AirToxScreen Glossary of Terms

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"N"-in-1 million cancer risk:

A lifetime cancer risk of "N"-in-1 million (where "N" is a number) means that, for every 1 million people who are continuously exposed to a certain level of a pollutant over 70 years, "N" people may develop cancer. For example, a 1-in-1 million risk means that one person may develop cancer for every 1 million people exposed. A 10-in-1 million risk means 10 people may develop cancer per 1 million exposed, etc. These risks are on top of any other cancer risks from other sources.

Activity pattern data:

Data that depict actual human physical activity, the location of the activity, and the time of day it takes place.

Adverse health effect:

A change in body chemistry, body function or cell structure that could lead to disease or health problems.

Air toxics:

Pollutants known to cause or suspected of causing cancer or other serious health effects. Air toxics are also known as toxic air pollutants or hazardous air pollutants.

Air Toxics Data Update:

EPA’s regular update of air toxics information, including the National Emissions Inventory and AirToxScreen.

Air Toxics Screening Assessment (AirToxScreen):

EPA's ongoing thorough review of air toxics in the United States. AirToxScreen results help scientists focus on pollutants, emission sources, and places that may need further study to better understand risks. AirToxScreen also spurs improvements in what we know about U.S. air toxics. This includes expanding air toxics monitoring, improving and updating emission inventories, improving air quality modeling, driving research on health effects and exposures to both ambient and indoor air, and improving assessment tools. (Note: AirToxScreen is the successor to the National Air Toxics Assessment, or NATA).

AMS/EPA Regulatory Model (AERMOD):

EPA’s preferred model for calculating air pollutant concentrations from most source types. AERMOD simulates how pollutant emissions move and disperse in the air. Learn more about AERMOD.

Ambient:

Surrounding, as in the surrounding environment. In AirToxScreen, ambient air refers to the outdoor air.

Ambient air monitoring:

Process of collecting outdoor air samples to determine how much of an air pollutant is present at a location.

Area sources:

Sources of air pollution that, by themselves, generally have lower emissions than “major sources” of air pollution (like factories). Area sources can include smaller facilities, such as gas stations, or widespread sources like smoke from home fireplaces.

In air quality modeling, area sources are those modeled in two dimensions (with length and width), as compared to point sources modeled at a single point location.

Assessment System for Population Exposure Nationwide (ASPEN):

A computer model that uses dispersion and mapping to estimate toxic air pollutant concentrations. Learn more about ASPEN.

Atmospheric transformation (secondary formation):

The process by which chemicals are transformed into other chemicals in the air (atmosphere). When a chemical is transformed the original pollutant no longer exists; it is replaced by one or more new chemicals. Compared to the original chemical, the transformed chemical(s) can have more, less or the same toxicity. In AirToxScreen, we use both “atmospheric transformation” and “secondary formation;” they mean the same thing.

Background concentrations:

The amount of a pollutant that exists in the air that does not come from a specific source. These pollutants may come from a natural source or from distance sources.  Background concentrations can explain pollutant concentrations found even without recent human-caused emissions.

Biogenic emissions:

Emissions from natural sources. In AirToxScreen, biogenic emissions are those from trees, plants and soil microbes.

Cancer Risk:

The probability of contracting cancer over the course of a lifetime, assuming continuous exposure (assumed in AirToxScreen to be 70 years).

Carcinogen:

A chemical, physical, or biological agent that can cause cancer.

Carcinogenicity:

Ability to produce cancer cells from normal cells.

Chemical Abstracts Service (CAS) number:

A unique number assigned to a chemical by the Chemical Abstracts Service, a service of the American Chemical Society. The purpose is to make database searches easier, as chemicals often have many names.

Census blocks:

Statistical areas bounded by visible features such as roads, streams, and railroad tracks. Generally small in area. In a city, a census block looks like a city block bounded on all sides by streets. Census blocks in suburban and rural areas may be large, irregular, and bounded by a variety of features, such as roads, streams, and transmission lines. In remote areas, census blocks may encompass hundreds of square miles. Census blocks are not delineated based on population, and many census blocks do not have any population.

Census tracts:

Land areas defined by the U.S. Census Bureau. Tracts usually contain from 1,200 to 8,000 people, with most having close to 4,000 people. Census tracts are usually smaller than 2 square miles in cities, but are much larger in rural areas.

