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Health Effects Glossary

Glossary of Health, Exposure, and Risk Assessment Terms
and Definitions of Acronyms

Absorption barrier: Any of the exchange barriers of the body that allow differential diffusion of various substances across a boundary. Examples of absorption barriers are the skin, lung tissue, and gastrointestinal tract wall.

ACGIH: American Conference of Governmental and Industrial Hygienists

ACGIH STEL: American Conference of Governmental and Industrial Hygienists' threshold limit value short-term exposure limit; a 15-min time-weighted-average exposure which should not be exceeded at any time during a workday.

ACGIH TLV: American Conference of Governmental and Industrial Hygienists' threshold limit value expressed as a time-weighted average; the concentration of a substance to which most workers can be exposed without adverse effects.

Accuracy: The measure of the correctness of data, as given by the difference between the measured value and the true or standard value.

Agent: A chemical, physical, mineralogical, or biological entity that may cause deleterious effects in an organism after the organism is exposed to it.

AIHA: American Industrial Hygiene Association

AIHA ERPG: American Industrial Hygiene Association's emergency response planning guidelines. ERPG 1 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed up to one hour without experiencing other than mild transient adverse health effects or perceiving a clearly defined objectionable odor; ERPG 2 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed up to one hour without experiencing or developing irreversible or other serious health effects that could impair their abilities to take protective action.

Ambient measurement: A measurement (usually of the concentration of a chemical or pollutant) taken in an ambient medium, normally with the intent of relating the measurement value to the exposure of an organism that contacts the medium.

 Ambient medium: One of the basic categories of material surrounding or contacting an organism, e.g., outdoor air, indoor air, water, or soil, through which chemicals or pollutants can move and reach the organism.

Arithmetic mean: The sum of all the measurements in a data set divided by the number of measurements in the data set.

ATSDR: Agency for Toxic Substances and Disease Registry

ATSDR MRL: An estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse noncancer health effects over a specified exposure duration.

Background level (environmental): The concentration of substance in a defined control area during a fixed period of time before, during, or after a data-gathering operation.

 Breathing zone: A zone of air in the vicinity of an organism from which respired air is drawn. Personal monitors are often used to measure pollutants in the breathing zone.

Bias: A systematic error inherent in a method or caused by some feature of the measurement system.

Biological medium: One of the major categories of material within an organism, e.g., blood, adipose tissue, or breath, through which chemicals can move, be stored, or be biologically, physically, or chemically transformed.
 
Biologically effective dose: The amount of a deposited or absorbed chemical that reaches the cells or target site where an adverse effect occurs, or where that chemical interacts with a membrane surface.

Blank (blank sample): An unexposed sampling medium, or an aliquot of the reagents used in an analytical procedure, in the absence of added analyte. The measured value of a blank sample is the blank value.

Body burden: The amount of a particular chemical stored in the body at a particular time, especially a potentially toxic chemical in the body as a result of exposure. Body burdens can be the result of long-term or short-term storage, for example, the amount of a metal in bone, the amount of a lipophilic substance such as PCB in adipose tissue, or the amount of carbon monoxide (as carboxyhemoglobin) in the blood.

Bounding estimate: An estimate of exposure, dose, or risk that is higher than that incurred by the person in the population with the highest exposure, dose, or risk. Bounding estimates are useful in developing statements that exposures, doses, or risks are "not greater than" the estimated value.

California EPA: California Environmental Protection Agency

California EPA reference exposure level:  A concentration at or below which adverse health effects are not likely to occur.

Comparability: The ability to describe likenesses and differences in the quality and relevance of two or more data sets.

Data Quality Objectives (DQO): Qualitative and quantitative statements of the overall level of uncertainty that a decisionmaker is willing to accept in results or decisions derived from environmental data. DQOs provide the statistical framework for planning and managing environmental data operations consistent with the data user's needs.

Dose: The amount of a substance available for interaction with metabolic processes or biologically significant receptors after crossing the outer boundary of an organism. The potential dose is the amount ingested, inhaled, or applied to the skin. The applied dose is the amount of a substance presented to an absorption barrier and available for absorption (although not necessarily having yet crossed the outer boundary of the organism). The absorbed dose is the amount crossing a specific absorption barrier (e.g., the exchange boundaries of skin, lung, and digestive tract) through uptake processes. Internal dose is a more general term denoting the amount absorbed without respect to specific absorption barriers or exchange boundaries. The amount of the chemical available for interaction by any particular organ or cell is termed the deliverable dose for that organ or cell.

Dose rate: Dose per unit time, for example in mg/day, sometimes also called dosage. Dose rates are often expressed on a per-unit-bodyweight-basis, yielding units such as mg/kg/d. They are often expressed as averages over some time period, for example a lifetime.
 
Dose-response curve: A graphical representation of the quantitative relationship between the administered, applied, or internal dose of a chemical or agent, and a specific biological response to that chemical or agent.

