ATSDR Glossary of Terms
The Agency for Toxic Substances and Disease Registry
(ATSDR) is a federal public health agency with headquarters in Atlanta,
Georgia, and 10 regional offices in the United States. ATSDR's mission is
to serve the public by using the best science, taking responsive public
health actions, and providing trusted health information to prevent harmful
exposures and diseases related to toxic substances. ATSDR is not a regulatory
agency, unlike the U.S. Environmental Protection Agency (EPA), which is
the federal agency that develops and enforces environmental laws to protect
the environment and human health.
This glossary defines words used by ATSDR in communications with the
public. It is not a complete dictionary of environmental health terms.
If you have questions or comments, call ATSDR's toll-free telephone number,
The process of taking in. 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.
Occurring over a short time [compare with chronic].
Contact with a substance that occurs once or for only a short time (up
to 14 days) [compare with intermediate
duration exposure and chronic exposure].
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
Adverse health effect
A change in body function or cell structure that might lead to disease
or health problems
Requiring oxygen [compare with anaerobic].
Surrounding (for example, ambient air).
Requiring the absence of oxygen [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 to hazardous substances
and disease by testing scientific hypotheses.
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].
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.
Decomposition or breakdown of a substance through the action of microorganisms
(such as bacteria or fungi) or other natural physical processes (such
of exposure study
A study that uses (a) biomedical testing
or (b) the measurement of a substance [an analyte],
its metabolite, or another marker of exposure
in human body fluids or tissues to confirm human exposure to a hazardous
substance [also see exposure investigation].
Measuring hazardous substances in biologic materials (such as blood, hair,
urine, or breath) to determine whether exposure has occurred. A blood
test for lead is an example of biologic monitoring.
The transfer of substances from the environment to plants, animals, and
Testing of persons to find out whether a change in a body function might
have occurred because of exposure to a hazardous substance.
Plants and animals in an environment. Some of these plants and animals
might be sources of food, clothing, or medicines for people.
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.
CAP [see Community
Any one of a group of diseases that occur when cells in the body become
abnormal and grow or multiply out of control.
A theoretical risk for getting cancer if exposed to a substance every
day for 70 years (a lifetime exposure). The true risk might be lower.
A substance that causes cancer.
A medical or epidemiologic evaluation of one person or a small group of
people to gather information about specific health conditions and past
A study that compares exposures of people who have a disease or condition
(cases) with people who do not have the disease or condition (controls).
Exposures that are more common among the cases may be considered as possible
risk factors for the disease.
CAS registry number
A unique number assigned to a substance or mixture by the American
Chemical Society Abstracts Service.
Central nervous system
The part of the nervous system that consists of the brain and the spinal
CERCLA [see Comprehensive
Environmental Response, Compensation, and Liability Act of 1980]
Occurring over a long time [compare with acute].
Contact with a substance that occurs over a long time (more than 1 year)
[compare with acute exposure and intermediate
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.
Community Assistance Panel
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.
Comparison value (CV)
Calculated concentration of a substance in air, water, food, or soil that
is unlikely to cause harmful (adverse) health effects in exposed people.
The CV is used as a screening level during the public health assessment
process. Substances found in amounts greater than their CVs might be selected
for further evaluation in the public health assessment process.
Completed exposure pathway
[see exposure pathway].
Environmental Response, Compensation, and Liability Act of 1980 (CERCLA)
CERCLA, also known as Superfund, 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.
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
Delayed health effect
A disease or an injury that happens as a result of exposures that might
have occurred in the past.
Referring to the skin. For example, dermal absorption means passing through
Contact with (touching) the skin [see route
The study of the amount and distribution of a disease in a specified population
by person, place, and time.
The lowest concentration of a chemical that can reliably be distinguished
from a zero concentration.
Measures used to prevent a disease or reduce its severity.
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,
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.
The relationship between the amount of exposure [dose]
to a substance and the resulting changes in body function or health (response).
Soil, water, air, biota (plants and animals), or
any other parts of the environment that can contain contaminants.
media and transport mechanism
Environmental media include water, air, soil, and biota (plants and animals).
Transport mechanisms move contaminants from the source to points where
human exposure can occur. The environmental media and transport mechanism
is the second part of an exposure pathway.
United States Environmental Protection Agency.
[see Public health surveillance].
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
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
The process of finding out how people come into contact with a hazardous
substance, how often and for how long they are in contact with the substance,
and how much of the substance they are in contact with.