Cohort:

A group of people assumed to have identical exposures during a certain period.

Community Multi-scale Air Quality (CMAQ) modeling system:

An air quality model used in AirToxScreen. CMAQ estimates how pollutants move and disperse in the air. It includes the effect of atmospheric chemistry – or how pollutants react in the air. Learn more about CMAQ.

Concentration:

A way to describe how much of a pollutant is in the air. Concentration is usually shown as an amount, or mass, of pollutant per certain volume of air. In AirToxScreen, most concentrations are in micrograms (µg) of air pollutant per cubic meter (m3) of air (a “box” of air one meter on each side).

Consolidated Human Activity Database (CHAD):

An in-depth EPA database of human activity. CHAD includes data from over 20 activity studies dating to 1982. It also includes data from other assessments of human exposure, intake dose, and risk.

Diesel particulate matter:

A mixture of particles that is part of diesel exhaust. EPA lists diesel exhaust as a mobile-source air toxic due to the health effects linked to exposure to whole diesel exhaust.

Dispersion model:

A computerized set of equations that uses emissions and meteorological data to simulate how air pollutants behave and move in the air. A dispersion model estimates outdoor concentrations of individual air pollutants at chosen locations (called receptors).

Emission Inventory System (EIS):

An EPA information system for collecting emission inventory data and generating emission inventories. Learn more about EIS.

Emissions:

Pollutants released into the air.

Emissions inventory

A listing, by source, of the location and amount of air pollutants released into the air during some period (in AirToxScreen, a single year).

Exposure assessment:

An exposure assessment is part of an air toxics risk assessment such as AirToxScreen. The assessment determines (or estimates):

  • how a person may be exposed to chemicals (for example, by breathing);
  • how much of a chemical to which a person is likely to be exposed;
  • how long and/or how often they will be exposed; and
  • how many people are likely to be exposed.

HAP:

Hazardous air pollutant; another name for air toxic.

Hazard index (HI):

The sum of hazard quotients for toxics that affect the same target organ or organ system. Because different air toxics can cause similar adverse health effects, combining hazard quotients from different toxics is often appropriate. A hazard index (HI) of 1 or lower means air toxics are unlikely to cause adverse noncancer health effects over a lifetime of exposure. However, an HI greater than 1 doesn’t necessarily mean adverse effects are likely. Instead, EPA evaluates this on a case-by-case basis. Read more about hazard indexes in Section 6 of the AirToxScreen Technical Support Document.

Hazard quotient (HQ):

The ratio of the potential exposure to a substance and the level at which no adverse effects are expected (calculated as the exposure divided by the appropriate chronic or acute value). A hazard quotient of 1 or lower means chronic adverse noncancer effects are unlikely, and thus can be considered to have negligible hazard. For HQs greater than 1, the potential for adverse effects increases, but we do not know by how much. Read more about hazard quotients in Section 6 of the AirToxScreen Technical Support Document.

Hazardous Air Pollutant Exposure Model (HAPEM):

A computer model designed to estimate inhalation exposure for specified population groups and air toxics. The model uses census data, human-activity patterns, ambient air quality levels, and indoor/outdoor concentration relationships to estimate an expected range of inhalation exposure concentrations for groups of people.

Human Exposure Model (HEM):

A computer model used primarily for conducting inhalation risk assessments for sources emitting air toxics to ambient air.

Inhalation exposure:

Introducing air toxics (or other pollutants) into the body via breathing. Once inhaled, air toxics can be deposited in the lungs, taken into the blood, or both.

Integrated Risk Information System (IRIS):

An EPA program that identifies and characterizes the health hazards of chemicals found in the environment. IRIS is EPA’s preferred source of toxicity information.

Lifetime cancer risk:

The probability of contracting cancer over the course of a lifetime (assumed to be 70 years for the purposes of AirToxScreen).

Major sources:

Defined by the Clean Air Act as facilities that emit or have the potential to emit 10 tons of any one toxic air pollutant or 25 tons of more than one toxic air pollutant per year.

Median:

The middle value of a set of ordered values (i.e., half the numbers are less than or equal to the median value). A median is the 50th percentile of the data.

