The definitions below are specific to how ATSDR typically applies the terms in the PHAGM and as part of the agency’s PHA processes and activities discussed throughout the manual. Readers can consult the NCEH/ATSDR’s Environmental Health Thesaurus and CDC’s Everyday Words for Public Health Communication for plain language alternatives and explanations for some of these terms. In addition, users of the manual can use the PHAGM search function to find a term and its context within the public health assessment process.

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Absorbed dose
The amount of the exposure dose that enters the body (i.e., penetrates barriers such as the skin, gastrointestinal tract, lung tissue). The route of exposure, type, and form of a substance, among other factors, influence how much of a substance is absorbed into the bloodstream. Also see internal dose.

Absorbed dose per event (DAevent) (for surface water dermal contact dose calculations)
Represents the dose of a contaminant absorbed through the skin per event.

The process of taking in a substance. For a person or an animal, absorption is the process of a substance getting into the body through the eyes, skin, stomach, intestines, or lungs.

The provision of Section 508 of the Rehabilitation Act, a federal law requiring agencies to provide individuals with disabilities equal access to electronic information and data comparable to those who do not have disabilities, unless an undue burden would be imposed on the agency.

Accredited offsite laboratories
Stationary facilities that receive samples collected in the field that are shipped offsite for analysis. These laboratories typically analyze the samples according to published, reputable methods and conduct extensive QA/QC on the measurements. They are also designed to minimize contamination of samples.

Active voice
A form of writing that is clear, concise, direct, and easy to understand. ATSDR prefers to use active voice in its documents whenever possible. This involves putting the subject (the “doer”) in a sentence before the verb rather than after. For example: “ATSDR conducted a public health assessment of the site” is active. “A public health assessment of the site was conducted by ATSDR” uses passive voice.

Occurring over a short time (0-14 days) [compare with intermediate and chronic].

Acute exposure
Contact with a substance that occurs once or for only a short time (0-14 days) [compare with intermediate exposure and chronic exposure].

Additive effect
A biologic response to exposure to multiple substances that equals the sum of responses of all the individual substances added together [compare with antagonistic effect and synergistic effect].

When the effect of the mixture can be estimated from the sum of the exposure levels (weighted for potency in dose or concentration additivity) or the probabilities of effect (response additivity) of the individual chemical mixture components. In dose additivity (also called concentration additivity), each chemical behaves as a dilution of every other chemical in the mixture. Most stringently, each chemical contributes to the production of a common adverse outcome via a common mechanism of action. Less stringently (for screening level assessments), each chemical contributes to the production of a common harmful outcome regardless of mechanism of action. In response additivity (also called independent action), components of a mixture act independently of each other, and probabilities of response to components are added.

Adverse health effect
A change in body function or cell structure that results in an abnormal or harmful effect, which might lead to disease or health problems.

Requiring oxygen (with air) [compare with anaerobic].

Age-dependent adjustment factors (ADAFs)
Children are more susceptible (see definition for susceptibility) to cancer and tumor development if exposed to carcinogens with a mutagenic mode of action (MOA). To account for this increased susceptibility, ATSDR applies age-dependent adjustment factors (ADAFs) to its cancer risk equation for these contaminants. ATSDR uses the following EPA-recommended ADAFs based on age: 10 for children 0 < 2 years, 3 for children 2 to < 16 years, and 1 for children ages 16 years and older and adults.

Agency for Toxic Substances and Disease Registry (ATSDR)
A federal public health agency that protects communities from harmful health effects related to exposure to natural and man-made hazardous substances. ATSDR responds to environmental health emergencies; investigates emerging environmental health threats; conducts research on the health impacts of hazardous waste sites; and builds capabilities of and provides actionable guidance to state and local health partners. ATSDR is part of the Department of Health and Human Services and is based with the Centers for Disease Control and Prevention in Atlanta, Georgia.

Surrounding (for example, ambient air).

Requiring the absence of oxygen (without air) [compare with aerobic].

A substance measured in the laboratory. A chemical for which a sample (such as water, air, or blood) is tested in a laboratory. For example, if the analyte is mercury, the laboratory test will determine the amount of mercury in the sample.

Analytic epidemiologic study
A study that evaluates the association between exposure and disease by testing scientific hypotheses. The key feature of analytic epidemiology is a comparison group. The comparison group allows epidemiologists to compare the observed pattern among case-patients (or a group of exposed persons) with the expected pattern among non-cases (or unexposed persons). By comparing the observed with expected patterns, epidemiologists can determine whether the observed pattern differs substantially from what should be expected and, if so, by what degree. [compare with descriptive epidemiologic study]

When the effect of a mixture is less than that estimated by additivity. The use of “less-than-additive” is preferred over the use of the term antagonism.

Antagonistic effect
A biologic response to exposure to multiple substances that is less than would be expected if the known effects of the individual substances were added together [compare with additive effect and synergistic effect].

Anthropogenic levels
Contaminant concentrations in the environment as a result of human-generated, non-site-related sources (e.g., benzene in ambient air as a result of a city’s motor vehicle traffic, radiation in sediments that resulted from fallout from past use and testing of nuclear weapons).

Aqueous photolysis
The alteration of a chemical species in water due to the absorption of light. Contaminants in water can change due to this process, potentially altering their fate and transport and making them more or less toxic.

ATSDR Petition Program
Established as part of ATSDR’s mandate and allows any concerned person or entity to petition ATSDR to investigate specific environmental health concerns of a hazardous waste site or release. ATSDR treats all petitions in a confidential manner. In addition, ATSDR has established criteria to evaluate each petition objectively.

ATSDR’s Partnership to Promote Local Efforts to Reduce Environmental Exposure (APPLETREE)
ATSDR’s cooperative agreement program that gives funding and provides guidance and resources to a number of states. These states use the ATSDR funding to build their ability to assess, evaluate, and respond to site-specific environmental public health issues involving exposure to hazardous substances in the environment and conduct ATSDR’s evaluations in their own states.

Averaging time
When performing exposure dose calculations during the PHA process, this is the average exposure time period (minutes, hours, days) used to arrive at a time weighted exposure factor. This factor is used to express exposure in terms of an average daily dose or air concentration.

B ratio (for surface water dermal contact dose calculations)
The dimensionless ratio of the permeability coefficient of a contaminant through the stratum corneum relative to its permeability coefficient across the viable epidermis.

B-qualified organic compound data
For organic compounds, a contaminant concentration with this qualifier means that the contaminant was detected both in the original field sample and in one or more blank samples. In these cases, environmental scientists typically compare the magnitude of the B-qualified concentration to the levels of blank contamination to determine if the data are usable. Refer to Section 5.5 in EPA’s Risk Assessment Guidance for Superfund, Part A for details on what to do with sampling data sets with blank contamination.

Background level
An average or expected amount of a substance or radioactive material in a specific environment, or typical amounts of substances that occur naturally in an environment.

Behavioral scientist
Studies human behavior and its relationship with disease. Develops community-level and site-specific strategies for healthy behaviors to reduce harmful exposures and studies the effectiveness of preventative measures.

Benchmark concentration (BMC)
A concentration that produces a specific magnitude of changes for a particular adverse response. For example, a BMC10 would be the concentration corresponding to a 10% benchmark response.

Benchmark concentration level (BMCL)
The lower bound of the confidence interval for the benchmark concentration. It is the lower confidence limit that corresponds to a concentration that produces a specific magnitude of changes for a particular adverse response.

Benchmark dose (BMD)
A dose that produces a specific magnitude of changes for a particular adverse response. For example, a BMD10 would be the dose corresponding to a 10% benchmark response.

Benchmark dose level (BMDL)
The lower bound of the confidence interval for the benchmark dose. It is the lower confidence limit that corresponds to a dose that produces a specific magnitude of changes for a particular adverse response.

The amount of a contaminant released from an environmental medium (e.g., food, soil, water) that enters the body (e.g., gastrointestinal tract) and is available for absorption (see definition for “absorption”).

Bioavailability factor
The amount of a contaminant that is absorbed into a person’s body. It is the percent of the total amount of a contaminant ingested, inhaled, or dermally contacted that enters the bloodstream and is available to potentially harm a person. When performing exposure dose calculations during the PHA process, ATSDR assumes this factor to be 1 (i.e., all of a contaminant to which a person is exposed is assumed to be absorbed) for all exposure pathways and contaminants, with one exception: arsenic via soil ingestion. Health assessors should not adjust the bioavailability factor of 1 for any contaminant other than arsenic without first consulting with a toxicologist and getting the approval of the Associate Director for Science group.

Bioconcentration factor (BCF)
A measure of the extent of chemical partitioning at equilibrium between a biological medium, such as fish or plant tissue, and an external medium, such as water.

Decomposition or breakdown of a substance through the action of microorganisms (such as bacteria or fungi) or other natural physical processes (such as sunlight). Biodegradation is a significant environmental process in soil.

Biologic changes
The chemical changes causing damage to tissues following a toxic exposure and an absorbed dose.

Biologic uptake
The transfer of substances from the environment to plants, animals, and humans.

Biological monitoring
During the PHA process, if so warranted, ATSDR may conduct an exposure investigation that involves measuring a biomarker (a chemical or its metabolite) in biologic materials (such as blood, hair, urine, or breath) in a given individual to determine whether exposure has occurred in the human body. A blood test for lead is an example of biologic monitoring that ATSDR may perform during an exposure investigation.

Biological sampling [see biological monitoring]

Biological testing [see biological monitoring]

Biologically effective dose
The amount of the absorbed dose reaching the cells or target sites where adverse effect occurs and needed to produce a biologic response. The potential for an observed adverse effect once a biologic response is elicited is dependent on a host of factors including the type of action, repair mechanisms, metabolism, etc.

Biomedical testing
Testing of persons to find out whether a change in a body function might have occurred.

Plants and animals in an environment. Some of these plants and animals might be sources of food, clothing, or medicines for people.

