Accuracy – Closeness of a measured or computed value to its “true” value, where the true value is obtained with perfect information. Due to the natural heterogeneity and stochasticity of many environmental systems, this true value exists as a distribution rather than a discrete value. In these cases, the true value will be a function of spatial and temporal aggregation.
Acid Deposition – A comprehensive term for the various ways acidic compounds precipitate from the atmosphere and deposit onto surfaces. It can include (1) wet deposition by means of acid rain, fog, and snow; and (2) dry deposition of acidic particles (aerosols).
Acute Exposure – One or a series of short-term exposures generally lasting less than 24 hours.
Acute Health Effect – A health effect that occurs over a relatively short period of time (e.g., minutes or hours). The term is used to describe brief exposures and effects that appear promptly after exposure.
Air Toxics – Also known as toxic air pollutants or hazardous air pollutants (HAPs) are those pollutants known to or suspected of causing cancer or other serious health problems. The Clean Air Act Amendments of 1990 listed 189 of these air toxics as HAPs because of their potential to be carcinogens, respiratory toxicants, neurotoxicants, or cause other harmful effects. They are differentiated from criteria air pollutants under the air quality management system laid out by the Clean Air Act.
Algorithm – A set of mathematical steps or procedures used for solving a problem.
Ambient Air – The air outside of structures. Often used interchangeably with “outdoor air.”
Analytical Models – Models that can be solved mathematically in closed form. For example, some model algorithms that are based on relatively simple differential equations can be solved analytically to provide a single solution.
Application Niche – The set of conditions under which the use of a model is scientifically defensible.
Bayesian analysis – An approach to statistical analysis that is based on Bayes’s Theorem, which states that the posterior probability of a parameter p is proportional to the prior probability of parameter p multiplied by the likelihood of p derived from the data collected. The Bayesian approach attempts to keep track of how a priori expectations about some phenomenon of interest can be refined and how observed data can be integrated with such a priori beliefs, to arrive at updated posterior expectations about the phenomenon. The Bayesian approach to decision making incorporates new information or data into the decision process. It allows the analyst to use both sample (data) and prior (expert-judgment) information in a logically consistent manner in making inferences. As further information becomes available, the original assumptions are refined and corrected.
Bias – Systematic deviation between a measured (observed) or computed value and its “true” value. Bias is affected by faulty instrument calibration and other measurement errors, systematic errors during data collection, and sampling errors, such as incomplete spatial randomization during the design of sampling programs.
Biologically Based Dose-Response (BBDR) Model – A predictive model that describes biological processes at the cellular and molecular level linking the target organ dose to the adverse effect. BBDR models predict dose-response relationships on the basis of principles of biology, pharmacokinetics, and toxicology.
Boundaries – The spatial and temporal conditions and practical constraints under which environmental data are collected. Boundaries specify the area or volume (spatial boundary) and the time period (temporal boundary) to which a decision will apply.
Boundary Conditions – The physical conditions at the boundaries of a system or at the edges of the region being modeled.
Calibration – The process of adjusting model parameters within physically defensible ranges until the resulting predictions give the best possible fit to the observed data.
Catalytic Converter – A mobile-source emissions-control device designed to reduce emissions of nitrogen oxides, hydrocarbons, and carbon monoxide.
Chronic Exposure – Long-term exposure usually lasting 1 year to a life-time.
Chronic Health Effect – A health effect that occurs over a relatively long period of time (e.g., months or years).
Clean Air Act (CAA) – Federal legislation administered by EPA that serves as the primary means of regulating ambient air quality in the United States. The original Clean Air Act in the United States was passed in 1963, but most of the national air pollution control program is based on the 1970 version of the law. The 1990 Clean Air Act Amendments are the most recent revisions of the law.
Clean Water Act (CWA) – Federal legislation administered by EPA that serves as the primary means of regulating the surface water quality of the United States. The original legislation was passed in 1972 as the Federal Water Pollution Control Act and became known as the Clean Water Act after Congress passed amendments to it in 1977.