Metropolitan statistical area (MSA):

A region with a relatively high population density at its core and close economic ties throughout the area. As defined by the U.S. Census Bureau, an MSA must have at least one urban area of 50,000 or more inhabitants.

Microenvironment:

A small space in which human contact with a pollutant takes place. AirToxScreen uses estimated cohort activities in several indoor, outdoor and in-vehicle microenvironments:

Indoor locations:

  • Residence
  • Office
  • Store
  • School
  • Restaurant
  • Church
  • Manufacturing facility
  • Auditorium
  • Healthcare facility
  • Service station
  • Other public building
  • Garage

Outdoor locations:

  • Parking lot/garage
  • Near road
  • Motorcycle
  • Service station
  • Construction site
  • Residential grounds
  • School
  • Sports arena
  • Park/golf course

In-vehicle locations:

  • Car
  • Bus
  • Truck
  • Train/subway
  • Airplane
  • Other

Microgram:

One-millionth of a gram. One gram is about one twenty-eighth of an ounce.

Mobile source:

Air pollution sources that can move from place to place, like cars or trucks. Mobile sources are divided into two categories: on-road and nonroad vehicles/engines.

Monitoring:

See Ambient air monitoring.

Motor Vehicle Emission Simulator (MOVES):

An emissions modeling system that estimates emissions for mobile sources at the national, county, and project level for criteria air pollutants, air toxics, and greenhouse gases.

National-Scale Air Toxics Assessments (NATA):

The predecessor to AirToxScreen. EPA performed six National Air Toxics Assessments for emissions years 1996, 1999, 2002, 2005, 2011 and 2014.

National Emissions Inventory (NEI):

A national database of air emissions data. EPA prepares NEI with input from many state and local air agencies, from tribes, and from industry. This database contains information on stationary and mobile sources that emit criteria air pollutants and their precursors, as well as hazardous air pollutants. NEI includes estimates of annual emissions, by source, of air pollutants in each area of the country. NEI includes emission estimates for all 50 states, the District of Columbia, Puerto Rico, and the U.S. Virgin Islands.

National Mobile Inventory Model (NMIM):

Computer tool containing EPA’s NONROAD model for estimating county level inventories of nonroad mobile emissions.

Noncancer risks:

Risks associated with health effects other than cancer.

Nonroad mobile sources:

Mobile sources not used on roads and highways for transportation of passengers or freight. Nonroad sources include:

  • aircraft;
  • heavy equipment;
  • locomotives;
  • marine vessels;
  • recreation vehicles (snowmobiles, all-terrain vehicles, etc.); and
  • small engines and tools (lawnmowers, etc.). 

On-road mobile sources:

Mobile sources used on roads and highways for transportation of passengers or freight. On-road sources include:

  • passenger cars and trucks;
  • commercial trucks and buses; and
  • motorcycles.

Percentile:

Any one of the points dividing a set of values into parts that each contain 1/100 of the values. For example, the 75th percentile is a value such that 75 percent of the values are less than or equal to it.

Polycyclic organic matter (POM):

A broad class of compounds that form mainly from combustion and are present in the air as particles. Sources of POM emissions include:

  • vehicle exhaust;
  • forest fires and wildfires;
  • asphalt roads;
  • coal;
  • coal tar;
  • coke ovens;
  • agricultural burning;
  • residential wood burning; and

hazardous waste sites.

Reference concentration (RfC):

An estimate of a continuous inhalation exposure unlikely to cause adverse health effects during a person's lifetime. This estimate includes sensitive groups such as children, asthmatics, and the elderly.

Risk:

The probability that adverse effects to human health or the environment will occur due to a given hazard (such as exposure to a toxic chemical or mixture of toxic chemicals). We can measure or estimate some risks in numerical terms (for example, one chance in a hundred).

Rural:

A county is considered “rural” if it does not contain a metropolitan statistical area with a population greater than 250,000 and the U.S. Census Bureau designates 50 percent or less of the population as “urban.”

Science Advisory Board (SAB):

A panel of scientists, engineers, and economists who provide EPA with independent scientific and technical advice.

Secondary formation:

See "Atmospheric transformation (Secondary Formation)"

Secondary sources:

See "Atmospheric transformation (Secondary Formation)"

Sparse Matrix Operator Kernel Emissions (SMOKE):

A modeling system that processes emissions data for use in air quality models. It uses the Biogenic Emission Inventory System (BEIS) to model biogenic emissions. It also has a feature to use MOVES emission factors, activity data and meteorological data to compute hourly gridded on-road mobile emissions.