Body burden
The total amount of a substance in the body. Some substances build up in the body because they are stored in fat or bone or because they leave the body very slowly.

A method used during an oral exposure study to administer a dose. It involves administering a large dose of a contaminant during a short time period.

Any one of a group of diseases that occur when cells in the body become abnormal and grow or multiply out of control.

Cancer classification
Categories assigned based on the strength of the scientific evidence linking a contaminant with cancer outcomes under reported testing conditions. ATSDR uses the cancer classification systems established by the National Toxicology Program, the U.S. Environmental Protection Agency, the International Agency for Research on Cancer, and the National Institute for Occupational Safety and Health.

Cancer effect level (CEL)
The lowest dose level of a contaminant in a study or group of studies observed to produce a significant increase in the incidence of cancer or tumors between the exposed population and its appropriate control (as shown in human epidemiologic or experimental animal studies).

Cancer potency factor [see cancer risk values]

Cancer risk (CR)
A theoretical chance for getting cancer if exposed to a contaminant. Cancer risk is calculated for carcinogens with available cancer risk values (oral cancer slope factors [CSFs], inhalation unit risks [IURs]). Cancer risk is obtained by multiplying an oral CSF by an estimated exposure dose and by multiplying an IUR by an air concentration.

Cancer Risk Evaluation Guides (CREGs)
A type of ATSDR-derived comparison values (CVs) for cancer health effects. They are used to identify concentrations of cancer-causing contaminants that are unlikely to result in increased cancer risks to people exposed every day over their lifetime. CREGs for some media, like water and soil, are derived using EPA’s oral CSFs and default exposure assumptions. CREGs for air are derived using EPA’s IURs and default exposure assumptions.

Cancer risk values
Oral cancer slope factors (CSFs) and inhalation unit risks (IURs) developed by the U.S. Environmental Protection Agency or others from toxicology or epidemiology studies (with safety factors applied) that are protective of human health. These values are also referred to as cancer potency factors. These cancer risk values serve as the basis for ATSDR’s cancer comparison values. For example, ATSDR uses the IUR cancer values as the basis for its cancer risk evaluation guides (CREGs) for air.

A substance that causes cancer.

Case study
A medical or epidemiologic evaluation of one person or a small group of people to gather information about specific health conditions and past exposures.

Cation exchange capacity
A predictive measure of how a piece of soil will retain cation(s) (an ion or group of ions possessing a positive charge)  from a contaminant. Cation exchange capacity is one type of soil property that can influence the movement of contaminants in soil, such as whether contaminants will move from soil to groundwater or the soil will retain contaminants or nutrients.

Centers for Disease Control and Prevention (CDC)
The national public health agency of the United States. It is a federal agency, under the Department of Health and Human Services. CDC focuses national attention on developing and applying disease prevention and control, environmental health, and health promotion and health education activities designed to improve the health of the people of the United States.

Central nervous system
The part of the nervous system that consists of the brain and the spinal cord.

Central tendency exposure (CTE)
Refers to individuals who have average or typical exposure to a contaminant. For instance, at an example site where children < 21 years old are exposed to a chemical via incidental surface water ingestion, the estimated CTE lifetime cancer risk is 1.6E-06 – based on the average swimming ingestion intake rate of 0.049 liter per hour (L/hour) and exposure duration of 12 years. This can be compared to a reasonable maximum exposure (RME) lifetime cancer risk for the same population of 5.4E-05, which is based on the 95th percentile swimming ingestion intake rate of 0.12 L/hour and exposure duration of 33 years.

CERCLA [see Comprehensive Environmental Response, Compensation, and Liability Act of 1980]

Chain-of-custody requirements
Outline steps that will be followed for collecting, shipping, and analyzing samples during a field program. These requirements will include outlining the use of a chain-of-custody form. The form is used to track specific identifying information that pertains to each sample, such as the sample ID, the media sampled, the sample collection date, the person who collected the sample, and the location where the sample was collected. The chain-of-custody form accompanies each sample from collection through laboratory analysis.

Chemical Abstracts Service Registration Number (CASRN)
A unique number assigned to a substance or mixture by the American Chemical Society Abstracts Service. Also referred to as a CAS registry number.

Chemical transformation
The physical, chemical, and biological changes in a contaminant over time resulting from hydrolysis, oxidation, photolysis, and biodegradation. A key transformation process for organic pollutants is aqueous photolysis, often in the form of photochemical reactions. Reaction rate constants and half-lives are both types of chemical transformation rates.

Occurring over a long time (365 or more days) [compare with acute and intermediate].

Chronic exposure
Contact with a substance that occurs over a long time (365 or more days) [compare with acute exposure and intermediate exposure].

Citizen science
An approach that encourages members of the public to voluntarily participate in the scientific process. Whether by asking questions, making observations, conducting experiments, collecting data, or developing low-cost technologies and open- source code, members of the public can help advance scientific knowledge and benefit society. Other terms used are civic science, community science, and crowd science.

Civic science [see citizen science]

Clear language
Wording that explains scientific principles and health information in a way that readers can easily understand and remember the first time they read or hear it.

Cluster investigation
A review of an unusual number, real or perceived, of health events (for example, reports of cancer) grouped together in time and location. Cluster investigations are designed to confirm case reports; determine whether they represent an unusual disease occurrence; and, if possible, explore possible causes and contributing environmental factors.

Coalition, alliance, or forum
Names for community groups that are more formalized with specific procedures and policies.

People who may be directly affected by site contamination because they currently live near the site or have lived near the site in the past. Community members may include, for example, residents, members of local action groups, local officials, tribal members, health professionals, and local media. The community is at the heart of all public health process activities.

Community assistance panel (CAP)
A group of people from a community and from health and environmental agencies who work with ATSDR to resolve issues and problems related to hazardous substances in the community. CAP members work with ATSDR to gather and review community health concerns, provide information on how people might have been or might now be exposed to hazardous substances, and inform ATSDR on ways to involve the community in its activities.

Community engagement
The process ATSDR uses to work with communities during the public health assessment (PHA) process. There are four phases of ATSDR’s community engagement efforts at sites: setting the stage (informing planning and goal setting), getting started (laying the foundation for productive partnership), keeping it going (maintaining trust and communication over time), and wrapping up (sharing findings and empowering the community).

Community health concerns
ATSDR is required by law to address the community’s health-related environmental concerns at hazardous waste sites on the NPL list. Therefore, it is important to meet with community members to obtain their concerns and address in the PHA report along with any other health questions the community may have related to the site.

Community health education
Programs designed to provide information to communities to help them understand and reduce exposure and health risks. Topics may include types of contaminant exposures, exposure routes and pathways, health effects, treatment options, and methods to prevent and minimize environmental exposures. This education could be done through providing materials (e.g., fact sheets, online products) or interacting with the communities themselves.

Community involvement specialist
Coordinates and oversees outreach to provide opportunities for community engagement in the PHA process.

Community needs assessment
A systematic process for gathering, analyzing, and reporting data and information about the characteristics, capacity, needs, and concerns of a community. ATSDR site teams use this information during the agency’s public health assessment process to protect public health.

Community science [see citizen science]

Community stress education
Teaching designed to help members cope with the stress of potential environmental contaminant exposure.

A measure of the similarity between different data sets, such as by comparing data results for the same contaminant collected from different instruments. Some factors to consider when determining similarity between data sets are the location, time period, method, and sensitivity.

Comparison value (CV)
Media- and contaminant-specific screening levels developed by the Agency for Toxic Substances and Disease Registry (ATSDR) and used during the screening analysis to identify contaminants requiring further evaluation. CVs are calculated concentrations of a substance in particular media (e.g., air, water, soil) that are unlikely to cause harmful health effects in exposed people.

Completed exposure pathway
All five elements of a pathway are present: contaminant source, environmental fate and transport, exposure point, exposure route, and potentially exposed population. A completed exposure pathway exists when there is direct evidence or, in the judgment of the health assessor, a strong likelihood that people have in the past, are presently, or could in the future come into contact with site-related contaminants.

Composite sampling
A technique that involves taking multiple discrete environmental samples (e.g., soil, sediment, biota) and combining them into a single sample for the purposes of analysis. The resultant concentration is meant to represent contaminant concentrations for the area and volume of material from which the combined samples were collected. For example, picture a five-acre commercial property that was divided into 10 half-acre plots. One sampling strategy for surface soil could involve collecting a composite sample in each of the half-acre plots; and perhaps each composite sample would be comprised of five discrete samples. In this case, 10 composite samples would be sent to the laboratory for analysis, rather than 50 discrete samples.

Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA)
Enacted by the U.S. Congress on December 11, 1980, and also known as Superfund. It is the federal law that concerns the removal or cleanup of hazardous substances in the environment and at hazardous waste sites. ATSDR, which was created by CERCLA, is responsible for assessing health issues and supporting public health activities related to hazardous waste sites or other environmental releases of hazardous substances. This law was later amended by the Superfund Amendments and Reauthorization Act (SARA).

The amount of a substance present in a certain amount of soil, water, air, food, blood, hair, urine, breath, or any other media.

Conceptual site model
A diagram that helps you visualize how contaminants move in the environment at your site and how people might come into contact with these contaminants. The diagram should indicate the various ways in which contaminants, based on site-specific conditions, can move from the source through the environmental medium and to the points of exposure. Also see site conceptual model.

Conclusion categories
ATSDR uses various hazard determinations in its documents to state whether people could be harmed by exposures to contaminants or other conditions at the site in the past, present, or future. One or more conclusion categories might be appropriate for each site.

Confounding risk factor
Variables that can influence an association between a chemical exposure and health outcome because they are also related with the health outcome. For instance, examples of confounding risk factors between lead exposure and mortality include age, sex, alcohol consumption, smoking status, hypertension, diabetes, and co-exposure with other metals (i.e., arsenic or cadmium).

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. Contaminants can be chemical (e.g., lead) or radiological (e.g., iodine-131).

