emission factors from “Compilation of Air Pollutant Emission Factors.” Online at

Allowable / Permitted emissions: (we may be able to get a definition from air permits and as opposed to full potential to emit.)

Background: Background air quality is the general concentration of pollutants in the air, not including the pollutants contributed by the source or sources under review.

Cancer risk: Cancer risk from exposure to air emissions from a given facility is the probability that a hypothetical human receptor will develop cancer based on an assumed set of exposure, model and toxicity assumptions. For example, a risk of 1x10-5 is interpreted to mean that a hypothetical individual has up to a one in 100,000 chance of developing cancer during their lifetime from exposure to pollutants emitted from the facility under evaluation.

Carcinogen: An agent capable of inducing a cancer response. Carcinogenic air toxics may act by initiation, promotion, and conversion.

CAS number: Chemical Abstracts Service registry number. Each chemical has a CAS registry number in order to ensure unique identification.

Ceiling value: Acute HRVs and California Reference Exposure Levels with developmental endpoints need to be considered ceiling values not to be exceeded. Adverse developmental effects could occur upon short term exposure to these chemicals at concentrations above inhalation health benchmarks.

Chemicals of potential interest (COPI): The chemicals known or reasonably expected to be emitted by a facility.

Chronic exposure: Multiple exposures occurring over an extended period of time or a significant fraction of an individual’s lifetime. For the purpose of AERA, chronic exposure is evaluated using an annual averaged ambient air concentration of a contaminant.

Conservatism: In addressing uncertainty and variability, the AERA methodology includes assumptions that may increase risk estimates. This is called “conservatism” and is incorporated into human health risk assessments to ensure that they are as health protective as is reasonable.

Conservative: There are never perfect data sets upon which to base a risk assessment and so many assumptions must be made. In risk assessment, decisions are made that increase risk estimates if there is uncertainty in a value. Risk assessments are completed in this way so that risk management decisions made incorporating this information are protective of public health. This maximizing of risk estimates when there are uncertainties is called “conservatism”. These assumptions are referred to as “conservative”.

Criteria pollutants: The pollutants for which USEPA has established national ambient air quality standards. The criteria pollutants are: particulate matter less than 10 microns in diameter (PM10), particulate matter less than 2.5 microns in diameters (PM2.5), sulfur dioxide (SO2), oxides of nitrogen (NOx), carbon monoxide (CO), ozone, and lead (Pb).

Cumulative risk assessments: An analysis, characterization, and possible quantification of the combined risks to health or the environment from multiple agents or stressors.

Developmental toxicants: Chemicals with acute Health Risk Values (HRVs) and acute Reference Exposure Levels (RELs) with “reproductive/developmental” listed as endpoint of concern (or toxicologic endpoint). Exposure of a developing fetus or newborn to these chemicals for short periods of time during a critical period of development can result in severe adverse effects.

Dispersion factor: a numerical value that represents the proportional relationship between an emissions rate from a stack or vent and the resulting ambient air concentration.

Estimated future actual emissions: The mass of pollutants emitted under an operating scenario that is reflected by some future business case that is not the “potential to emit” for the emissions source or facility.

Facility: a business or source of air emissions

Facility project team: an MPCA review team generally including an air permit engineer, an air dispersion modeling and a risk assessor.

Hazard: numeric value representing the potential for adverse health effects from non-cancerous pollutants; value can be described as a hazard index or hazard quotient, depending on whether a single pollutant or multiple pollutants are represented.

Hazard index: The sum of more than one hazard quotient for multiple substances with the same or similar toxic endpoints. For AERA purposes, at the screening level it is assumed all noncarcinogens have the same or similar toxic endpoint. A hazard index less than one poses no appreciable likelihood of adverse health effects to potential receptors, including sensitive populations.

Hazard quotient: The ratio of a single substance exposure level to an inhalation health benchmark (IHB) for that substance derived from a similar exposure period (e.g., Conc/IHB, where Conc is the air concentration for a particular contaminant, and the IHB is the inhalation health benchmark. A hazard quotient less than one poses no appreciable likelihood of adverse health effects to potential individuals, including sensitive populations, if there are no other pollutants present.