Stationary sources:

Sources of air emissions that do not move. Stationary sources include large industrial sources such as power plants and refineries, smaller industrial and commercial sources such as dry cleaners, and residential sources such as residential wood burning and consumer products usage.

Susceptibility:

The increased likelihood of an adverse effect. Susceptibility is often discussed in terms of relationship to a factor describing a human population (for example, life stage, demographic feature or genetic trait).

Toxicity-weighting:

A way to prioritize pollutant emissions based on risk. Pollutants that are more harmful have a higher toxicity factor. Toxicity weighting is very useful if the number of pollutants is large, helping risk assessors focus on pollutants that contribute the most to risk.

Typical:

In a risk assessment, this describes the average person doing the types of things (indoors and outdoors) that most people living in an area would do.

Unit risk estimate (URE):

The upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an air toxic at a concentration of 1 microgram per cubic meter (µg/m3) in air. You can interpret the URE as follows: If the URE = 3 x 10-6 per µg/m3, as many as three more people might be expected to develop cancer per one million people exposed daily for a lifetime to 1 microgram (µg) of the chemical in 1 cubic meter (m3) of air. UREs are considered upper-bound estimates designed to keep us from underestimating risks. The true risk may be lower and is considered unlikely to be higher. In AirToxScreen, we multiply the model-output concentrations for an air toxic by that pollutant’s URE to calculate exposure risks from that air toxic.

Upper-bound:

A likely upper limit to the true value of a quantity.

Upper-bound lifetime cancer risk:

A likely upper limit to the true probability that a person will contract cancer over a 70-year lifetime due to a given hazard (such as exposure to a toxic chemical). This risk can be measured or estimated in numerical terms (for example, one chance in a hundred).

Urban:

A county is considered “urban” if it either includes a metropolitan statistical area with a population greater than 250,000 or the U.S. Census Bureau designates more than 50 percent of the population as “urban.” 

Volatile organic compounds (VOCs):

Chemicals emitted as gases from certain solids or liquids. VOCs are known for being common indoor air pollutants. EPA regulates VOCs in the outdoor air because some cause adverse health effects and because they can react with other pollutants to form ozone and secondary air toxics. Cars and trucks, some industries, and even plants and trees emit VOCs.

Weight-of-evidence for carcinogenicity:

Weight-of-evidence for carcinogenicity (WOE):

A system used by the EPA for characterizing the extent to which available data support the hypothesis that an agent causes cancer in humans. The approach, outlined in EPA’s Guidelines for Carcinogen Risk Assessment (2005), considers all scientific information in determining the WOE. Five standard descriptors are used as part of the WOE narrative:

  1. Carcinogenic to humans.
  2. Likely to be carcinogenic to humans.
  3. Suggestive evidence of carcinogenic potential.
  4. Inadequate information to assess carcinogenic potential.
  5. Not likely to be carcinogenic to humans.

Each of these descriptors is explained in its own glossary entry. You can read more details about each of these WOE narratives in the AirToxScreen Technical Support Document.

Carcinogenic to humans:

This descriptor indicates strong scientific evidence of causing cancer in humans This descriptor is appropriate when there is a convincing scientific link between human exposure and cancer.


Likely to be carcinogenic to humans:

This descriptor is appropriate when the weight of evidence is enough to show the potential to cause cancer in humans but does not meet all conditions necessary to be called “carcinogenic to humans.”

Suggestive evidence of carcinogenic potential:

This descriptor is appropriate when the weight of evidence suggests carcinogenicity, raising concern for potential cancer effects in humans, but the data are judged insufficient for a stronger conclusion.

Inadequate information to assess carcinogenic potential:

This descriptor is appropriate when available data are judged inadequate for applying one of the other descriptors. Additional studies generally would be expected to provide further insights.

Not likely to be carcinogenic to humans:

This descriptor is appropriate when the available data are considered strong enough for deciding that there is no basis for cancer concerns for humans.

Weather Research and Forecasting (WRF) model:

A computer weather-prediction system for atmospheric research and weather forecasting. Learn more about WRF.

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Last updated on May 23, 2024