Contaminant of concern
A label given to a contaminant if its concentration meets or exceed screening levels, or its estimated exposure dose or cancer risk levels exceed acceptable levels. It is important to note that chemicals flagged as potential contaminants of concern do not necessarily pose a health threat. Further evaluation is needed to evaluate contaminants flagged as potential contaminants of concern. For example, trichloroethylene detected in air above the ATSDR comparison value would represent a potential contaminant of concern and require further evaluation.

Contaminant source
The place where the chemical substance or radiation is released to the environment. For example, a sanitary landfill, a waste pile, and a smokestack are common contaminant sources. For the PHA process, health assessors evaluate the contaminant source to determine, among other things, the affected environmental media and how contaminants might reach populations at or near the site.

Contaminant-specific information
The physical, toxicological, biologic, and physiological properties of the hazardous substances evaluated at a site. ATSDR uses this information to support the scientific evaluation components of the PHA process.

Contract Required Quantitation Limit (CRQL)
The minimum level of quantitation that can be reliability achieved under the contract Statement of Work. These values are used as reporting limits.

Cooperative Agreement Program
Cooperative agreements are mechanisms used to transfer federal resources in exchange for services that help fulfill an agency’s mission and are a direct benefit to the public. ATSDR’s cooperative agreement program funds state health departments, tribal governments, and U.S. territories to evaluate and respond to environmental public health issues. Also see APPLETREE definition.

Crowd science [see citizen science]

CSW (for surface water dermal contact dose calculations)
The chemical concentration in surface water.

Cultural contact
Acts as a bridge between the community and the site team and provides guidance to the site team on the most culturally appropriate, constructive, and productive ways for learning from, informing, and involving the community.

Data accuracy
Indicates the extent to which measurements represent their corresponding “true” or “actual” values.

Data completeness
The fraction of attempted sampling events that have valid results. It is often expressed as a percentage (e.g., 92% of the samples collected were valid). The completeness of a sampling program is a rough measure of how successfully it was implemented.

Data gap [see limitation]

Data precision
Characterizes the repeatability of measurements. Precision is usually quantified by collecting duplicate samples in the field or by analyzing the sample twice in the laboratory (replicate). Ideally, concentrations of contaminants in duplicate and replicate samples will be in close agreement, and within the bounds specified by the published sampling and analytical method. Precise sampling data will have relatively low differences in concentrations between duplicate and replicate samples. Precision is usually reported as a relative percent difference (RPD), and most sampling and analytical methods specify acceptable ranges of RPDs.

Data quality objectives (DQOs)
Quantitative and qualitative measures used to ensure that the quality of the collected data is sufficient to achieve the project goals. Operational DQOs could be for elements such as where to conduct sampling and the number of sampling locations, while technical DQOs could refer to elements such as measurement completeness, accuracy, and precision.

Data representativeness
Refers to the degree that data are sufficient to identify the concentration and location of contaminants that may be available for contact at a site. It also refers to how well the data adequately characterize the exposure pathways of concern during the time frame of interest (i.e., when exposures occurred).

Data validation
An analyte- and sample-specific process performed by a chemist that determines the analytical quality of a data set. The chemist validates the data using QA/QC parameters provided by the laboratory, such as matrix spikes and blanks.

Data validation draft (DVD)
A version of an ATSDR document reviewed by data providers and stakeholders. This review ensures that the data used in the document is current, complete, and presented accurately.

Data verification
A process generally completed by an analytical laboratory. The data are evaluated for completeness, correctness, and compliance with laboratory methods and contract requirements.

Default exposure assumptions
Standard exposure parameters for various inputs (e.g., intake rate) used to estimate exposures via ingestion, dermal contact, and inhalation.

Delayed health effect
A disease or an injury that happens as a result of exposures that might have occurred in the past.

Demographic data
Various information on the characteristics of a community, such as race, ethnicity, age, gender, and socioeconomic status. This information helps to better define the size, characteristics, locations, and possible susceptibility of known populations potentially affected by the site. ATSDR generally develops maps using these types of data to evaluate the potentially exposed populations during the PHA process.

Density of liquid
A liquid’s mass per volume.

Referring to the skin. For example, dermal absorption means passing through the skin.

Dermal contact
Contact with (touching) the skin [see route of exposure].

Dermally absorbed dose
The amount of the exposure dose that is absorbed into the body through the skin. The type and form of a contaminant, among other factors, influence how much of a contaminant is absorbed.

Dermal absorption fraction (ABSd) (for soil/sediment and surface water dermal contact dose calculations)
The amount of a contaminant absorbed into the body through the skin. Included to adjust for the amount of a contaminant that is absorbed into the body through the skin after dermal contact.

Dermal permeability coefficient (Kp) (for surface water dermal contact dose calculations)
A factor describing the speed (in centimeters per hour) at which a contaminant in water permeates the skin; it is an important parameter in estimating the amount of a chemical that is absorbed into the body.

Pertaining to the skin of a person or animal.

Descriptive epidemiologic study
The study of the amount and distribution of a disease in a specified population by person, place, and time. For example, health outcome data analyses or reviews conducted during the PHA process are descriptive epidemiologic analyses. [compare with analytic epidemiologic study]

Detection limit
The lowest concentration of a chemical that can be reliably distinguished from a zero concentration. This may also be reported as the limit of detection (LOD) or method detection limit (MDL).

Direct evidence
Information or other types of proof that clearly establish something to be a fact.

Discrete sample
An individual environmental sample from a given point and time that is independent of other samples.

Disease prevention
Measures used to prevent a disease or reduce its severity.

The process of how and where a contaminant travels within the body or through the environment (also see fate and transport).

Disease registry
A system of ongoing registration of all cases of a particular disease or health condition in a defined population.

United States Department of Defense.

United States Department of Energy.

Dose (for chemicals that are not radioactive)
The amount of a substance to which a person is exposed over some time period. Dose is a measurement of exposure. Dose is often expressed as milligram (amount) per kilogram (a measure of body weight) per day (a measure of time) when people eat or drink contaminated water, food, or soil. In general, the greater the dose, the greater the likelihood of an effect. An “exposure dose” is how much of a substance is encountered in the environment. An “absorbed dose” is the amount of a substance that actually got into the body through the eyes, skin, stomach, intestines, or lungs.

Dose (for radioactive chemicals)
The radiation dose is the amount of energy from radiation that is actually absorbed by the body. This is not the same as measurements of the amount of radiation in the environment.

Dose-response relationship
The relationship between the amount of exposure [dose] to a substance and the resulting changes in body function or health (response). For example, acute toxicity to a chemical at <30 microgram per deciliter (µg/dL) resulted in no adverse health effects, but signs and symptoms of gastrointestinal and neurological toxicity were observed as the chemical concentrations increased to >30 µg/dL, with severity of symptoms rising as chemical concentrations increased.

Dosimetry models
Used to estimate the dose from exposure to contaminants. For instance, in some studies, ATSDR will use dosimetric modeling to convert a 95% lower confidence limit on the benchmark dose (BMDL) to a human equivalent dose (HED).

Subject to change and evolve.

Ecologic study
An epidemiological study focused on the effects on groups rather than effects on individuals.

Any change in physiologic function or cellular structure as a result of an exposure to an environmental contaminant.

Eliminated exposure pathway
At least one of the five elements of a pathway is not present and either will never be present or is extremely unlikely to ever be present: contaminant source, environmental fate and transport, exposure point, exposure route, and potentially exposed population. Health assessors classify an exposure pathway as eliminated after determining that exposure has not occurred in the past, is not occurring currently, and is extremely unlikely to occur in the future.

The process of a contaminant leaving the body (e.g., excretion).

Engineering controls
Physical methods at hazardous waste sites used to prevent or reduce contact with contaminants. These can include physical controls and barriers put in place, such as permanent fences and gates, as well as technologies such as water filtration systems, landfill liners, leachate collection systems, wastewater treatment systems, and air filtering techniques.

Environmental health scientist
Evaluates data and conducts investigations to examine whether human exposures to environmental contaminants can result in potential harmful health effects. Makes conclusions about potential impacts to public health associated with environmental exposures, including exposure pathways, sampling data, site-specific information, population characteristics, and contaminant-specific properties.

Environmental justice
Advancing health equity and eliminating environmental health disparities through the fair treatment and meaningful involvement of all people in environmental health policies, research, and programs and ensuring equal access to a healthy environment.

Environmental media
Soil, water, air, biota (plants and animals), or any other parts of the environment that can contain contaminants.

Environmental fate and transport [see fate and transport]

Environmental Media Evaluation Guides (EMEGs)
ATSDR comparison values that are based on ATSDR’s MRLs for noncancer health effects. They represent estimated contaminant concentrations below which humans exposed during a specific timeframe (acute, intermediate, or chronic) are not expected to experience noncarcinogenic health effects.  Air EMEGs are the same as their corresponding inhalation MRLs. Drinking water EMEGs are derived from the corresponding oral MRLs using conservative assumptions of intake rate and body weight. Soil EMEGs for typical scenarios are derived from the corresponding oral MRLs using conservative assumptions of intake rate and body weight. Soil EMEGs for acute and intermediate pica scenarios are derived from the corresponding oral MRLs using conservative assumptions of intake rate, exposure frequency, and body weight, appropriate for the young age groups who exhibit pica behavior.

Environmental odors
Caused by a substance in the air that you can smell. Sometimes communities are affected by smells related to environmental contamination that can impact their quality of life. Environmental odors can come from various sources, including human activities, animals, nature, vehicles, and industries.

Environmental sampling data
Data obtained by sampling environmental media, such as soil, water, air, or biota (plants and animals), and used to characterize contamination present where people live, spend time, play, or may otherwise come into contact with contaminants under investigation.

EPA (U.S. Environmental Protection Agency)
The primary federal agency responsible for environmental issues, including research, monitoring, enforcement, and cleanup functions. ATSDR (using its public health assessment process) and EPA (using its risk assessment process) evaluate spills or releases of hazardous substances in the environment at Superfund sites to protect public health.

Epidemiologic study
A 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.

