• Air Modeling:
    • The use of computer programs, known emissions data, and meteorological data to estimate pollutant concentrations in the area around a source or series of sources.
  • Air Monitoring:
    • The act of collecting physical air samples for analysis at an environmental laboratory or, in some cases, on-site at the location of the monitor.
  • Air Monitoring Network:
    • A collection of air monitoring equipment spread across a region or state whose readings are used to better understand the air quality in that region or state.
  • Air Pollutant:
    • Anything emitted to the air which could have a detrimental effect on human health or the environment.
  • Air Sample:
    • The small collection of ambient air that gets analyzed by a laboratory.
  • Air Toxics:
    • A generic term used to denote any chemical present in the air which could have a toxic effect on a person breathing it. Air toxics are sometimes confused with hazardous air pollutants (HAPs) but HAPs generally only refer to those pollutants specifically mentioned in Title III of the federal Clean Air Act Amendments of 1990.
  • Ambient Air:
    • The air located outside of buildings; outdoor air
  • Animal Study:
    • A toxicity study performed by administering various concentrations of a chemical to a test subject to determine, what, if any, toxic effect the chemical has on a group of animals.
  • Attainment:
    • An area is classified as being in attainment if it meets the air quality standard for a criteria pollutant (under NAAQS).


  • Calibration:
    • The process of testing and adjusting laboratory equipment to help ensure that results from the equipment are accurate.
  • Calibration Curve:
    • Samples with varying known quantities of a pollutant are measured and plotted on a graph. The calibration curve is a straight line that is drawn to be as close to all the data points as possible. This helps ensure that the laboratory instrument is able to accurately measure varying concentrations of a pollutant.
  • Cancer Risk:
    • The estimated likelihood of contracting cancer from a given concentration of a pollutant or group of pollutants. Usually expressed as a decimal number without units. Example: 0.000004 = 4 in a million cancer risk.
  • Carbon Monoxide (CO):
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. It has a molecular formula of CO and contains a single carbon atom bonded to a single oxygen atom. (More Info)
  • Carcinogen:
    • A pollutant that causes cancer. A pollutant can be a known carcinogen, a probable carcinogen, or a possible carcinogen based on how confident scientists are in the information available.
  • Census Block:
    • The smallest grouping used by the U.S. Census Bureau. A census block is usually around 10 houses in size.
  • Census Tract:
    • Land areas defined by the U.S. Census Bureau. Tracts can vary in size but each typically contains about 4,000 residents. Census tracts are usually smaller than 2 square miles in cities, but are much larger in rural areas.
  • Class I Area:
    • A Class 1 area, as defined under the Clean Air Act, is one in which visibility is protected more rigorously than under the national ambient air quality standards. Class I areas include national parks (over 6,000 acres), national wilderness areas, national memorial parks (over 5,000 acres), national monuments, and other areas of special national and cultural significance.
  • Concentration:
    • The amount of pollutant present in a given amount of air. For example: 10 parts per billion volume (ppbv) means that there are 10 parts of the pollutant for every billion parts of total air. 10 µg/m3 means that there are 10 µg (micrograms) of pollutants in every cubic meter of air.
  • Criteria Pollutants:
    • Pollutants for which National Ambient Air Quality Standards (NAAQS) have been established in order to facilitate maintenance at low levels to protect public health. These are Carbon Monoxide, Lead, Nitrogen Dioxide, Sulfur Dioxide, Ozone, and Particulate Matter. (More Info)


  • Emissions:
    • The pollutants emitted from a source into the air.
  • Emissions Inventory:
    • A detailed listing and count of all pollutants emitted by a source.
  • Evacuated:
    • In the context of Summa canisters, evacuated means to have all or most of the air removed so that there is a vacuum inside the canister.
  • Exposure:
    • The amount of contact that a person has with the pollutant. Example: How many days a year they breathe contaminated air.


  • Facility:
    • A building or collection of adjoining buildings which are collectively treated as a single entity for permitting purposes.


