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See also Abbreviations and Acronyms

A , B, C, D , E, F, G, H , I, J, K, ,L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Abatement - The reduction in degree or intensity of pollution

Absolute risk – Absolute risk is the risk of developing a disease or condition over a time-period

Absolute lethal concentration, LC100 -  Lowest concentration of a substance in an environmental medium which kills 100 % of test organisms or species under defined conditions. This value is dependent on the number of organisms used in its assessment.

Absolute lethal dose (LD100) - Lowest amount of a substance that kills 100 % of test animals under defined conditions.

Absorbed dose (of a substance) - Amount (of a substance) taken up by an organism or into organs or tissues of interest. See absorption, systemic.

Absorption (in biology) -   Penetration of a substance into an organism by various processes, some specialized, some involving expenditure of energy (active transport), some involving a carrier system, and others involving passive movement down an electrochemical gradient: in mammals, absorption is usually through the respiratory tract, gastrointestinal tract, or skin.

Absorption barrier - Any exposure surface that may retard the rate of penetration of an agent into a target. Examples of absorption barriers are the skin, respiratory tract lining, and gastrointestinal tract wall.

Absorption, systemic - Uptake to the blood and transport via the blood of a substance to an organ or compartment in the body distant from the site of absorption.

Absorption coefficient (in biology) - Ratio of the absorbed quantity (uptake) of a substance to the administered quantity (intake).

Acceptable Daily Intake - Estimated maximum amount of an agent, expressed on a body mass basis, to which individuals in a (sub) population may be exposed daily over their lifetimes without appreciable health risk.

Acceptable Risk - This is a risk management term. The acceptability of the risk depends on scientific data, social, economic, and political factors, and the perceived benefits arising from exposure to an agent. A risk whose probability of occurrence is so small, whose consequences are so slight, or whose benefits (perceived or real) are so great, that persons or groups in society are willing to take or be subjected to that risk.  

Acceptable Risk Level - When it is not possible to completely eliminate human exposure to a hazardous chemical or agent, a decision must be made as to how large a risk can be accepted in order to allow the use of that chemical or agent.  >Acceptable= levels of environmental hazards will vary with the specific application and substance and, in principle, should be as low as reasonable achievable, taking into account not only the hazard but also the social and economic benefits and the available technology.  Depending on circumstances, acceptable levels of risk vary from one extra death per 10 000 (1 x 10-4) people exposed to the contaminant, to one extra cancer death per million people exposed (1 x 10-6).{McColl, Hicks, et al. 2000 206 /id}

Accepted risk - Probability of suffering disease or injury that is accepted by an individual.

Accident - That occurrence in a sequence of events which usually produces unintended injury, death or property damage.

Accidental exposure - Unintended contact with a substance or change in the physical environment (including, e.g., radiation) resulting from an accident.

Accumulation (in biology) - See bioaccumulation.

Accuracy - The degree of agreement between a measured value and the true value; usually expressed as +/- percent of full scale.

Action level
1. Concentration of a substance in air, soil, water, or other defined medium at which specified emergency counter-measures, such as the seizure and destruction of contaminated materials, evacuation of the local population or closing down the sources of pollution, are to be taken.
2. Concentration of a pollutant in air, soil, water, or other defined medium at which some kind of preventive action (not necessarily of an emergency nature) is to be taken.

Active ingredient - Component of a mixture responsible for the biological effects of the mixture.

Active metabolite - Metabolite causing biological and (or) toxicological effects.

Activity pattern data - Information on human activities used in exposure assessments. These may include a description of the activity, frequency of activity, duration spent performing the activity, and the microenvironment in which the activity occurs.

1. Of short duration, in relation to exposure or effect. In experimental toxicology, acute refers to studies where dosing is either single or limited to one day, although the total study duration may extend to two weeks.
2. In clinical medicine, sudden and severe, having a rapid onset.