Epidemiologic surveillance [see public health surveillance]

Someone who studies the distribution and determinants of disease in a particular population.

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.

Cause of disease; the factors that contributes to occurrence of a disease.

Event duration (tevent) (for surface water dermal contact dose calculations)
The duration of exposure for each event in units of hours. The value is site-specific.

Expert in tribal affairs
Provides expertise to the site team on issues affecting American Indian and Alaska Native (AI/AN) populations. Works with AI/AN populations to identify and evaluate environmental health concerns and empower individual tribes to make informed decisions that benefit their communities.

Exposed population
The exposed or potentially exposed population (the fifth element in an exposure pathway) is the group or groups of people that may come or may have come in contact with site-related contaminants.

Contact with a substance by swallowing, breathing, or touching the skin or eyes. Exposure may be short-term [acute exposure], of intermediate duration, or long-term [chronic exposure].

Exposure assessment
The process of finding out how people come into contact with a hazardous substance, how often (frequency) and for how long (duration) they are in contact with the substance, and how much (amount) of the substance they are in contact with.

Exposure dose
The estimate of how much of a contaminant a person might contact based on his or her actions and habits. It is generally expressed as milligrams of contaminant per kilogram of body weight per day (mg/kg/day). Estimating an exposure dose requires identifying how much, how often, and how long a person or population might come in contact with some concentration of a contaminant (i.e., the exposure point concentration) in a specific medium. ATSDR calculates exposure doses based on acute, intermediate, and chronic exposure durations. For example, the exposure dose formula for drinking water is the concentration x the intake rate x the exposure factor divided by the body weight. Let’s say the birth to <1 year exposure group (weighing 7.8 kilograms) on a daily chronic basis (exposure factor of 1) drank 1.113 liters per day (the reasonable maximum exposure intake rate) of water with an EPC of 3.5 milligrams per liter (mg/L) for a chemical. Using this drinking water equation, this example results is a reasonable maximum exposure dose of 0.50 mg/kg/day.

Exposure Dose Guidance (EDG)
Developed by ATSDR to provide health assessors with the most updated guidance for health assessors to perform the scientific components of the public health assessment process. ATSDR’s EDGs are housed on the PHAGM Resources page.

Exposure dose reconstruction model
A type of modeling that uses analytical methods and computational tools to quantify fate and transport of contaminants. Dose reconstruction modeling can be used to estimate past, current, and future contaminants’ levels; estimate distributions of contaminants; and identify potentially exposed populations. Air and groundwater are the most common media evaluated using exposure dose reconstruction modeling.

Exposure duration
The length of time (a few hours, days, weeks, years) a population has been exposed to site contaminants.

Exposure factor (EF)
Expresses how often (frequency) and how long (duration) a person could contact a contaminant in the environment over a certain amount of time (averaging time). It is calculated by multiplying the exposure frequency and the exposure duration, and then divided by the averaging time. The default exposure factor value is usually 1, which indicates exposure occurs daily for acute, intermediate, and chronic durations, but health assessors can change the exposure factor based on site-specific exposures.

Exposure frequency
How frequently exposure occurs, often measured in days per week and weeks per year.

Exposure investigation (EI)
The collection and analysis of site-specific environmental and/or biological data (when appropriate) to determine whether people have been exposed to hazardous substances. Conducted to identify and sample the most highly exposed individuals (through biologic monitoring) or the most contaminated locations (through environmental sampling). An EI might be deemed necessary to assess possible impacts to public health when complete environmental or biological data are lacking. An EI is an example of one way to fill data gaps at a site during the public health assessment process.

Exposure investigation-health consultation (EI-HC)
An ATSDR document that summarizes the findings of ATSDR’s exposure investigation, which involves examining environmental and/or biological data (blood/urine). Also see exposure investigation.

Exposure parameters
Variables in the exposure dose equations. Health assessors can use conservative default exposure parameters, or refine their analysis using more realistic exposure parameters consistent with what is known about site-specific exposures.

Exposure pathway
The route 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 elements: 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.

Exposure pathways evaluation
One of four scientific components in the public health assessment process. Involves studying the available environmental and biological data and developing a conceptual model of the site to determine the specific ways in which people might contact site-related contamination. During the evaluation, health assessors classify past, current, and future exposures as completed, potential, or eliminated exposure pathways. Exposure units also can be defined during this step.

Exposure point
The environmental point is the third part of an exposure pathway. It is the specific location(s) where people might come into contact with a contaminated medium.

Exposure point concentration (EPC)
A representative contaminant concentration. When appropriate, ATSDR calculates an EPC for each exposure unit or area, individual completed and potential exposure pathway, and exposure duration: acute (0-14 days), intermediate (15-364 days), or chronic (365 or more days) durations.

Exposure point concentration (EPC) and exposure calculations evaluation
One of four main scientific evaluations ATSDR performs during the public health assessment process. It involves calculating EPCs for each contaminant in each completed and potential exposure pathway (by exposure unit, if appropriate), as well as using default or site-specific exposure conditions to estimate exposure doses, adjusted air concentrations, hazard quotients, and cancer risks. Includes identifying contaminants that exceed acceptable non-cancer and cancer levels, have no health guidelines or cancer risk values, or pose other issues (e.g., a community concern) and need an in-depth toxicological effects analysis.

Exposure Point Concentration (EPC) Tool
ATSDR’s web application for health assessors to calculate exposure point concentrations for discrete environmental data during the public health assessment process. The EPCs are either 95 percent upper confidence limits of the arithmetic mean (95UCLs) or maximum values for datasets where 95UCLs cannot be calculated.

Exposure registry
A system of ongoing follow-up of people who have had documented environmental exposures.

Exposure route
The fourth part of an exposure pathway. It is the path by which contaminants enter the body (dermal, inhalation, ingestion).

Exposure unit
A geographically defined point or area where a person is expected to contact an environmental medium, such as soil, surface water, groundwater, air, or food items (e.g., fruits, vegetables, fish, game). Health assessors define exposure units for each potential contaminant of concern identified in a completed exposure pathway or potential exposure pathway.

External radiation
Exposure that occurs when a person is exposed to a source of penetrating radiation (beta particles of specific energies and gamma radiation).

Fate and transport
A mechanism that is the second part of an exposure pathway. Transport involves the movement of gases, liquids, and particulate solids within a given medium and across interfaces between water, soil, sediment, air, plants, and animals. Transport mechanisms move contaminants from the source to points where human exposure can occur. Fate refers to what eventually happens to contaminants released to the environment. Some fraction of the contaminants might simply move from one location to the next; other fractions might be physically, biologically, or chemically transformed; and others still might accumulate in one or more media.

Feasibility study
A study by EPA to determine the best way to clean up environmental contamination. A number of factors are considered, including health risk, costs, and what methods will work well.

Federal Advisory Committee Act (FACA) Committee
Under FACA, federal government agencies can convene committees to provide consensus advice and recommendations. FACA committees must meet the requirements of FACA. They are much more resource-intensive than community assistance panels (CAPs), and they require a much greater time commitment on the part of community members than CAPs.

Field screening
A sampling technique that involves using instruments onsite to obtain real-time indications of levels of contamination. Field screening (or direct reading instrumentation) is typically used during preliminary investigations to determine whether contamination is present. Some examples include photoionization detectors (PIDs), flame ionization detectors (FIDs), X-ray fluorescence (XRF) to measure lead and other metals, and chemical test kits.

Fraction absorbed (FA) (for surface water dermal contact dose calculations)
The portion of contaminant on the skin that is absorbed by the body. Without contaminant-specific information, ATSDR uses a default value of one.

Gastrointestinal absorption fraction (ABSGI) (for soil/sediment and surface water dermal contact dose calculations)
The fraction of a contaminant absorbed by the gastrointestinal tract. Used to convert an absorbed dermal dose to an equivalent administered oral dose, which allows health assessors to combine doses from the ingestion and dermal exposure pathway for the same medium (soil/sediment, surface water) to create a total dose, and use the combined dose to evaluate potential for noncancer or cancer effects. For most contaminants, ATSDR assumes 100% of the contaminant is absorbed through the GI tract, thus using an ABSGI of 1. For some inorganic compounds (e.g., antimony, manganese), however, ATSDR adjusts the dermal dose using specific absorption factors.

A method used during an oral exposure study to administer a dose. It involves administering a contaminant dose to a subject through a feeding tube that goes into the stomach.

The study of heredity.

Geographic information system (GIS)
A mapping system that uses computers to collect, store, manipulate, analyze, and display data. For example, GIS can show the concentration of a contaminant within a community in relation to points of reference such as streets and homes.

Geographic information system (GIS) scientist
Applies science and GIS (a mapping system to collect, store, manipulate, analyze, and display data) technology to manage geographic relationships and integrate information.

The intentional ingestion of earth. Unlike pica, it is usually associated with cultural practices and eating clay from sources of depth, and not attributed to children’s exploratory behavior (also refer to pica).

Grand rounds
Training sessions for physicians and other health care providers about health topics.

Water beneath the earth’s surface in the spaces between soil particles and between rock surfaces [compare with surface water].

Groundwater recharge
When groundwater supplies are filled back up by rain and snow melt.

Half-life (t½)
The time it takes for half the original amount of a substance to disappear. In the environment, the half-life is the time it takes for half the original amount of a substance to disappear when it is changed to another chemical by bacteria, fungi, sunlight, or other chemical processes. In the human body, the half-life is the time it takes for half the original amount of the substance to disappear, either by being changed to another substance or by leaving the body. In the case of radioactive material, the half-life is the amount of time necessary for one half the initial number of radioactive atoms to change or transform into another atom (that is normally not radioactive). After two half-lives, 25% of the original number of radioactive atoms remain.

A source of potential harm from past, current, or future exposures.

Hazard index (HI)
A sum of noncancer hazard quotients used when examining multiple chemical exposures. The hazard index approach uses the assumption of dose additivity to assess the noncancer health effects of a contaminant mixture from the data on the mixture components. ATSDR estimates separate hazard indexes for each pathway and exposure duration of concern. For a given duration, hazard indexes are summed across pathways that affect the same receptor population.