Code – Instructions, written in the syntax of a computer language, which provide the computer with a logical process. Code may also be referred to as “computer program.” The term “code” describes the fact that computer languages use a different vocabulary and syntax than algorithms that may be written in standard language.
Code of Federal Regulations (CFR) – Document that codifies all rules of the executive departments and agencies of the federal government. It is divided into 50 volumes, known as titles. Title 40 of the CFR (referenced as 40 CFR) lists all environmental regulations.
Community Multiscale Air Quality (CMAQ) Model – An air quality model designed to simulate and model a wide range of physical and chemical processes relating to air quality at particular scales in the lower atmosphere over a regional and subregional scale.
Computational Model – A model that is expressed in formal mathematics using equations, statistical relationships, or a combination of the two. Although values, judgment, and tacit knowledge are inevitably embedded in the structure, assumptions, and default parameters, computational models are inherently quantitative, relating phenomena through mathematical relationships and producing numerical results.
Computational Toxicology – The application of mathematical and computer models to predict the effect of an environmental agent and elucidate the cascade of events that result in an adverse response. It uses technologies developed in computational chemistry (computer-assisted simulation of molecular systems), molecular biology (characterization of genetics, protein synthesis, and molecular events involved in biological response to an agent), bioinformatics (computer-assisted collection, organization, and analysis of large data sets of biological information), and systems biology (mathematical modeling of biological systems and phenomena). The goals of using computational toxicology are to set priorities among chemicals on the basis of screening and testing data and to develop predictive models for quantitative risk assessment.
Conceptual Model – An abstract representation that provides the general structure of a system and the relationships within the system that are known or hypothesized to be important. Many conceptual models have as a key component a graphical or pictorial representation of the system.
Contaminant – A substance that is either present in an environment where it does not belong or is present at levels that might cause harmful (adverse) health effects.
Corroboration (Model) – Quantitative and qualitative methods for evaluating the degree to which a model corresponds to reality. In some disciplines, this process has been referred to as validation. In general, the term “corroboration” is preferred because it implies a claim of usefulness and not truth.
Criteria Air Pollutants – An air pollutant for which National Ambient Air Quality Standards have been set. There are six common air pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide) that have been designated as criteria pollutants. The Clean Air Act states that the presence of criteria pollutants in the ambient air results from numerous or diverse mobile or stationary sources.
Cumulative Risk – The combined risks from aggregate exposures to multiple agents or stressors.
Design Standard – A technology-based standard that requires emitters to use a specific technology to control emissions of a pollutant. These can also be called engineering standards.
Deterministic Model – A mathematical model that contains no random (stochastic) components; consequently, each component and input is determined exactly. Because this type of model does not explicitly simulate the effects of data uncertainty or variability, changes in model outputs are solely due to changes in model components.
Domain (Spatial and Temporal) – The limits of space and time that are specified within a model’s boundary conditions (see Boundary Conditions).
Domain Boundaries (Spatial and Temporal) – The spatial and temporal domain of a model are the limits of extent and resolution with respect to time and space for which the model has been developed and over which it should be evaluated.
Dose – The amount of a contaminant that is absorbed or deposited in the body of an exposed person for an interval of time—usually from a single medium. Total dose is the sum of doses received by interactions with all environmental media that contain the contaminant. Units (mass) of dose and total dose are often converted to units of mass or contaminant per volume of physiological fluid or mass of tissue.
Dose-Response Relationship – The relationship between a quantified exposure (or dose) and a quantified effect
Emission Rate – The weight of a pollutant emitted per unit of time (e.g., tons/year).
Emissions Budget – Allowable emissions levels identified as part of a state implementation plan for pollutants emitted from mobile, industrial, stationary, and area sources. These emissions levels are used for meeting emission-reduction milestones, attainment, or maintenance demonstrations.
Emissions Factor – For mobile sources, the emission factor is the relationship between the amount of pollution produced and the number of vehicle miles traveled. For stationary sources, the relationship between the amount of pollution produced and the amount of raw material processed or burned. By using the emission factor of a pollutant and specific data regarding activities (quantities of materials used by a given source or number of miles traveled), it is possible to compute emissions for the source.