Acute Effects - Effects that occur rapidly following exposure and are of short duration.

Acute Exposure – A contact between an agent and a target occurring over a short time, generally less than a day.

Acute Toxicity - The adverse effects occurring within a short time of administration of a single dose or multiple doses given within 24 hours.

Adaptive Effect - Effects that enhance an organism's performance as a whole and/or its ability to withstand a challenge (i.e., homeostatic mechanism).

Adaptive Response - A physiological response to environmental chemical exposure that helps to protect the organism from the harmful effects of toxic substances, or to compensate for physiological disturbances caused by toxic agents.

Added risk - Difference between the incidence of an adverse effect in a treated group (of organisms or a group of exposed humans) and a control group (of the same organisms or the spontaneous incidence in humans).

Adverse effect - Change in morphology, physiology, growth, development, reproduction, or lifespan of an organism, system, or (sub)population that results in an impairment of functional capacity or impairment of capacity, an impairment of the capacity to compensate for additional stress or an increase in susceptibility to other influences.

Adverse event - Occurrence that causes an adverse effect.

Aerosol - System in which the dispersion medium is a gas and the dispersed phase (composed of solid particles or liquid droplet) does not settle out under the influence of gravity.

Aerosol particles - Solid particles <10-6 m in diameter, dispersed in gas.

After-effect of a poison - Ability of a poison to produce a change in an organism after cessation of contact.

Agent – A chemical, biological, or physical entity that contacts a target.

Air emissions - The release or discharge of a pollutant (from a stationary source) into the ambient air. For anthropogenic sources this may involve release (1) by means of a stack or (2) as a fugitive dust, mist or vapor as a result inherent to the manufacturing or formulating process. Pollutants may also be discharged from mobile sources, from area sources such as roads and fields, and from non-manufacturing, stationary sources.

Air monitoring - The continuous sampling for, and measuring of, pollutants present in the atmosphere.

Air pollutant - Dust, fumes, mist, smoke and other particulate matter, vapor, gas, odorous substances, or any combination thereof; any air pollution agent or combination of such agents, including any physical, chemical, biological, radioactive (including source material, special nuclear material, and by-product material) substance or matter which is emitted into or otherwise enters the ambient air.

Air pollution - Presence of substances in the atmosphere resulting either from human activity or natural processes, in sufficient concentration, for a sufficient time and under circumstances such as to interfere with comfort, health, or welfare of persons or to harm the environment.

Air quality criteria - The levels of pollution and lengths of exposure above which adverse effects may occur on health and welfare.

Air quality standards - The level of pollutants prescribed by law or regulation that cannot be exceeded during a specified time in a defined area.

Air sampling - The collection and analysis of air samples for detection or measurement of radioactive substances, particulate matter, or chemical pollutants.

ALARA - As Low As Reasonably Achievable - A concept which asserts that environmental exposure to toxic substances should be kept as low as is reasonably achievable, using pollution control equipment and industrial processes that can be installed and operated at reasonable cost.
Allergen - Immunostimulant antigenic substance that may or may not cause a clinically significant effect but which is capable of producing immediate hypersensitivity.

Allergy - Symptoms or signs occurring in sensitized individuals following exposure to a previously encountered substance (allergen) which would otherwise not cause such symptoms or signs in non-sensitized individuals.  The most common forms of allergy are rhinitis, urticaria, asthma, and contact dermatitis.

Ambient - Surrounding (applied to environmental media such as air, water, sediment, or soil).

Ambient level - The level (of pollutant) in the general environment as characterized by an average over a suitably long time and large volume.

Ambient monitoring - Continuous or repeated measurement of agents in the environment to evaluate ambient exposure and health risk by comparison with appropriate reference values based on knowledge of the probable relationship between exposure and resultant adverse health effects.

Analysis - Detailed examination of anything complex, made in order to understand its nature or to determine through an appropriate risk assessment.

Anecdotal Data - Data based on descriptions of individual cases rather than on controlled studies.