Hazard quotient (HQ)
A calculation to evaluate the potential for noncancer health hazards to occur from exposure to a contaminant with available noncancer health guidelines (MRLs, RfDs, RfCs). Typically, the hazard quotient is obtained by dividing the duration-specific (acute, intermediate, or chronic) exposure dose or concentration by the noncancer health guideline for the same duration for your exposure groups of interest.

Hazardous waste
Potentially harmful substances that have been released or discarded into the environment.

Health advisory (HA)
An ATSDR document that serves as an official notice given to EPA. The advisory identifies an immediate threat to human health from a hazardous substance, includes recommendations to reduce exposure and any threat to human health, and notifies appropriate state and local health and environmental agencies about any public health problems. Also called a public health advisory.

Health assessor
Often the leader of the site team, coordinating the PHA process activities and working with subject matter experts (SMEs) as needed to gather and evaluate data. Usually, they are also responsible for writing the document that summarizes the findings of the PHA process.

Health communication
The use of communication strategies and messages to best meet the needs of the community with culturally appropriate public health information and materials. Health communication may include public meetings, fact sheets, media support, translation, etc.

Health communications specialist
Analyzes and applies communication strategies to inform and influence community members’ decisions that enhance health.

Health consultation (HC)
An ATSDR document that presents a review of available information or collection of new data that generally responds to a specific public health issue or question, contaminant, health condition, or technical interpretation. HCs are typically focused on a specific exposure issue.

Health Education Activity Tracking (HEAT)
A form for health assessors to record health education at sites.

Health education
Any planned combination of learning experiences designed to predispose, enable, and reinforce voluntary behavior conducive to health in individuals, groups, or communities.

Health educator
Provides guidance on health education programs, projects, and activities. Specifies appropriate instructional strategies and media to use for reaching desired populations. Proposes and refines programs and establishes methods for monitoring public health programs.

Health guidelines
Values that serve as the basis for ATSDR’s non-cancer comparison values. They consist of oral human doses and air concentrations developed from toxicology or epidemiology studies (with safety factors applied) that are protective of human health.

Health hazard
One of three overall conclusion categories used in ATSDR’s documents for sites where there is a reasonable possibility that harmful health effects have occurred or are likely to occur due to exposure levels being high enough to potentially cause an effect in members of the exposed population.

Health investigation
The collection and evaluation of information about the health of community residents. This information is used to describe or count the occurrence of a disease, symptom, or clinical measure and to evaluate the possible association between the occurrence and exposure to hazardous substances.

Health outcome data
Measure disease mortality or morbidity. ATSDR is required by CERCLA to consider the evaluation of mortality and morbidity data during the PHA process.

Health physicist
Detects, analyzes, and evaluates radiological agents in food, human tissue, and the environment.

Health professional education
Information for doctors, nurses, or other health care providers about environmental exposures and their prevention, potential for substance-specific exposure effects, community health warning signs, or special diagnostic techniques for detecting possible site-related illnesses.

Health promotion
The process of enabling people to increase control over, and to improve, their health.

Health statistics review
The analysis of existing health information (i.e., from death certificates, birth defects registries, and cancer registries) to determine if there is excess disease in a specific population, geographic area, and time period. A health statistics review is a descriptive epidemiologic study.

Hematopoietic system
Pertaining to the organs, tissues, or cells that form blood.

Henry’s Law Constant
A measure of the tendency for a chemical to pass from an aqueous solution to the vapor phase. It is a function of molecular weight, solubility, and vapor pressure. For example, a high Henry’s Law Constant corresponds to a greater tendency for a chemical to volatilize to air.

Pertaining to the liver of a person or animal.

Holding time
The maximum amount of time that may pass between when a sample is collected and when the extraction and analysis begin before the analytes start to degrade. There can be different holding times for extraction and analysis. Holding times need to be monitored to ensure timely extraction and analysis, and appropriate reporting of analytical results. For example, there was a holding time of 7 days between when the soil sample collection effort occurred and when the laboratory completed its analysis of the samples.

Human equivalent concentration (HEC)
A concentration equivalent to a human concentration. When possible, exposures are converted to their human equivalents through the use of toxicological models. For example, an acute-duration inhalation LOAEL for animals for a chemical was based on 0.14 parts per million (ppm), which was used to calculate a human equivalent concentration of 30 ppm.

Human equivalent dose (HED)
A dose equivalent to a human dose. When possible, exposures are converted to their human equivalents through the use of toxicological models. For example, a human equivalent dose of 0.00221 milligrams per kilogram per day was calculated for a chemical based on an animal LOAEL of 31.9 micrograms per milliliter.

Evaluates groundwater characteristics (e.g., depth, direction, type of flow) to see how hydrogeologic conditions might influence groundwater movement and the possibility of contaminants reaching people. Can research the location, size, and movement of underground water reservoirs and the impact of contamination on these waters. Often uses advanced computer modeling programs and imaging technology to evaluate groundwater flow and to map groundwater reservoirs.

A chemical’s reaction with water.

The number of new cases of disease in a defined population over a specific time period [contrast with prevalence].

Incremental sampling
A structured form of composite sampling, designed to characterize the overall average contamination level within an area of interest, where numerous incremental soil samples (an absolute minimum of 30; ideally at least 50 to 100) are collected and combined to form a single ISM sample for laboratory analysis.

In-depth toxicological effects analysis
One of four main scientific evaluations ATSDR performs during the PHA process. It involves closely analyzing toxicological information for contaminants that exceeded acceptable noncancer and cancer levels to determine whether people could possibly have health problems from their exposure. It also involves examining contaminants that warrant further evaluation because they have no health guidelines or cancer risk values or pose other issues (e.g., a community concern).

Industrial hygienist
During the PHA process, evaluates factors related to commercial releases that can cause illness, impair health, or affect the well-being of potentially exposed community members. An ATSDR industrial hygienist prepares consultations, conducts surveillance, and performs environmental testing; helps interpret sampling data results; recommends strategies to prevent exposure; and provides information to health care providers investigating possible toxic exposures.

Information mapping
A way to structure and organize information in a manner that increases clarity for readers. This involves using a block and label format to categorize information that allows readers to identify the information that is relevant to them.

The act of swallowing something through eating, drinking, or mouthing objects. A hazardous substance can enter the body this way [see route of exposure].

The act of breathing. A hazardous substance can enter the body this way [see route of exposure].

Inhalation unit risks (IURs)
Derived by EPA to measure the potency of various carcinogens for inhalation exposures. EPA develops IURs as a result of quantitative evaluation of inhalation exposure to a suspected carcinogenic contaminant. IURs are estimates of increases in cancer cases in a human population per unit increase in exposure concentration. For example, the inhalation unit risk for a chemical is 7.8E-06 for each microgram per cubic meter increase in exposure to this chemical which is based on leukemia cancers in humans.

When a chemical mixture component that does not have a toxic effect on a particular biological system decreases the apparent effect of a second chemical on that organ system.

Institutional controls
Prevent and reduce exposure to hazardous contaminants through administrative means, such as deed restrictions, building permits, ordinances that prevent construction of private wells in areas with contaminated groundwater, requirements for routine sampling, and building restrictions.

Intake rate
The amount of a contaminated medium to which a person is exposed during a specified period of time. The amount of water, soil, and food ingested on a daily basis; the amount of air inhaled; or the amount of water or soil that a person may contact through dermal exposures are all examples of intake rates.

The way that any combination of two or more contaminants may jointly contribute to actual or potential effects in an exposed population. Also referred to as interactive effects.

Occurring between a short and long time (15-364 days) [compare with acute and chronic].

Intermediate exposure
Contact with a substance that occurs for more than 14 days and less than a year (15-364 days) [compare with acute exposure and chronic exposure].

Internal dose
The amount of the exposure dose that actually enters the body (i.e., penetrates barriers such as the skin, gastrointestinal tract, lung tissue). The route of exposure, type, and form of a substance, among other factors, influence how much of a substance is absorbed into the bloodstream. Also see absorbed dose.

Internal radiation
Exposure that occurs when a person inhales or ingests radioactive materials or they are absorbed through the skin or taken in through wounds.

Ionizing radiation
Any one of several types of particles and rays given off by radioactive material, some specialized high-voltage equipment, nuclear reactions, and stars. The types that could have the greatest effect on your health are alpha particles, beta particles, x-rays, and gamma rays. Alpha and beta particles are small, fast-moving bits of atoms that a radioactive atom gives off when it changes into another substance. X-rays and gamma rays are types of electromagnetic radiation. These radiation particles and rays carry enough energy to knock out electrons from atoms and molecules (such as water, protein, and DNA) that they hit or pass near. This process is called ionization, which is why this radiation is called “ionizing radiation.”

In vitro
In an artificial environment outside a living organism or body. For example, some toxicity testing is done on cell cultures or slices of tissue grown in the laboratory, rather than on a living animal [compare with in vivo].

In vivo
Within a living organism or body. For example, some toxicity testing is done on whole animals, such as rats or mice [compare with in vitro].

J-qualified data
Data that generally indicate that the reported concentration is an estimated value. A J qualifier indicates that the contaminant was detected, but the concentration is estimated because it is below the Contract Required Quantitation Limit (CRQL), but greater than or equal to the Method Detection Limit (MDL). For example, methylene chloride was detected in four well samples. Two of the four sample results were  “J” qualified, which means these two sampling results represented estimated values.

The study of movement.

Lag time per event (τevent) (for surface water dermal contact dose calculations)
The contaminant loss due to desquamation as the contaminant crosses the stratum corneum during the absorption process.

Latency period
The time from when exposure occurs to the manifestation of disease. The latency period can range from immediate to delayed (hours or days) to prolonged (decades).