Hazardous Air Pollutants (HAPs): The 188 chemicals identified in the Clean Air Act. The specific list can be found at:

Hierarchy of toxicity data sources: MPCA and MDH guidance for the preferred order for the selection of toxicity data sources. Specific MDH guidance > MDH HRVs > USEPA IRIS > CalEPA (OEHHA) > PPRTV.

Incremental cancer risk: Excess risk to an individual, over background risk of cancer, attributed to lifetime exposure to a cancer-causing chemical.

Inhalation Health Benchmark (IHB): A chronic IHB is a concentration in ambient air at or below which an air toxic is unlikely to cause an adverse health effect to the general public when exposure occurs daily throughout a person’s lifetime. An acute IHB is a concentration in ambient air at or below which an air toxic is unlikely to cause an adverse health effect to the general public when exposure occurs over a prescribed period of time. For implementation purposes, acute IHBs are compared to one-hour averaged concentrations. A subchronic IHB is the concentration in ambient air at or below which the air toxics is unlikely to cause an adverse health effect to the general public when exposure occurs on a continuous basis over a less than lifetime exposure. For implementation purposes, subchronic IHBs are compared to a monthly averaged concentration.

Modification: The definition for “modification” is provided in Minn. R. 7007.0100, subp. 14.

MPCA air pollutant identification numbers: MPCA has developed a system of applying identifying numbers to groups of air toxics that do not have CAS numbers.

Potential to emit: The maximum capacity of a stationary source to emit a pollutant under its physical and operational design. Any physical or operational limitation on the capacity of the source to emit a pollutant, including air pollution control equipment and restriction on hours of operation or on the type and amount of fuel combusted, stored, or processed, shall be treated as part of its design if the limit or the effect it would have on emissions is federally enforceable. (Minn. R. 7055.0100, subp. 35a)

Project: a way of referring to a facility that is undergoing an AERA, this generally refers to the change in the facility that results in a change in air emissions.

Qualitative analysis: Refers to any pertinent information not represented by the estimated “risk” values generated by the RASS. The AERA qualitative analysis may include qualitative, semi-qualitative, and quantitative components.

Quantitative analysis: The estimation of cancer risks and hazard indices using the RASS.

Reference concentration (RfC): Pollutant concentration in air that is expected to cause no harm when exposure occurs on a daily basis for a 70 year lifetime

Risk: Characterizes estimated cancer risks and non-cancer health endpoints.

Speciation: Chemicals are often a part of a larger group or class, such as polyaromatic hydrocarbons (PAHs). Speciation is a process in which individual chemicals emitted at a facility are identified and removed from the larger group or class for individual assessment.

Surrogate: In the AERA process, IHBs for specific chemicals have been applied to compounds, groups, or mixtures containing a fraction of that specific chemical. When a value that was developed for one specific chemical is a applied to other chemicals, that value is known as a surrogate value.

Toxic endpoint: The endpoint of cancer for carcinogens or the organ or physiological system(s) affected by exposure to non-carcinogens. For carcinogenic air toxics, the organ or physiological systems are not differentiated, but all treated as a single endpoint.

Uncertainty: Uncertainty refers to our inability to know for sure - it is often due to incomplete data. For example, when assessing the potential for risks to people, toxicology studies generally involve dosing of sexually mature test animals such as rats as a surrogate for humans. Since we don't really know how differently humans and rats respond, U.S. EPA often employs the use of an uncertainty factor to account for possible differences. Additional consideration may also be made if there is some reason to believe that the very young are more susceptible than adults, or if key toxicology studies are not available. (from U.S. EPA risk assessment glossary)

Unit risk: The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 μg/m3 in air.

Variability: This refers to the range of toxic response or exposure - for example, the dose that might cause a toxic response can vary from one person to the next depending on factors such as genetic differences and preexisting medical conditions. Exposure may vary from one person to the next depending on factors such as where one works, time spent indoors or out, where one lives, and how much people eat or drink. (from U.S. EPA risk assessment glossary)

Volatile organic compounds (VOCs): Compounds identified as participating in atmospheric reactions that contribute to the concentration of ozone in the ambient air. VOC is defined by U.S. EPA definition 40 CFR 51.100(s).