A method of estimating the amount of people's past exposure to hazardous
substances. Computer and approximation methods are used when past information
is limited, not available, or missing.
The collection and analysis of site-specific information and biologic
tests (when appropriate) to determine whether people have been exposed
to hazardous substances.
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 parts: a source of contamination
(such as an abandoned business); an environmental
media and transport mechanism (such as movement through groundwater);
a point of exposure (such as a private
well); a route of exposure (eating, drinking,
breathing, or touching), and a receptor
population (people potentially or actually exposed). When all five
parts are present, the exposure pathway is termed a completed exposure
A system of ongoing followup of people who have had documented environmental
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.
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
Training sessions for physicians and other health care providers about
Water beneath the earth's surface in the spaces between soil particles
and between rock surfaces [compare with surface
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.
Hazardous Substance Release and Health Effects
The scientific and administrative database system developed by ATSDR to
manage data collection, retrieval, and analysis of site-specific information
on hazardous substances, community health concerns, and public health
Potentially harmful substances that have been released or discarded into
A review of available information or collection of new data to respond
to a specific health question or request for information about a potential
environmental hazard. Health consultations are focused on a specific exposure
issue. Health consultations are therefore more limited than a public health
assessment, which reviews the exposure potential of each pathway and chemical
[compare with public health assessment].
Programs designed with a community to help it know about health risks
and how to reduce these risks.
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.
The process of enabling people to increase control over, and to improve,
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.
The category used in ATSDR's public health assessment documents when a
professional judgment about the level of health hazard cannot be made
because information critical to such a decision is lacking.
The number of new cases of disease in a defined population over a specific
time period [contrast with prevalence].
The act of swallowing something through eating, drinking, or mouthing
objects. A hazardous substance can enter the body this way [see route
The act of breathing. A hazardous substance can enter the body this way
[see route of exposure].
Contact with a substance that occurs for more than 14 days and less than
a year [compare with acute exposure and
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
Within a living organism or body. For example, some toxicity testing is
done on whole animals, such as rats or mice [compare with in
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.
A set of medical tests and physical exams specifically designed to evaluate
whether an individual's exposure could negatively affect that person's
The conversion or breakdown of a substance from one form to another by
a living organism.
Any product of metabolism.
Milligram per kilogram.
Milligram per square centimeter (of a surface).
Milligram per cubic meter; a measure of the concentration of a chemical
in a known volume (a cubic meter) of air, soil, or water.
Moving from one location to another.
Minimal risk level (MRL)
An ATSDR estimate of daily human exposure to a hazardous substance at
or below which that substance is unlikely to pose a measurable risk of
harmful (adverse), noncancerous effects. MRLs are calculated for a route
of exposure (inhalation or oral) over a specified time period (acute,
intermediate, or chronic). MRLs should not be used as predictors of harmful
(adverse) health effects [see reference dose].
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)
A substance that causes mutations (genetic damage).
A change (damage) to the DNA, genes, or chromosomes of living organisms.
National Priorities List
for Uncontrolled Hazardous Waste Sites (National Priorities List or
EPA's list of the most serious uncontrolled or abandoned hazardous waste
sites in the United States. The NPL is updated on a regular basis.
National Toxicology Program
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.
No apparent public
A category used in ATSDR's public health assessments for sites where human
exposure to contaminated media might be occurring, might have occurred
in the past, or might occur in the future, but where the exposure is not
expected to cause any harmful health effects.
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.
No public health hazard
A category used in ATSDR's public health assessment documents for sites
where people have never and will never come into contact with harmful
amounts of site-related substances.
NPL [see National Priorities
List for Uncontrolled Hazardous Waste Sites]
Physiologically based pharmacokinetic 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.
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 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).
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.
Parts per billion.
Parts per million.
The number of existing disease cases in a defined population during a
specific time period [contrast with incidence].
The measure of the current level of disease(s) or symptoms and exposures
through a questionnaire that collects self-reported information from a
Actions that reduce exposure or other risks, keep people from getting
sick, or keep disease from getting worse.
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 action
A list of steps to protect public health.
Public health advisory
A statement made by ATSDR to EPA or a state regulatory agency that a release
of hazardous substances poses an immediate threat to human health. The
advisory includes recommended measures to reduce exposure and reduce the
threat to human health.
Public health assessment
An ATSDR document that 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
[compare with health consultation].