Emissions Inventory – An estimate of the amount of a pollutant emitted into the atmosphere from major mobile, stationary, areawide, and natural sources over a specific period of time, such as a day or a year.
Empirical Model – An empirical model is one where the structure is determined by the observed relationship among experimental data. These models can be used to develop relationships that are useful for forecasting and describing trends in behavior but may not necessarily be mechanistically relevant.
Environmental Regulatory Model – A computational model used to inform the environmental regulatory process. Some models are independent of a specific regulation, such as water quality or air quality models that are used in an array of application settings. Other models are created to provide a regulation-specific set of analyses completed during the development and assessment of specific regulatory proposals. The approaches can range from single parameter linear relationship models to models with thousands of separate components and many billions of calculations.
Epidemiology – The study of the distribution and determinants of disease or health status in a population; the study of the occurrence and causes of health effects in humans.
Evaluation (Model) – The process used to generate information to determine whether a model and its results are of a quality sufficient to serve as the basis for a regulatory decision.
Evaporative Emissions – Hydrocarbon emissions that do not come from the tailpipe of a car but come from evaporation, permeation, seepage, and leaks in a vehicle’s fueling system. The term is sometimes used interchangeably with nontailpipe emissions.
Ex Ante – Analysis of the effects of a policy based only on information available before the policy is undertaken. Also termed prospective analysis.
Ex Post – Analysis of the effects of a policy based on information available after the policy has been implemented and its performance observed. Also termed retrospective analysis.
Exceedance – An air pollution event in which the ambient concentration of a pollutant exceeds the National Ambient Air Quality Standards.
Expert Elicitation – A process for obtaining expert beliefs about subjective quantities and probabilities. Typically, structured interviews and questionnaires are used to elicit the necessary knowledge. Expert elicitations may also include “coaching” techniques to help the expert conceptualize, visualize, and quantify the knowledge being sought.
Exposure – Contact between an agent and a target. Contact takes place at an exposure surface over an exposure period, which is the time of continuous contact between an agent and a target
Exposure Assessment – The process of characterizing the magnitude, frequency, and duration of exposure to an agent, along with the number and characteristics of the population exposed. Ideally, it describes the sources, pathways, routes, and uncertainties in the assessment.
Exposure Pathway – The course a substance takes from its source (where it began) to its end point (where it ends), and how people can come into contact with (or get exposed to) it. An exposure pathway has five parts: a source of contamination (such as an abandoned business); an environmental media and transport mechanism (such as movement through groundwater); a point of exposure (such as a private well); a route of exposure (eating, drinking, breathing, or touching), and a receptor population (people potentially or actually exposed). When all five parts are present, the exposure pathway is termed a completed exposure pathway.
Genomics – The study of genes and their function.
Greenhouse Gas – Atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, ozone, and water vapor that slow the passage of re-radiated heat through the earth’s atmosphere.
Hazard Assessment – The process of determining whether exposure to an agent can cause an increase in the incidence or severity of a particular health effect (e.g., cancer, birth defect).
Hazardous Air Pollutants (HAPs) – Air toxics listed under section 112(b) of the Clean Air Act Amendments of 1990.
Informatics (Bioinformatics) – The science of managing and analyzing vast amounts of biological data using advanced computing techniques. Especially important in analyzing genomic research data.
Marginal Benefit – The additional benefit gained from one more unit of output. In terms of reducing emissions, it represents the benefits from reducing emissions by one more unit.
Marginal Cost – The additional cost associated with producing one more unit of output. In terms of reducing emissions, it represents the cost of reducing emissions by one more unit.
Model – A simplification of reality that is constructed to gain insights into select attributes of a particular physical, biological, economic, or social system. Models can be of many different forms. They can be computational. Computational models include those that express the relationships among components of a system using mathematical relationships. They can be physical, such as models built to analyze effects of hydrodynamic or aeronautical conditions or to represent landscape topography. They can be empirical, such as statistical models used to relate chemical properties to molecular structures or human dose to health responses. Models also can be analogs, such as when nonhuman species are used to estimate health effects on humans. And they can be conceptual, such as a flow diagram of a natural system showing relationships and flows among individual components in the environment or a business model that broadly shows the operations and organization of a business. The above definitions are not mutually exclusive. For example, a computational model may be developed from concep-
tual and physical models, and an animal analog model can be the basis for an empirical model of human health impacts.
Module – An independent or self-contained component of a model that is used in combination with other components and forms part of one or more larger programs.
National Ambient Air Quality Standards (NAAQS) – Standards set by EPA for the maximum levels of criteria air pollutants that can exist in the outdoor air without adverse effects on human health or the public welfare. There are four elements of a NAAQS (1) the pollutant indicator (such as PM2.5), (2) the concentration of the indicator in the air, (3) the time over which measurements are made or averaged, and (4) the statistical form of the standard used to determine the allowable number of exceedances (such as the fourth highest value over a 3-year period).
National Pollution Discharge Elimination System (NPDES) – Federal regulations that regulate discharge of wastewater to surface waters, such as streams, rivers, lakes, and estuaries. An NPDES permit is required for any project involving the construction, alteration, and/or operation of any sewer system, treatment works, or disposal system and for construction of certain storm water runoff structures that would result in a discharge into surface waters.
Nonattainment Area – A geographic area designated by EPA to have concentrations of a criteria pollutant in excess of the NAAQS. A single geographic area may have acceptable levels of some criteria air pollutants but unacceptable levels of others; thus, an area can be both an attainment area for one pollutant and a nonattainment area for another.
Nonpoint Source Pollution – Sources of water pollution not associated with a distinct discharge source; includes rainwater, erosion, run-off from roads, farms, and parking lots, and seepage from soil-based wastewater disposal systems.
Parameters – Terms in the model that are fixed during a model run or simulation but can be changed in different runs as a method for conducting sensitivity analysis or to achieve calibration goals.
Photochemical Reaction – A term referring to a chemical reaction brought about by sunlight, such as the formation of ozone from the interaction of oxygen and nitrogen oxides and/or hydrocarbons in the presence of sunlight.
Physiologically Based Pharmacokinetic (PBPK) Model – A model that estimates the dose to a target tissue or organ by taking into account the rate of absorption into the body, distribution among target organs and tissues, metabolism, and excretion.
Plume – A volume of a substance that moves from its source to places farther away from the source. Plumes can be described by the volume of air or water they occupy and the direction they move. For example, a plume can be a column of smoke from a chimney or a substance moving with groundwater.
Point Source Pollution – A specific discharge to a water body, ambient air, or land that is traceable to a distinct source (e.g., pipe, smokestack, and container) such as those from wastewater treatment plants, power plants, or industrial facilities.
Precision – The quality of being reproducible in amount or performance. With models and other forms of quantitative information, precision refers specifically to the number of decimal places to which a number is computed as a measure of the preciseness or exactness with which a number is computed.
Proteomics – The study of the full set of proteins encoded by a genome.
Regulatory Impact Analysis (RIA) – An analysis document produced by EPA for each major rulemaking listing the expected impacts of the rule, including environmental impacts, health impacts, cost–benefit analyses, economic impacts, and small business impacts.
Reliability – The confidence that (potential) users have in a model and in the information derived from the model such that they are willing to use the model and the derived information. Specifically, reliability is a function of the performance record of a model and its conformance to best available, practicable science.
Risk Assessment (in the context of human health) – The evaluation of scientific information on the hazardous properties of environmental agents (hazard identification), the dose-response relationship (dose-response assessment), and the extent of human exposure to those agents (exposure assessment). The product of the risk assessment is a statement describing the populations or individuals that are likely to be harmed and to what degree (risk characterization).