  • Gas Chromatograph/Mass Spectrometer:
    • Laboratory instrument used to measure the concentrations of specific pollutants in an air sample.
  • Greenhouse Gases:
    • Gases that trap heat in the atmosphere are called greenhouse gases. The four main greenhouse gases are carbon dioxide, methane, nitrous oxide, and fluorinated gases. (More Info)


  • Hazard:
    • A number indicating whether a non-carcinogenic hazard is possible from a given concentration of a certain pollutant or group of pollutants. A hazard below 1.0 indicates that an effect is unlikely, while 1.0 or above indicates the possibility of an effect.
  • Hazardous Air Pollutant (HAPs):
    • A subset of air toxics that are known to cause cancer and other serious health impacts. These 187 air toxics are regulated by the U.S. EPA and by extension IDEM. (More info)
  • Health-Protective:
    • When uncertainty exists in part of a risk assessment calculation, health protective refers to making choices that result in numbers which are more protective of human health than reality. For example, assuming 24 hours per day / 365 days per year exposure when actual exposure is likely much less.
  • Human Exposure Concentration:
    • A number created for NATA which adjusts the actual modeled air concentrations based on likely human exposure.


  • Laboratory:
    • A place where air samples are analyzed.
  • Lead:
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. Lead is a heavy metal with the atomic symbol Pb. (More Info)


  • Maintenance Plan:
    • Nonattainment areas that are later designated to attainment are considered maintenance areas and must submit a maintenance plan, which outlines the steps to maintain air quality for that area.
  • Mean:
    • The "average" of a set of values. Calculated by adding all values together and dividing by the number of values in the set.
  • micrograms per meter cubed (ug/m3):
    • A measure of the concentration of a pollutant in the air. 1 ug/m3 means that for every cubic meter of air there is 1 microgram of the pollutant present.


  • National Ambient Air Quality Standard (NAAQS):
    • NAAQS are set by the U.S. Environmental Protection Agency (U.S. EPA) for six “criteria” pollutants in order to protect human health and the environment, as required by the Clean Air Act. The six pollutants are carbon monoxide, lead, ozone, particulate matter, nitrogen dioxide, and sulfur dioxide.
  • Neurological Hazard:
    • A number indicating whether a non-carcinogenic neurological hazard is possible from a given concentration of a certain pollutant or group of pollutants. A neurological hazard below 1.0 indicates that an effect is unlikely, while 1.0 or above indicates the possibility of an effect.
  • Nitrogen Oxides (NOx):
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. Nitrogen Oxides play a major role in the formation of ozone, as well as contributing to acid rain. (More Info)
  • Non-Parametric:
    • A set of statistical tools that don't require any assumptions be made about the distribution of the dataset from which a sample came.
  • Nonattainment:
    • An area is classified as being in nonattainment if it fails to meet the air quality standard for a criteria pollutant (under NAAQS).


  • Outlier:
    • A number in a set of numbers which is so much higher or lower than other numbers in the set that it doesn't belong in the set.
  • Ozone (O3):
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. Ozone has the chemical formula O3 and consists of 3 oxygen atoms. Ozone is unique among the criteria pollutants in that very little ozone is emitted directly from sources but is rather the product of secondary formation from other pollutants. (More Info)


  • Particulate Matter (PM):
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. Particulate matter is made up of many different compounds and is divided into categories based on the size of the particles of which it is made. (More Info)
  • Parts per billion (ppb):
    • A measure of the concentration of a pollutant in the air. 1 ppb means that for every one billion parts of air there is one part of the pollutant. For air pollutants this is usually done on a volume basis (ppbv).
  • Primary Standards:
    • Primary standards protect public health, including protecting the health of "sensitive populations" like asthmatics, children, and the elderly.
  • Probabilities:
    • The likelihood of something happening. In air toxics terms, usually refers to the chance of a pollutant concentration causing cancer. Example: the probability of rolling a 1 on a six-sided die is 1-in-6 or 0.167.