Antagonism - Agents are antagonistic when the observed effect from combined exposure is less than that expected from a simple summation of single exposures. They produce opposite effects, hence then lack of simple summation response.

Association - The non-random occurrence of a disease in relation to an exposure.  An observed association suggests that a cause-effect relationship exists between an exposure and health effect, but cannot by itself establish causality.

Anticarcinogen - A substance or agent that opposes the action of carcinogens.

1. Caused by or influenced by human activities.
2. Describing a conversion factor used to calculate a dose or concentration affecting a human that has been derived from data obtained with another species (e.g., the rat).

Anthropogenic Hazard - An environmental hazard attributable to human activity, most commonly ascribed to synthetic (>man-made=) chemicals and their by-products, but also encompassing natural agents released by human activities.

Area source - Widespread origin of emissions.

Artefact - Observation, effect, or result which is inaccurate because it is produced by the methodology used in scientific investigation or by experimental error.

1. Process of quantitative or qualitative analysis of a component of a sample.
2. Results of a quantitative or qualitative analysis of a component of a sample.
3. To carry out quantitative or qualitative analysis of a component of a sample.

Assessment - Evaluation or appraisal of an analysis of facts and the inference of possible consequences concerning a particular object or process.

Assessment end-point - Quantitative/qualitative expression of a specific factor with which a risk may be associated as determined through an appropriate risk assessment.

Assessment factor - Numerical adjustment used to extrapolate from experimentally determined (dose-response) relationships to estimate the agent exposure below which an adverse effect is not likely to occur.

Assimilation - Uptake and incorporation of substances by a living organism.

Attributable risk - The rate of a disease in exposed individuals that can be attributed to the exposure. This measure is derived by subtracting the rate (usually incidence or mortality) of the disease among non-exposed persons from the corresponding rate among exposed individuals.

Background levelThe amount of an agent in a medium (e.g., water, soil) that is not attributed to the source(s) under investigation in an exposure assessment. Background level(s) can be naturally occurring or the result of human activities. (Note: Natural background is the concentration of an agent in a medium that occurs naturally or is not the result of human activities.)

Baseline Risk - The level of risk that exists, or is predicted to exist, prior to the introduction of remedial or preventive control actions intended to reduce risk to a lower level.

Benchmark concentration (BMC) - Statistically calculated lower 95 % confidence limit on the concentration that produces a defined response (called the benchmark response or BMR, usually 5 or 10 %) for an adverse effect compared to background, often defined as 0 or 5 %.

Benchmark response - Response expressed as an excess of background, at which a benchmark dose or benchmark concentration is set.

Benefit - Advantage to or improvement in condition of an individual or a population.

Best Available Control Technology (BACT) - Technical equipment or industrial processes that are intended to reduce the emission of chemical contaminants into the environment.  The term >best-available= is often seen as a commitment to employ state-of-the-art control technology, where the effectiveness of control measures is deemed more important than its capital and operating costs or economic efficiency.

Best Practicable Control Technology (BPCT) - Technical equipment or industrial processes that are intended to reduce the emission of chemical contaminants into the environment, using technology where cost of control and economic efficiency are considered in conjunction with overall effectiveness of the control measures.

Best Practical Technology - That technology (machinery, software etc.) that is easily available and affordable which will fulfil the needs of the proposed health strategies.

Bias - Deviation of results or inferences from the truth, or processes leading to such deviation.  Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth.
Biased sample - Any sample that is not a random sample.

Bioaccessible - Able to come in contact with a living organism and interact with it.

Bioaccessibility - Potential for a substance to come in contact with a living organism and then interact with it. This may lead to absorption.

Bioaccumulation - The process by which the amount of a substance in a living organism (or its parts) increases with time.

Bioaccumulation potential - Ability of living organisms to concentrate a substance obtained either directly from the environment or indirectly through its food.