Dosimeter: Instrument to measure dose; many so-called dosimeters actually measure exposure rather than dose.

Ecological exposure: Exposure of a nonhuman receptor or organism to a chemical, or radiological or biological agent.

Effluent: Waste material being discharged into the environment, either treated or untreated. Effluent generally is used to describe water discharges to the environment, although it can refer to stack emissions or other material flowing into the environment.

Environmental fate model: In the context of exposure assessment, any mathematical abstraction of a physical system used to predict the concentration of specific chemicals as a function of space and time subject to transport, intermedia transfer, storage, and degradation in the environment.

Environmental medium: One of the major categories of material found in the physical environment that surrounds or contacts organisms, e.g., surface water, ground water, soil, or air, and through which chemicals or pollutants can move and reach the organisms.

Exposure concentration: The concentration of a chemical in its transport or carrier medium to the point of contact.

Exposure pathway: The physical course a chemical or pollutant takes from the source to the organism exposed.

Exposure route: The way a chemical or pollutant enters an organism after contact, e.g., by ingestion, inhalation, or dermal exposure.

Exposure scenario: A set of facts, assumptions and inferences about how exposure takes place that aids the exposure assessor in evaluating, estimating, or quantifying exposures.

Fixed-location monitoring: Sampling of an environmental or ambient medium for pollutant concentration at one location continuously or repeatedly over some length of time.
 
Geometric mean: The nth root of the product of n values.

Guidelines: Principles and procedures to set basic requirements for general limits of acceptability for assessments.

High-end exposure (dose) estimate: A plausible estimate of individual exposure or dose for those persons at the upper end of an exposure or dose distribution, conceptually above the 90th percentile, but not higher than the individual in the population who has the highest exposure or dose.

High-end Risk Descriptor: A plausible estimate of the individual risk for those persons at the upper end of the risk distribution, conceptually above the 90th percentile but not higher than the individual in the population with the highest risk. Note that persons in the high end of the risk distribution have high risk due to high exposure, high susceptibility, or other reasons, and, therefore, persons in the high end of the exposure or dose distribution are not necessarily the same individuals as those in the high end of the risk distribution.

Intake: The process by which a substance crosses the outer boundary of an organism without passing an absorption barrier, e.g., through ingestion or inhalation.

Internal dose: The amount of a substance penetrating across the absorption barriers (the exchange boundaries) of an organism via either physical or biological processes.

LC50 (Lethal Concentration50): A calculated concentration of a chemical in air to which exposure for a specific length of time is expected to cause death in 50% of a defined experimental animal population.

LD50 (Lethal Dose50): A calculated dose of a chemical in water to which exposure for a specific length of time is expected to cause death in 50% of a defined experimental animal population.

Limit of Detection (LOD) (or Method Detection Limit [MDL]): The minimum concentration of an analyte that, in a given matrix and with a specific method, has a 99% probability of being identified, qualitatively or quantitatively measured, and reported to be greater than zero.
 
LOAEL (lowest-observed-adverse-effect level): The lowest dose of a chemical in a study or group of studies that produce statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control.

Matrix: A specific type of medium (e.g., surface water, drinking water) in which the analyte of interest may be contained.

Maximally exposed individual (MEI): The single individual with the highest exposure in a given population (also, maximum exposed individual). This term has historically been defined in various ways, including as defined here and also synonymously with worst case or bounding estimate.

Maximum exposure range: A semiquantitative term referring to the extreme uppermost portion of the distribution of exposures. For consistency, this term should refer to the portion of the individual exposure distribution that conceptually falls above the 98th percentile of the distribution but is not higher than the individual with the highest exposure.

Median value: The value in a measurement data set such that half the measured values are greater and half are less.

Microenvironment method: A method used in predictive exposure assessments to estimate exposures by sequentially assessing exposure for a series of areas (microenvironment) that can be approximated by constant or well-characterized concentrations of a chemical or other agent.

Mode: The value in the data set that occurs most frequently.

Monte Carlo technique: A repeated random sampling from the distribution of values for each of the parameters in a generic (exposure or dose) equation to derive an estimate of the distribution of (exposures or doses in) the population.

NIOSH: National Institute of Occupational Safety and Health

NIOSH ceiling limit: NIOSH's recommended 15-min exposure limit, which should not be exceeded.

NIOSH IDLH--NIOSH's immediately dangerous to life and health; NIOSH concentration representing the maximum level of a pollutant from which an individual could escape within 30 minutes without escape-impairing symptoms or irreversible health effects.

NIOSH REL--NIOSH's recommended exposure limit; NIOSH-recommended exposure limit for an 8- or 10-h time-weighted-average exposure and/or ceiling.

NIOSH STEL--NIOSH's recommended short-term exposure limit; a 15-minute TWA exposure which should not be exceeded at any time during a workday.

NOAEL (no-observed-adverse-effect level): That dose of chemical at which there are no statistically or biologically significant increases in frequency or severity of adverse effects seen between the exposed population and its appropriate control. Effects may be produced at this dose, but they are not considered to be adverse.

Nonparametric statistical methods: Methods that do not assume a functional form with identifiable parameters for the statistical distribution of interest (distribution-free methods).

OSHA: Occupational Safety and Health Administration

OSHA PEL: Occupational Safety and Health Administration's permissible exposure limit expressed as a time-weighted average; the concentration of a substance to which most workers can be exposed without adverse effect averaged over a normal 8-h workday or a 40-h workweek.

Pathway: The physical course a chemical or pollutant takes from the source to the organism exposed.

Personal measurement: A measurement collected from an individual's immediate environment using active or passive devices to collect the samples.

Point-of-contact measurement of exposure: An approach to quantifying exposure by taking measurements of concentration over time at or near the point of contct between the chemical and an organism while the exposure is taking place.

Potential dose: The amount of a chemical contained in material ingested, air breathed, or bulk material applied to the skin.

Precision: A measure of the reproducibility of a measured value under a given set of conditions.

Probability samples: Samples selected from a statistical population such that each sample has a known probability of being selected.

Quality assurance (QA): An integrated system of activities involving planning, quality control, quality assessment, reporting and quality improvement to ensure that a product or service meets defined standards of quality with a stated level of confidence.

Quality control (QC): The overall system of technical activities whose purpose is to measure and control the quality of a product or service so that it meets the needs of its users. The aim is to provide quality that is satisfactory, adequate, dependable, and economical.

Quantification limit (QL): The concentration of analyte in a specific matrix for which the probability of producing analytical values above the method detection limit is 99 percent.

Random samples: Samples selected from a statistical population such that each sample has an equal probability of being selected.

Range: The difference between the largest and smallest values in a measurement data set.

Reasonable worst case: A semiquantitative term referring to the lower portion of the high end of the exposure, dose, or risk distribution. The reasonable worst case has historically been loosely defined, including synonymously with maximum exposure or worst case. As a semiquantitative term, it is sometimes useful to refer to individual exposures, doses, or risks that, while in the high end of the distribution, are not in the extreme tail. For consistency, it should refer to a range that can conceptually be described as above the 90th percentile in the distribution, but below about the 98th percentile.

Reconstruction of dose: An approach to quantifying exposure from internal dose, which is in turn reconstructed after exposure has occurred, from evidence within an organism such as chemical levels in tissues or fluids or from evidence of other biomarkers of exposure.

Representativeness: The degree to which a sample is, or samples are, characteristic of the whole medium, exposure, or dose for which the samples are being used to make inferences.

RfC (inhalation reference concentration): An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure of a chemical to the human population through inhalation (including sensitive subpopulations), that is likely to be without risk of deleterious noncancer effects during a lifetime.

RfD (oral reference dose): An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure of a chemical to the human population (including sensitive subpopulations) that is likely to be without risk of deleterious noncancer effects during a lifetime.

Sample: A small part of something designed to show the nature or quality of the whole. Exposure-related measurements are usually samples of environmental or ambient media, exposures of a small subset of a population for a short time, or biological samples, all for the purpose of inferring the nature and quality or parameters important to evaluating exposure.

Sampling frequency: The time interval between the collection of successive samples.

Sampling plan: A set of rules or procedures specifying how a sample is to be selected or handled.

Scenario evaluation: An approach to quantifying exposure by measurement or estimation of both the amount of a substance contacted, and the frequency/duration of contact, and subsequently linking these together to estimate exposure or dose.

Source characterization measurements: Measurements made to characterize the rate of release of agents into the environment from a source of emission such as an incinerator, landfill, industrial or municipal facility, consumer product, etc.

Standard operating procedure (SOP): A procedure adopted for repetitive use when performing a specific measurement or sampling operation.

Statistical control: The process by which the variability of measurements or of data outputs of a system is controlled to the extent necessary to produce stable and reproducible results. To say that measurements are under statistical control means that there is statistical evidence that the critical variables in the measurement process are being controlled to such an extent that the system yields data that are reproducible within well-defined limits.

Statistical significance: An inference that the probability is low that the observed difference in quantities being measured could be due to variability in the data rather than an actual difference in the quantities themselves. The inference that an observed difference is statistically significant is typically based on a test to reject one hypothesis and accept another.

Surrogate data: Substitute data or measurements on one substance used to estimate analogous or corresponding values of another substance.

TWA (time-weighted average): An allowable exposure concentration averaged over a normal 8-h workday or a 40-h workweek.

Uptake: The process by which a substance crosses an absorption barrier and is absorbed into the body.

Worst case: A semiquantitative term referring to the maximum possible exposure, dose, or risk, that can conceivably occur, whether or not this exposure, dose, or risk actually occurs in a specific population.  

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