  • Quality Assurance:
    • The process by which IDEM maintains a high level of quality in the air toxics monitoring process. This includes regular calibration and maintenance of equipment as well as ensuring that standard procedures are developed and followed.


  • Redesignation:
    • Once an area is designated as attainment, nonattainment, or unclassifiable, it can be redesignated if it meets certain conditions. For example, an area can be redesignated from nonattainment to attainment if the area has monitored attainment of the air quality standard and U.S. EPA has determined that the improvement in air quality is due to permanent and enforceable reductions in emissions, among other requirements.
  • Regulatory Agencies:
    • Governmental agencies responsible for managing a specific aspect of human activity. Example: Indiana Department of Environmental Management (IDEM) and the U.S. Environmental Protection Agency (U.S. EPA) are responsible for managing human activity as it relates to environmental quality.
  • Respiratory Hazard:
    • A number indicating whether a non-carcinogenic respiratory hazard is possible from a given concentration of a certain pollutant or group of pollutants. A respiratory hazard below 1.0 indicates that an effect is unlikely, while 1.0 or above indicates the possibility of an effect.
  • Risk:
    • In environmental terms, generally refers to the risk of developing cancer as the result of exposure to a particular pollutant or group of pollutants. In almost all cases, risks reported are based on very health protective assumptions, and represent a high-end estimate of likely cancer risk.


  • Screening Levels:
    • A set of concentrations calculated which are compared to actual measured concentrations. If a measured concentration exceeds a screening level that indicates that further investigation of that measured concentration may be needed.
  • Secondary Standards:
    • Secondary standards protect public welfare, including protecting against decreased visibility and damage to animals, crops, vegetation, and buildings.
  • Source:
    • A company or facility which emits pollution to the air.
  • Standard:
    • In relation to laboratory equipment, a standard is a sample with a known concentration of a pollutant used to test the accuracy of the equipment. For example, if a standard known to have 1 ppb of benzene is analyzed on a gas chromatograph/mass spectrometer and the instrument reads 1.2 ppb then adjustments can be made to increase the accuracy of the equipment.
  • Standard Deviation:
    • A statistic used to describe how "spread-out" a data set is. Often used in conjunction with the mean to describe a data set. Example: {2,6,10,14,18} and {8,9,10,11,12} both have a mean of 10 but the first set has much more space between the numbers and thus has a larger standard deviation.
  • State Implementation Plan (SIP):
    • A SIP is a document, prepared by individual states and subject to approval by the United States Environmental Protection Agency (U.S. EPA), that identifies actions and programs to be undertaken to control pollution within state boundaries. In part, SIPs detail steps needed to achieve air quality standards and reasons for these steps. Each state has one SIP that contains many smaller documents. A state’s SIP is constantly changing as documents are added to it, updates are made and standards change.
  • Sulfur Dioxide (SO2):
    • One of the criteria pollutants regulated by U.S. EPA under the Clean Air Act. Sulfur Dioxide has the chemical formula SO2 and consists of two oxygen atoms bonded to a sulfur atom. Sulfur Dioxide is a major source of acid rain. (More Info)
  • SUMMA Canisters:
    • Stainless steel sealed canisters used to collect air samples.


  • Toxicity:
    • How strong of a negative health effect a pollutant has. For carcinogenic effects, it refers to a pollutant's likelihood of causing cancer, for non-carcinogenic effects it refers to how high a concentration is necessary to cause an adverse health effect.


  • Unclassifiable Area:
    • An unclassifiable area is any area that cannot be classified (attainment or nonattainment) on the basis of available information as meeting or not meeting a national primary or secondary ambient air quality standard for the pollutant. For instance, there may be incomplete monitoring data available to make a firm classification determination.


  • Volatile Organic Compound (VOC):
    • A chemical in such substances as gasoline fumes and oil-based paints that react with nitrogen oxides in the presence of sunlight to form ozone.