Public health hazard
A category used in ATSDR's public health assessments for sites that pose
a public health hazard because of long-term exposures (greater than 1
year) to sufficiently high levels of hazardous substances or radionuclides
that could result in harmful health effects.
Public health hazard
Public health hazard categories are statements about whether people could
be harmed by conditions present at the site in the past, present, or future.
One or more hazard categories might be appropriate for each site. The
five public health hazard categories are no
public health hazard, no
apparent public health hazard, indeterminate
public health hazard, public health
hazard, and urgent public health
Public health statement
The first chapter of an ATSDR toxicological profile. 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.
A public forum with community members for communication about a site.
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)]
People who could come into contact with hazardous substances [see exposure
Reference dose (RfD)
An EPA estimate, with uncertainty or safety factors built in, of the daily
lifetime dose of a substance that is unlikely to cause harm in humans.
A systematic collection of information on persons exposed to a specific
substance or having specific diseases [see exposure
registry and disease registry].
The CERCLA process of determining the type and extent of hazardous material
contamination at a site.
and Recovery Act (1976, 1984) (RCRA)
This Act regulates management and disposal of hazardous
wastes currently generated, treated, stored, disposed of, or distributed.
RCRA Facility Assessment. An assessment required by RCRA to identify potential
and actual releases of hazardous chemicals.
RfD [see reference dose]
The probability that something will cause injury or harm.
Actions that can decrease the likelihood that individuals, groups, or
communities will experience disease or other health conditions.
The exchange of information to increase understanding of health risks.
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].
Safety factor [see uncertainty
SARA [see Superfund
Amendments and Reauthorization Act]
A portion or piece of a whole. A selected subset of a population or subset
of whatever is being studied. For example, in a study of people the sample
is a number of people chosen from a larger population [see population].
An environmental sample (for example, a small amount of soil or water)
might be collected to measure contamination in the environment at a specific
The number of units chosen from a population or an environment.
A liquid capable of dissolving or dispersing another substance (for example,
acetone or mineral spirits).
Source of contamination
The 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.
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.
A person, group, or community who has an interest in activities at a hazardous
A branch of mathematics that deals with collecting, reviewing, summarizing,
and interpreting data or information. Statistics are used to determine
whether differences between study groups are meaningful.
A program of research designed to fill important data needs for specific
hazardous substances identified in ATSDR's toxicological
profiles. Filling these data needs would allow more accurate assessment
of human risks from specific substances contaminating the environment.
This research might include human studies or laboratory experiments to
determine health effects resulting from exposure to a given hazardous
Superfund [see Comprehensive
Environmental Response, Compensation, and Liability Act of 1980 (CERCLA)
and Superfund Amendments and Reauthorization Act (SARA)
Superfund Amendments and Reauthorization Act
In 1986, SARA amended the Comprehensive
Environmental Response, Compensation, and Liability Act of 1980 (CERCLA)
and expanded the health-related responsibilities of ATSDR. 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
Water on the surface of the earth, such as in lakes, rivers, streams,
ponds, and springs [compare with groundwater].
Surveillance [see public
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
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].
A substance that causes defects in development between conception and
birth. A teratogen is a substance that causes a structural or functional
Chemical or physical (for example, radiation, heat, cold, microwaves)
agents that, under certain circumstances of exposure, can cause harmful
effects to living organisms.
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
The study of the harmful effects of substances on humans or animals.
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).
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. These factors are applied to the lowest-observed-adverse-effect-level
(LOAEL) or the no-observed-adverse-effect-level (NOAEL)
to derive a minimal risk level (MRL).
Uncertainty factors are used to account for variations in people's sensitivity,
for differences between animals and humans, and for differences between
a LOAEL and a NOAEL. Scientists use uncertainty factors when they have
some, but not all, the information from animal or human studies to decide
whether an exposure will cause harm to people [also sometimes called a
Urgent public health hazard
A category used in ATSDR's public health assessments 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.
Volatile organic compounds
Organic compounds that evaporate readily into the air. VOCs include substances
such as benzene, toluene, methylene chloride, and methyl chloroform.
Other glossaries and dictionaries:
Environmental Protection Agency
Library of Medicine (NIH)
For more information on the work of ATSDR, please contact
NCEH/ATSDR Office of Communication, Information Services Center
1600 Clifton Road, N.E. (MS E-29)
Atlanta, GA 30333