Risk Characterization (in the context of human health) – The integration of information on hazard, dose-response, and exposure to provide an estimate of the likelihood that any of the identified adverse effects will occur in exposed people.
Risk Management (in the context of human health) – A decision-making process that accounts for political, social, economic, and engineering implications together with risk-related information to develop, analyze, and compare management options and select the appropriate managerial response to a potential adverse health risk.
Robustness – The capacity of a model to perform equally well across the full range of environmental conditions for which it was designed.
Safe Drinking Water Act (SDWA) – Legislation to ensure safe drinking water. Passed by Congress in 1974 and amended in 1986, it directs EPA to establish and enforce water quality standards to protect public health.
Screening Model – A type of model designed to provide a “conservative” or risk-averse answer. Because screening models can be used with limited information and are conservative, they can be used to determine whether more refined models would be useful or whether the screening model results are sufficient to make decisions without proceeding to a refined model.
Sensitivity – The degree to which the model outputs are affected by changes in a selected input parameters.
State Implementation Plan (SIP) – A detailed description of the scientific methods and emission-reduction programs a state will use to carry out its responsibilities under the Clean Air Act for complying with the NAAQS. The Clean Air Act requires that EPA approve each SIP after the public has had an opportunity to participate in its review and approval.
Stochastic Model – A model that includes variability (see definition) in model parameters. This variability is a function of (1) changing environmental conditions, (2) spatial and temporal aggregation within the model framework, and (3) random variability. The solutions obtained by the model or output is therefore a function of model components and random variability.
Susceptibility – Increased likelihood of an adverse effect, often discussed in terms of relationship to a factor that can be used to describe a human subpopulation (e.g., life stage, demographic feature, and genetic characteristic).
Susceptible Subgroups – May refer to life stages (e.g., children and the elderly) or to other segments of the population (e.g., people who have asthma or who are immune compromised), but are likely to be chemical-specific and may not be consistently defined in all cases.
Technology-Based Standards – A type of standard that dictates polluters use specific techniques (e.g., a particular type of pollution abatement equipment) or follow a specific set of operating procedures and practices.
Technology Forcing – The establishment by a regulatory agency of a requirement to achieve an emissions limit, within a specified time frame, that can be reached through use of unspecified technology or technologies that have not yet been developed for widespread commercial applications and have been shown to be feasible on an experimental or pilot-demonstration basis.
Total Maximum Daily Load (TMDL) – The total waste (pollutant) loading from point and nonpoint sources that a water body can assimilate while still maintaining its water quality classification and standards.
Toxic Substances Control Act (TSCA) – Federal legislation administered by EPA that regulates the manufacture, labeling, and distribution of chemicals outside of pesticides and drugs. It requires tests of chemicals that may harm human health or the environment, reviews of new chemical substances, limits on the availability of some existing chemicals, and import certification standards to ensure that imported chemicals comply with domestic rules.
Toxicogenomics – The study of how genomes respond to environmental stressors or toxicants. Combines genomewide mRNA expression profiling with protein expression patterns using bioinformatics to understand the role of gene-environment interactions in disease and dysfunction.
Toxicology – The study of the harmful effects of substances on living organisms.
Transparency – The clarity and completeness with which data, assumptions and methods of analysis are documented.
Variability – Observed differences attributable to true heterogeneity or diversity and the result of natural random processes—usually not reducible by further measurement or study (although it can be better characterized).
Water Quality Criteria – Levels of water quality expected to render a water body suitable for its designated use. Criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, fish production, or industrial uses.
Water Quality Standards – Ambient standards for water bodies adopted by a state and approved by EPA that prescribe the use of the water body and establish the water quality criteria that must be met to protect designated uses. Water quality standards may apply to dissolved oxygen, heavy metals, pH, and other water constituents.
CARB at http://www.arb.ca.gov/html/gloss.htm.
Human Genome Project at http://www.ornl.gov/sci/techresources/Human_Genome/glossary/glossary.shtml; Agency for Toxic Substances and Disease Registry at http://www.atsdr.cdc.gov/glossary.html.