Bioassay - Procedure for estimating the concentration or biological activity of a substance by measuring its effect on a living system compared to a standard system.

Bioavailability – The rate and extent to which an agent can be absorbed by an organism and is available for metabolism or interaction with biologically significant receptors. Bioavailability involves both the release from a medium (if present) and absorption by an organism.

Bioconcentration - A process leading to a higher concentration of a chemical in the organism relative to its environment.

Biological Assessment of Exposure - Exposure to chemicals may be assessed by the analysis of specimens taken in the environment (air, water, food, etc.) or of specimens of biological material.  Most often, urine an blood are analyzed, but other materials such as expired air, faeces, saliva, bile, hair, and biopsy or autopsy material are sometimes analyzed.  In these samples, the content of the xenobiotic(s) or its metabolite(s) is determined and, on this basis, the exposure level (concentration in the air, absorbed amount of the substance) or the probability of health impairment due to exposure is derived.  Biochemical changes in the components of an organism can also be sued for this purpose (e.g., changes in enzyme activity or in the excretion of metabolic intermediates) if they show a relationship to the exposure.

Biological Cycle - The process through which a chemical substance passes in the biosphere.  It may involve transport through the various media (air, water, soil), followed by environmental transformation, and carriage through various ecosystems.  Chemical compounds that occur naturally have a natural biological cycle.

Biological effect monitoring (BEM) - Continuous or repeated measurement of early biological effects of exposure to a substance to evaluate ambient exposure and health risk by comparison with appropriate reference values based on knowledge of the probable relationship between ambient exposure and biological effects.

Biological exposure indices (BEI) - Guidance values recommended by ACGIH for assessing biological monitoring results.

Biological half life - For a substance, the time required for the amount of that substance in a biological system to be reduced to one half of its value by biological processes, when the rate of removal is approximately exponential.

Biological magnification - The concentration of certain substances up a food chain. A very important mechanism in concentrating pesticides and heavy metals in organisms such as fish.

Biological monitoring (biomonitoring) - Continuous or repeated measurement of any naturally occurring or synthetic chemical, including potentially toxic substances or their metabolites or biochemical effects in tissues, secreta, excreta, expired air, or any combination of these in order to evaluate occupational or environmental exposure and health risk by comparison with appropriate reference values based on knowledge of the probable relationship between ambient exposure and resultant adverse health effects.

Biomagnification - A sequence of processes in an ecosystem by which higher concentrations are attained in organisms of higher trophic level i.e., of higher levels in the food chain.

Biomarker/biological marker - Indicator of changes or events in biological systems. Biological markers of exposure refer to cellular, biochemical, analytical, or molecular measures that  are obtained from biological media such as tissues, cells, or fluids and are indicative of exposure to an agent.

Biomarker of effect - Biomarkerthat, depending on its magnitude, can be recognized as associated with an established or possible health impairment or disease.

Biomarker of exposure - Biomarkerthat relates exposure to a xenobioticto the levels of the substance or its metabolite, or of the product of an interaction between the substance and some target molecule or cell that can be measured in a compartment within an organism.

Biomarker of susceptibility - Biomarkerof an inherent or acquired ability of an organism to respond to exposure to a specific substance.

Biotransformation - Chemical conversionof a substance that is mediated by living organisms or enzyme preparations derived there from.

Blood–brain barrier - Barrier formed by the blood vessels and supporting tissues of the brain that prevents some substances from entering the brain from the blood.

Blood–placenta barrier - Physiological interface between maternal and fetal blood circulations that filters out some substances which could harm the fetus while favoring the passage of others such as nutrients: Many fat-soluble substances such as alcohol are not filtered out, and several types of virus can also cross this barrier. The effectiveness of the interface as a barrier varies with species and different forms of placentation.

Body burden - The total amount of a specific substance (for example, lead) in an organism, including the amount stored, the amount that is mobile, and the amount absorbed.

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

Bystander exposure - