95% Confidence Interval

A range that is calculated based on the standard error of a measurement and conveys how precise a measurement is. 95% confidence interval means that that the likelihood of the true mean falling within the interval is 95%.

95th Percentile

A value in a set of data where 95% of the other values are estimated to be smaller, or a value where only 5% of the other values are estimated to be larger. For example: In the National Exposure Report, the 95th percentile of a measurement indicates that 95% of the population is estimated to have lower concentration values and 5% of the population is estimated to have higher concentration values.


The process of a substance getting into the body through the eyes, skin, stomach, intestines, or lungs.


Occurring over a short time.

Acute exposure

Contact with a substance that occurs once or for a short time.

Acute toxicity

Adverse effects that occur shortly after an exposure, within a few hours to several days.

Adverse health effect

A change in body function or cell structure that may lead to disease or health problems.


A substance measured in a sample (e.g., water, air, blood) by laboratory analysis.

Analytical method

Laboratory procedure using various methods and instruments to measure the amount of a chemical in a human specimen such as blood or urine.

Blood lead level (BLL)

A measure of lead in the body usually reported as micrograms of lead per deciliter of blood (µg/dL).


An unintentional or secondary product formed when an intended product or chemical is being formed.


A substance that can cause cancer.

CAS registry number

A unique number assigned to a substance or mixture by the Chemical Abstracts Service (CAS) of the American Chemical Society.

Childbearing age

Range of ages during which a woman may become pregnant.For example: Can be defined as 16-49 years of age.

Chronic (health) effect

A health condition that develops and persists over a long period of time. For example: A health condition with symptoms that do not start immediately; symptoms may stay a long time (typically several weeks or more); or first symptoms may stop and then come back.

Chronic exposure

Contact with a substance that occurs for a long time, typically more than one year.

Clinical Laboratory Improvement Amendments (CLIA)

Federal requirements established to ensure the accuracy, reliability, and timeliness of laboratory test results. CLIA regulations apply to all facilities in the United States that perform laboratory testing on human specimens to assess health, diagnosis disease, or measure results of methods used to prevent or treat disease. All laboratories, including those at CDC, that perform these types of testing must meet the requirements specified by CLIA.


Substance composed by combining more than one part or element (or chemical if a “chemical compound”).


Amount of a substance present in a specific amount of any media (e.g., soil, water, air, food, blood, hair, urine, breath).


A substance that is either present in an environment where it does not belong, or is present at levels that might cause bad health effects.


Statistical measurement that describes the form, direction, and strength of a relationship between two things.


A substance that is a breakdown product of energy usage in muscle, filtered from the blood by the kidneys and eliminated from the body in urine.

Creatinine corrected

A technique used for calculating urine measurement results that can indicate whether urine was diluted or concentrated.


A metabolite (resulting product) formed when the body breaks down (metabolizes) nicotine.


For chemicals that are not radioactive: Amount of a toxic substance taken into the body from contaminated water, food, or soil over a given period of time; a measurement of exposure often expressed as milligram (amount) per kilogram (a measure of body weight) per day (a measure of time). For radioactive chemicals: The radiation dose is the amount of energy from radiation that is actually absorbed by the body. This measurement is not the same as the amount of radiation in the environment.


Naturally occurring substance (e.g., sodium, lead, arsenic, cadmium) that cannot be broken down by using chemical methods.

Environmental chemicals

Chemical compounds or elements present in the air, water, food, soil, or dust, or from other sources such as consumer products.

Environmental media

Soil, water, air, plants, animals, or any other part of the environment that can contain contaminants.

Environmental tobacco smoke (ETS)

Combination of tobacco smoke in the environment that comes from the burning end of a cigarette or cigar or smoked pipe tobacco, and the smoke exhaled by a smoker from any of these sources. Also referenced as second-hand smoke.


Protein that starts or speeds up a chemical reaction.

Epidemiologic Investigation

Collection and analysis of data related to the cause or spread of a disease, or health effects from a disease, in a population.


Process of eliminating a substance from the body.


Swallowing, breathing, or touching a substance through the skin or eyes. Exposure duration may be immediate, short term (14 days or less), intermediate, or long term (more than 1 year).