Letter health consultation (LHC)
An ATSDR document that focuses, in general, on one specific public health issue or question (like health consultations) but provides a much more focused response. It is sent in the form of a letter to a specific requestor.

Limit of detection (LOD) [see detection limit]

Limit of quantitation (LOQ)
The level above which quantitative results may be obtained with a specified degree of confidence. This may also be called the lower limit of quantitation. For example, during a soil sample collection effort where the limit of detection was defined as 5 milligrams per kilogram (mg/kg), the reported analytical result of arsenic was 25 mg/kg (at the LOQ, which was five times the limit of detection).

Missing, unknown, or assumed information, as part of the public health assessment process, that can be critical to make a public health call, help you understand what contaminants, and at what concentrations, people could be exposed to, and lend uncertainty to the public health conclusions.

Lowest-observed-adverse-effect level (LOAEL)
The lowest tested dose of a substance that has been reported to cause harmful (adverse) health effects in people or animals. For example, mice exposed to an air concentration of 250 parts per million (ppm) experienced shallow breathing, and the study reported 250 ppm as a LOAEL.

When the chemical mixture components produce opposite or functionally competing effects on the same biological system, and diminish the effects of each other, or one overrides the effect of the other.

Mechanisms of action
The specific cellular or molecular events that lead to a specific adverse effect. A more specific term than mode of action.

Medical monitoring
A set of medical tests and physical exams specifically designed to evaluate whether an individual’s exposure could negatively affect that person’s health.

The conversion or breakdown of a substance from one form to another by a living organism.

Any product of metabolism.

Method detection limit (MDL) [see detection limit]

Moving from one location to another.

Minimal risk level (MRL)
Health guidelines for non-cancer effects developed by ATSDR. They represent estimates of the daily human exposure to a contaminant that, based on ATSDR evaluations, should not cause non-cancer health effects during a specified exposure duration. MRLs are set below levels that might cause adverse health effects in most people, including sensitive populations. MRLs are derived for acute (1-14 days), intermediate (15-364 days), and chronic (365 days and longer).

Misclassification bias
Relates to when individuals were not properly categorized as to their disease risk or exposure during epidemiological studies. For instance, they may have been misdiagnosed with a disease or may have had a disease that was not diagnosed.

Mode of action
The overall means by which a chemical produces its adverse effect (e.g., enzyme inhibition, DNA adduct formation). A more general term than mechanism of action.

Modeled data
Information collected using mathematical tools and scientific methods to help answer questions that readily available information cannot. For instance, water modeling could be performed to help ATSDR estimate water system conditions during time periods when little or no data exist at a site. Another example is using air modeling to estimate contaminant concentrations over different time frames and over a wide range of locations.

State of being ill or diseased. Morbidity is the occurrence of a disease or condition that alters health and quality of life.

Death. Usually the cause (a specific disease, a condition, or an injury) is stated.

A disease resulting from the interaction of many factors, for example Type II diabetes, which is caused by several factors such as obesity, lack of exercise, etc.

A substance that causes mutations (genetic damage).

Mutagenic mode of action (MOA)
Carcinogens that act in such a way as to change (damage) the DNA, genes, or chromosomes of those exposed.

A contaminant’s capacity to change (damage) the DNA, genes, or chromosomes of those exposed. Mutagenicity is discussed in the Genotoxicity Section of ATSDR’s Toxicological Profiles.

A change (damage) to the DNA, genes, or chromosomes of living organisms.

National Priorities List for Uncontrolled Hazardous Waste Sites (National Priorities List or NPL)
EPA’s list of sites with known releases or threatened releases of hazardous substances, pollutants or contaminants that are listed as priority sites at the national level (that is, listed on the NPL).

National Toxicology Program (NTP)
Part of the Department of Health and Human Services. NTP develops and carries out tests to predict whether a chemical will cause harm to humans.

Natural attenuation
The reduction, breakdown, and transformation of contaminants via biological processes. An example of natural attenuation is when vinyl chloride is formed in the environment as a result of trichloroethylene being broken down by certain microorganisms.

No health hazard
One of three overall conclusion categories used in ATSDR’s documents. It applies to sites where harmful health effects are not likely in the population — exposures might be possible, but neither the duration nor the degree of exposure is sufficient to result in harmful health effects. It also applies to sites where there is no exposure to site-related hazardous substances.

No-observed-adverse-effect level (NOAEL)
The highest tested dose of a substance that has been reported to have no harmful (adverse) health effects on people or animals. For example, a study found no respiratory effects in male rats dosed at 50 mg/kg/, and documented this dose as the study’s NOAEL.

Non-ATSDR screening levels
Sometimes used by health assessors during the screening analysis. There are various types. These non-ATSDR values might be used in addition to ATSDR-derived CVs, or when ATSDR CVs are not available. An example of a non-ATSDR screening level is EPA’s maximum contaminant level (MCL).

Non-detect (ND)-qualified data
Data indicating that a sample was analyzed for a contaminant, but the contaminant was not detected at a concentration above the Method Detection Limit (MDL). Contaminants qualified as ND may also be called U- qualified data or may be presented as <MDL.

Non-discrete sample
Refers to composite sampling and the incremental sampling methodology (ISM).

NPL [see National Priorities List for Uncontrolled Hazardous Waste Sites]

Octanol/water partition coefficient (Kow)
A chemical’s potential to accumulate in animal fat by representing how a chemical is distributed at equilibrium between octanol and water.

Onsite mobile laboratories
Used to quickly analyze samples collected at a site to provide rapid results. An example of an onsite mobile laboratory is a mobile gas chromatography/ mass spectrometry (GC/MS) unit.

Oral cancer slope factors (CSFs)
Derived by EPA to measure the relative potency of various carcinogens for oral exposures. EPA performs quantitative evaluations of available animal and human studies, and uses various scientific methods (e.g., modeling) to develop oral CSFs based on those data. CSFs are estimates of increases in cancer cases in a human population associated with exposure to a suspected carcinogenic contaminant. For example, the oral cancer slope factor for a chemical is 0.055 milligrams per kilogram per day, based on leukemia cancers in humans.

Organic carbon partition coefficient (Koc)
The sorption affinity of a chemical for organic carbon and consequently the tendency for compounds to be adsorbed to soil and sediment. Often called the adsorption coefficient.

A chemical process that involves the adding of oxygen, resulting in a change in the chemical property.

Passive voice
A form of writing that involves putting the subject (the “doer”) in a sentence after the verb rather than before. ATSDR prefers writing in active voice rather than passive voice. For example, “ATSDR conducted a public health assessment of the site” is active. “A public health assessment of the site was conducted by ATSDR” uses passive voice.

The process of using the senses to obtain information about your surroundings.

The ability of a toxicant to pass through something, such as soil, skin, and the placenta. For instance, some toxicants more readily penetrate children’s skin, especially in the newborn period when the skin is more permeable.

Personal identifiable information (PII)
Any information that can be used to establish individual identity, directly or indirectly (for example, a person’s name, address, phone number, or Social Security number).

ATSDR, under its petition program, allows for any person or entity to request services. The petition process begins when the petitioner sends a petition requesting that ATSDR perform an evaluation to investigate specific environmental health concerns of a hazardous waste site or release. Once the letter is received, ATSDR works with the petitioner to better define the request, make a decision based on established criteria, and communicate the decision to the petitioner.

The study of biochemical and physiological effects of substances and their mechanisms of action.

Photochemical reaction
A reaction in the air driven by the sunlight.

Degradation by reacting with light.

Physiologically based pharmacokinetic model (PBPK model)
A computer model that describes what happens to a chemical in the body. This model describes how the chemical gets into the body, where it goes in the body, how it is changed by the body, and how it leaves the body.

A craving to eat nonfood items, such as dirt, paint chips, and clay. Some children exhibit pica-related behavior. It is associated with consumption of surface soil rather than geophagy, which involves the consumption of clay from sources at depth and is usually related to cultural practices (see geophagy).

Plain language
A style of communication that ensures the target audience can understand information the first time they read or hear it.  Plain language avoids the use of scientific jargon and overly technical language. Instead, it focuses on writing techniques that help readers find and understand information, and in turn,  they can use that information to address their needs. Techniques include use of simpler terms where possible (e.g., “breathe” instead of “inhale” and “eat” instead of “ingest”), active voice, short sentences, and reader-centered organization, among others. The Plain Writing Act requires federal agencies like ATSDR to write clearly for any documents issued to the public.

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 of departure
A modeled or estimated dose associated with a no- or lowest-observed-effect level. The point of departure is used as a stepping-off point for developing health guidelines.

Point of exposure
The place where someone can come into contact with a substance present in the environment [see exposure pathway].

A group or number of people living within a specified area or sharing similar characteristics (such as occupation or age).

The ability of a solid material to hold liquid or gas or allow liquid or gas to pass through it.

Potential exposure pathway
Indicates that exposure to a contaminant could have occurred in the past, could be occurring currently, or could occur in the future. A potential exposure exists when information about one or more of the five exposure pathway elements (contaminant source, environmental fate and transport, exposure point, exposure route, and potentially exposed population) is missing or uncertain, but the health assessor determines it is likely that the element exists.

Potentially exposed population
The fifth part of an exposure pathway. This represents the people who potentially have, do, or could come in contact with environmental contaminants.

Potentially responsible party (PRP)
A company, government, or person legally responsible for cleaning up the pollution at a hazardous waste site under Superfund. There may be more than one PRP for a particular site.

When a chemical mixture component that is not toxic to a particular biological system increases the effect of a second contaminant on that system.

The number of existing disease cases in a defined population during a specific time period [contrast with incidence].

Prevalence survey
The measure of the current level of disease(s) or symptoms and exposures through a questionnaire that collects self-reported information from a defined population.

Actions that reduce exposure or other risks, keep people from getting sick, or keep disease from getting worse.

Professional judgment
Using your experience and knowledge to interpret data and information to make decisions.

Public availability session
An informal, drop-by meeting at which community members can meet one-on-one with ATSDR staff members to discuss health and site-related concerns.