Geometric mean

Mathematical average calculated by taking the average of log-transformed data; often used to describe data sets that are not normally distributed.


The time required for half of the original amount of a substance to disappear. The substance may disappear by being changed to another chemical by bacteria, sunlight, or other chemical processes. In the human body, the substance may disappear by being changed to another substance and then leaving the body, or simply leaving the body.


A chemical, substance, or an exposure that can adversely affect health.

Human biomonitoring

Assessing human exposure to environmental chemicals by measuring chemicals or their metabolites, or reaction products in tissue or fluids such as blood and urine.

Inductively coupled plasma mass spectrometry (ICP-MS)

A complex laboratory method used to analyze metals and many inorganic chemicals at very low concentrations (as low as one part per billion or one part per trillion).


One or more items, characteristics, or markers to be assessed. Indicators provide information about a population’s health status, their environment, and other factors to help monitor trends, compare situations, and understand the link between environment and health.

Limit of detection (LOD)

The smallest amount of a substance that can be measured consistently by laboratory analysis.


Average of a list of numbers that is calculated by adding all the numbers and then dividing by the total number of items in the list.


The middle value in a set of values that are arranged in ascending or descending order.


The conversion or breaking down of a substance from one form to another by a living organism. Also refers to the conversion of chemicals into other products in the body.


A substance produced when the body breaks down a chemical or compound.

National Health and Nutrition Examination Survey (NHANES)

Ongoing survey of studies designed to assess the health and nutritional status of adults and children in the United States.


Chemical in tobacco that causes and maintains the powerful addicting effects of tobacco products.


An atom (tiny elements that make up everything, including humans) characterized by the number of protons and neutrons in the nucleus and the amount of energy it contains.

Nutritional indicator

Vitamins, trace elements, body iron stores, or other dietary measures that determine a person’s or population’s health status.


Term that describes a chemical that remains in the environment or the body for a long time (typically months to years).


Any substance released into the environment that adversely affects the health of humans, animals, or the environment.

Proficiency testing

Assessing a laboratory’s ability to accurately measure a substance sample against a known standard sample.


Energy moving in the form of particles or waves. Familiar radiations are heat, light, radio, and microwaves.


An unstable nuclide with excessive energy that is given off to form radiation.

Random sample

A process where members or items are chosen from a group (population) in no order or pattern.

Simple random sample

A sample (smaller group) of items or members chosen from a larger group so that all members within the larger group have an equal chance of being chosen.

Reference range

The highest and lowest values that will be used to measure or compare with other values. For example: The National Exposure Report provides reference ranges of environmental chemicals for the US population enabling researchers, physicians and health authorities to identify unusual exposure levels.

Reference values

Values used to compare other results of the same test. For example: Total cholesterol of 200 might be the reference value; above this value, a result is considered elevated or high, and likely to require intervention.

Rapid toxic screen (RTS)

A series of tests used at CDC’s Division of Laboratory Sciences to identify and measure a number of high-priority chemicals in human blood or urine.


A portion of blood, urine, or other body fluid or tissue taken for scientific and laboratory testing.


A portion or part of a population or whatever is being studied.For example: CDC National Exposure Report uses human blood and urine samples for analysis.


The liquid part of blood that remains after clotting proteins and blood cells are removed.

Special populations

People who are more sensitive to the effects of hazardous substances because of age, occupation, sex, or behaviors (e.g., cigarette smoking). Special populations can be children, pregnant women, and older adults.

Standardized reagents

A reagent is a substance or chemical used to prepare a solution used in chemical analysis. A standardized reagent is usually a solution with a known amount of the chemical that is used to measure unknown quantities of the same chemical.

Public Health Surveillance

Collection, analyzing, interpreting, and distributing health data continuously and logically.

Quality assurance/Quality control

A program designed to produce results that are valid, scientifically provable, precise, accurate, and fair.


The extent to which a substance is harmful.


Harmful or poisonous substance produced by an organism such as a bacteria, plant, or animal.

Volatile organic compounds (VOCs)

Contaminants that contain carbon atoms and easily become vapors or gases at room temperature. Some VOCs are released from burning fuel such as gasoline and coal.

Page last reviewed: April 7, 2017