Public comment period
An opportunity for the public to comment on agency findings or proposed activities contained in draft reports or documents. The public comment period is a limited time period during which comments will be accepted.

Public health advisory
An ATSDR document that serves as an official notice given to EPA. The advisory identifies an immediate threat to human health from a hazardous substance, includes recommendations to reduce exposure and any threat to human health, and notifies appropriate state and local health and environmental agencies about any public health problems. Also called a health advisory.

Public health assessment (PHA)
An ATSDR document that summarizes the results of ATSDR’s evaluation of all available information about a site and the potentially affected communities. For example, it examines hazardous substances, health outcomes, and community concerns at a hazardous waste site to determine whether people could be harmed from coming into contact with those substances. The PHA also lists actions that need to be taken to protect public health.

Public Health Assessment Site Tool (PHAST)
ATSDR’s multi-purpose tool recommended for health assessors to use when performing various components of the PHA process, including using the PHAST CV Module for screening data. If a contaminant meets or exceeds the ATSDR CV or non-ATSDR screening level during the screening analysis, health assessors carry the contaminant into the PHAST Dose Calculator Module. PHAST is an application available to ATSDR and its partner health assessors, and other individuals involved in the PHA process. Health assessors and others involved in the PHA process can gain access by contacting

Public Health Assessment Training (PHAT)
ATSDR’s online basic course that teaches learners the public health assessment process. The course contains eight modules that provide information about the process, and step through various components of the PHA process, such as the exposure pathways evaluation and screening analysis. Learners review a health case study, perform knowledge checks, and proceed through class exercises. The course is available online here.

Public health statement
The first chapter of some ATSDR toxicological profiles. The public health statement is a summary written in words that are easy to understand. The public health statement explains how people might be exposed to a specific substance and describes the known health effects of that substance.

Public health surveillance
The ongoing, systematic collection, analysis, and interpretation of health data. This activity also involves timely dissemination of the data and use for public health programs.

Public meeting
A public forum with community members to listen to their concerns and share information about a site.

Information that describes something, but does not use a measured amount or quantity.

Quality assurance project plan (QAPP)
Describes the procedures to ensure sampling data are collected in a way to meet desired data quality objectives (DQOs) and quality assurance/quality control (QA/QC) measures. QAPPs outline specific quality acceptance criteria that relate to measures such as precision, accuracy, sensitivity, completeness, comparability, and representativeness of data. They lay out the DQO quantitative and qualitative measures that need to be considered when determining the quality and appropriateness of the data.

Quality of life
The degree of enjoyment and satisfaction experienced in everyday life (as opposed to financial or material well-being).

Information consisting of data that can be counted (i.e., numerical data).

Radiation [see ionizing radiation]

An unstable or radioactive isotope (form) of an element that can change into another element by giving off radiation.

Any radioactive isotope (form) of any element.

RCRA [see Resource Conservation and Recovery Act (1976, 1984)]

Reasonable maximum exposure (RME)
Refers to people who are at the high end of the exposure distribution (approximately the 95th percentile), a scenario intended to assess exposures that are higher than average, but still within a realistic exposure range. For instance, at an example site where children < 21 years old are exposed to a chemical via incidental surface water ingestion, the estimated RME lifetime cancer risk is 5.4E-05, which is based on the 95th percentile swimming ingestion intake rate of 0.12 liter per hour (L/hour) and exposure duration of 33 years. This can be compared to the central tendency exposure (CTE) lifetime cancer risk for this population of 1.6E-06 – based on the average swimming ingestion intake rate of 0.049 L/hour and exposure duration of 12 years.

Receptor population
People who could come into contact with hazardous substances [see exposure pathway].

Reference concentration (RfC)
Health guidelines derived by EPA for non-cancer health effects related to exposures to contaminants in air. RfCs are estimates of daily inhalation exposures to a contaminant that are likely to be without a discernible risk of deleterious effects to the general human population, including sensitive subgroups, during a lifetime of exposure.

Reference dose (RfD)
Health guidelines derived by EPA to examine exposure doses associated with non-cancer health effects. They are estimates of daily oral exposures to a contaminant that are likely to be without a discernible risk of deleterious effects to the general human population, including sensitive subgroups, during a lifetime of exposure.

Reference Dose Media Evaluation Guide (RMEG)
An ATSDR-derived comparison value based on non-cancer health effects for chronic exposure duration only. RMEGs represent the concentration in a specific medium (e.g., water or soil) at which daily human exposure is unlikely to result in adverse noncarcinogenic effects. ATSDR derives RMEGs from EPA’s reference doses (RfDs) and reference concentrations (RfCs). For air, RMEGs are the same as corresponding inhalation RfCs. For drinking water and soil, RMEGs are derived from the corresponding oral RfDs using conservative assumptions of intake rate and body weight.

Regional representative
Individuals who are always included on the site team and kept informed of PHA process activities. They serve as the liaison between ATSDR, the EPA, other environmental and health agencies in their region, and the community; and are instrumental in the implementation of ATSDR programs in that region. They maintain current and historic knowledge of the sites and issues in the specific regions, provide and follow up on ATSDR recommendations, and sometimes review site-specific information.

A systematic collection of information on persons exposed to a specific substance or having specific diseases [see exposure registry and disease registry].

Relative percent difference (RPD)
Comparative statistic used to evaluate precision. Most sampling and analytical methods specify acceptable ranges of RPDs. Comparing the reported RPD to these ranges or to a program’s data quality objectives (DQOs) can provide insights on the precision of the sampling data.

Remedial investigation (RI)
The CERCLA process of determining the type and extent of hazardous material contamination at a site. EPA is responsible for conducting RIs at Superfund sites.

Resource Conservation and Recovery Act (1976, 1984) (RCRA)
Enacted in 1976 to manage hazardous waste storage or destruction facilities. In 1984, a series of RCRA amendments authorized ATSDR to conduct PHAs at these sites. RCRA and its amendments also authorize ATSDR to assist EPA in determining which substances should be regulated and the levels at which substances may pose a threat to human health.

RCRA Facility Assessment (RFA)
An assessment required by RCRA to identify potential and actual releases of hazardous chemicals.

The probability that something will cause injury or harm.

Risk assessment (RA)
A process used by EPA as authorized by Congress to reduce or eliminate pollutants that could harm the environment and human health, as well as to guide regulators in determining safe regulatory limits and prioritizing Superfund sites for remedial action (“cleanup”). This differs from ATSDR’s public health assessment (PHA) process. The agencies collaborate during their efforts, with ATSDR serving in an advisory role and EPA serving in a regulatory role. Early in the RA process, ATSDR can sometimes influence EPA’s sampling efforts and priorities to obtain the most appropriate data to determine exposure and possible health outcomes. In turn, EPA uses information from ATSDR’s PHA process to help guide actions at sites.

Risk communication
The exchange of information to increase understanding of health risks.

Risk reduction
Actions that can decrease the likelihood that individuals, groups, or communities will experience disease or other health conditions.

Route of exposure
The way people come into contact with a hazardous substance. Three routes of exposure are breathing [inhalation], eating or drinking [ingestion], or contact with the skin [dermal contact].

R-qualified data
Data that have been rejected for quality reasons, and the contaminant of interest may or may not have been present in the original sample.

Safety factor [see uncertainty factor]

For statisticians, this typically refers to all observations from a data set. For instance, if 100 out of 125 people submit a survey, the data sample would be the 100 completed surveys. For environmental scientists, this typically refers to a physical quantity of an environmental medium – soil, water, air, and food items – that is collected for measurement. For instance, six ounces of tap water collected in a vial for laboratory analysis is one environmental sample. A kilogram of soil collected and combined from fifty points in a large stockpile is also a single sample. Unless otherwise noted, all references to “sample” in PHAGM describe environmental samples.

Sample size
The number of units chosen from a population or an environment.

Sampling Analysis Plan (SAP)
Provides data quality details and include information on site conditions that can impact the implementation of sampling tasks. In addition, the sampling and analysis plan could include information on the environmental media to be sampled, analytes to be measured within these media, sampling and analytical methods, proposed sampling locations, sampling frequency and duration, data quality objectives, Quality Assurance/Quality Control (QA/QC) measures, and health and safety considerations for field personnel.

Scientific evaluations
ATSDR performs four different types of scientific evaluations during the public health assessment process: Exposure Pathways, Screening Analysis, EPC and Exposure Calculations, and In-Depth Toxicological Effects Analysis. After examining the available information and data, health assessors conduct an exposure pathways evaluation. For completed and potential exposure pathways, health assessors will perform a screening analysis using the available sampling data. Based on the screening, health assessors may need to conduct an evaluation of exposure point concentrations (EPCs) and exposure calculations (exposure doses, adjusted air concentrations, hazard quotients, and cancer risks). Then if necessary, health assessors perform an in-depth toxicological effects analysis.

Screening analysis
One of four main scientific evaluations ATSDR performs during the public health assessment process. It involves comparing contaminant concentrations to media-specific screening levels (ATSDR comparison values and non-ATSDR screening levels) to identify those that meet or exceed screening levels, pinpointing contaminants with no available screening levels, and evaluating other factors (e.g., a community concern) that warrant closer examination in the EPC and exposure calculation evaluation.

The ability of an analytical method to accurately measure the smallest possible concentration of a contaminant. Some terms you may see in laboratory reports related to sensitivity include the limit of quantitation (LOQ) [see limit of quantitation] and the limit of detection (LOD) [see limit of detection].

Shower and Household Water-use Exposure (SHOWER) Model
ATSDR’s tool for health assessors to use when evaluating inhalation and dermal exposure to indoor air concentrations of contaminants from household water use. The PHA process for evaluating shower model exposure consists of several steps, with the SHOWER Model used for elements of both the screening analysis and exposure calculation evaluation components of the PHA process. Anyone can request a copy of the SHOWER Model by emailing

Simple mixture
A combination of a relatively small number of chemicals (no more than 10) that have been identified and quantified.

Site conceptual model
A diagram that helps you visualize how contaminants move in the environment at your site and how people might come into contact with these contaminants. The diagram should indicate the various ways in which contaminants, based on site-specific conditions, can move from the source through the environmental medium and to the points of exposure. Also see conceptual site model.

Site impact assessment (SIA)
ATSDR’s system for health assessors to document critical demographic information for individual exposure pathways.

Site remediation
Refers to the cleanup of the site or the use of other methods to remove or contain a site’s toxic spill or hazardous materials.

Site tour
Includes an inspection of the area under investigation with an emphasis on evaluating exposures to hazardous substances.

Skin surface area (SA) (for soil/sediment and surface water dermal contact dose calculations)
The amount of skin surface available for dermal contact with contaminants through these media.

Soil adherence factor (AF) (for soil/sediment dermal contact dose calculations)
A measure of soil adherence to the skin. Factors that can impact the soil adherence factor include moisture content, soil type, and the skin barrier. This is one of many parameters in the equation ATSDR uses to calculate a soil/sediment administered dermal dose. In the absence of site-specific information, ATSDR uses a soil adherence factor of 0.2 milligrams per square centimeter (mg/cm2) for child receptors and 0.07 mg/cm2 for adult receptors.

A liquid capable of dissolving or dispersing another substance (for example, acetone or mineral spirits).

Source of contamination
A place where a hazardous substance comes from, such as a landfill, waste pond, incinerator, storage tank, or drum. A source of contamination is the first part of an exposure pathway. Some sites have just one contamination source, but many sites have numerous sources. Each source represents a location—a point or area—where contaminants were, are, or could be released into the environment. And at other sites, the detected contamination could be naturally occurring or from another nearby (not-site-related) location.

Special populations
People who might be more sensitive or susceptible to exposure to hazardous substances because of factors such as age, occupation, sex, or behaviors (for example, cigarette smoking). Children, pregnant women, and older people are often considered special populations. Also see susceptibility.

A person, group, or community who has an interest in activities at a hazardous waste site.

Statistical power
The probability or chance that a study will detect a difference (an effect) between two populations, if a difference really exists. If the test does not have enough statistical power for the situation, then the test result will probably not be interpreted as an increase in illness. The result could appear (falsely) that there is no difference between the exposed and unexposed group in the amount of illness. How much statistical power a study design has depends on how large the study population is; how rare or common the disease is; and how much of a change from the expected the difference is. The bigger the effect you are looking for, the easier it is to recognize (so less statistical power is needed).

Collects, analyzes, and interprets data or information. Examines these data to determine whether differences between study groups are meaningful. In terms of involvement during the PHA process, a statistician performs statistical analyses on collected data for use during the PHA process, develops sample and survey designs to conduct research in communities affected by hazardous substances, and evaluates if methods used in health research are valid and reliable, including for information analyzed during the PHA process.

A branch of mathematics that deals with collecting, analyzing, summarizing, and interpreting data or information. Statistics are used to determine whether differences between study groups are meaningful.

The reaction people may have when presented with demands and pressures that are not matched to their knowledge and abilities and which challenge their ability to cope.

Stress level
Extent to which local environmental contamination places stress on a community. For some community members, learning about local environmental contamination can contribute to psychological stress. Environmental contamination can also cause intra-family and community-wide social stress (e.g., conflict). For most people, these stress reactions are normal, not an indication of mental health problems. But, chronic stress poses health risks on top of those associated with potential exposures.

Subject matter expert (SME)
A person with knowledge and expertise in a specific subject area. During the PHA process, the health assessor might consult with a variety of SMEs. The mix of SMEs will vary from one site to another and depend on the nature and complexity of site issues. The SMEs can also change over the course of the PHA process as more information becomes available; the composition of the site team can also change.

A chemical.

Superfund [see Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) and Superfund Amendments and Reauthorization Act (SARA)]

Superfund Amendments and Reauthorization Act (SARA)
Passed by the U.S. Congress in 1986. SARA amended the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA). CERCLA and SARA direct ATSDR to look into the health effects from substance exposures at hazardous waste sites and to perform activities including health education, health studies, surveillance, health consultations, and toxicological profiles. SARA also broadened ATSDR’s responsibilities regarding PHAs and led to the establishment and maintenance of toxicologic databases, dissemination of information and medical education.

Surface soil
Defined by ATSDR as the top 3 inches of soil. Health assessors can use soil sampling results collected from deeper than 3 inches (e.g., EPA commonly collects samples at a depth of 0-6 inches) to evaluate exposures, as long as it is documented.

Surface water
Water on the surface of the earth, such as in lakes, rivers, streams, ponds, and springs [compare with groundwater].

Surveillance [see public health surveillance]

A systematic collection of information or data. A survey can be conducted to collect information from a group of people or from the environment. Surveys of a group of people can be conducted by telephone, by mail, or in person. Some surveys are done by interviewing a group of people [see prevalence survey].

The state of being more easily affected by something, such as to the effects of contaminant exposure. For some selected contaminants, children are no more susceptible, and are sometimes less susceptible, than adults to an adverse outcome. For most contaminants, however, documentation points to increased susceptibility to environmental hazards among children. Also see special populations.

When the effect of a mixture is greater than that estimated by additivity. Synergism is defined in the context of the definition of no interaction, which is usually dose additivity or response additivity. The use of “greater-than-additive” is preferred over the use of the term synergism.

Synergistic effect
A biologic response to multiple substances where one substance worsens the effect of another substance. The combined effect of the substances acting together is greater than the sum of the effects of the substances acting by themselves [see additive effect and antagonistic effect].

Target-organ toxicity doses (TTDs)
An approach used when examining chemical mixtures. It is based on the fact that a chemical has the potential to cause multiple effects in humans and other organisms as a function of dose. TTDs are developed for the chemicals that affect an endpoint at a dose higher than that for the critical effect for the same chemical. A TTD for each endpoint of concern is calculated using appropriate MRL (or RfD) methodology to develop a value akin to an effect-specific MRL or RfD, and then used in estimating the endpoint-specific hazard indexes. The MRL (or RfD) is used for the critical effect for each chemical and the TTD is used for the other endpoints of concern for the chemical.

Technical assist (TA)
A written response to a request for scientific environmental public health and education information from external agencies and the public. A TA does not draw a public health conclusion or comment on another entity’s conclusion, but it provides information to help the requestor make informed decisions.

A substance that causes defects in development between conception and birth. A teratogen is a substance that causes a structural or functional birth defect.

The starting point or point at which something begins or changes. For example, an odor threshold is when you first smell that odor. An example of a toxicological threshold is the minimal amount of a contaminant that an animal is exposed to in a study that results in a measurable adverse effect.

Time to steady state (t*) (for surface water dermal contact dose calculations)
The time to reach steady absorption across the skin. For organic contaminants, the equation used to determine DAevent depends on the exposure duration (i.e., the contaminant-specific t* versus the site-specific tevent).

Toxic agent
Chemical or physical (for example, radiation, heat, cold, microwaves) agents that, under certain circumstances of exposure, can cause harmful effects to living organisms.

Toxicological profile
An ATSDR document that examines, summarizes, and interprets information about a hazardous substance to determine harmful levels of exposure and associated health effects. A toxicological profile also identifies significant gaps in knowledge on the substance and describes areas where further research is needed.

The study of kinetics (movement) of toxic substances within the body. Specifically, the study of the absorption, distribution, metabolism (biotransformation), and excretion of a contaminant.

The study of the harmful effects that chemicals may have on living organisms. It involves understanding how a substance gets into the body, how it is able to exert what adverse effects, how to prevent or mitigate those effects, and the amount of substance required to result in each adverse effect (i.e., the dose-response relationship).

Studies the health effects from natural and human-made poisons in living organisms. Evaluates health effects that might occur from exposure to site-related toxic chemicals.

An abnormal mass of tissue that results from excessive cell division that is uncontrolled and progressive. Tumors perform no useful body function. Tumors can be either benign (not cancer) or malignant (cancer).

UJ-qualified data
Data that indicate that the analyte was not detected and the reporting limit is estimated.

Uncertain health hazard
One of three overall conclusion categories used in ATSDR’s documents when a professional judgment about the level of health hazard cannot be made because information critical to such a decision is lacking.

Uncertainty factor
Mathematical adjustments for reasons of safety when knowledge is incomplete. For example, factors used in the calculation of doses that are not harmful (adverse) to people. When deriving minimal risk levels (MRLs), for instance, ATSDR uses uncertainty factors to account for things such as variations in people’s sensitivity, for differences between animals and humans, and for differences between an effect level and a no-observed-adverse-effect-level (NOAEL). Scientists use uncertainty factors when they have some, but not all, of the information from animal or human studies to decide whether an exposure will cause harm to people [also sometimes called a safety factor].

95 percent upper confidence limit of the arithmetic mean. Health assessors use statistical tools such as EPA’s ProUCL and R to calculate the 95UCL for the exposure point concentration (EPC).

Urgent public health hazard
A category used by ATSDR for sites where short-term exposures (less than 1 year) to hazardous substances or conditions could result in harmful health effects that require rapid intervention.

Vapor intrusion
The migration of vapor-forming chemicals from any subsurface source into an overlying building. Vapor-forming chemicals may include volatile organic compounds (VOCs), such as trichloroethylene and benzene; select semi-volatile organic compounds (SVOCs), such as naphthalene, elemental mercury; and some polychlorinated biphenyls and pesticides.

Vapor pressure
A measure of the volatility of a chemical in its pure state.

Volatile organic compounds (VOCs)
Organic compounds that evaporate readily into the air. VOCs include substances such as benzene, toluene, methylene chloride, and methyl chloroform.

Water solubility
The maximum concentration of a chemical that dissolves in a given amount of pure water.

Wind rose
Graphical displays of the statistical distribution of wind speeds and measurements of the direction that winds are coming from (for example, measurements of southerly winds are winds coming from the south to the north).

Page last reviewed: July 12, 2022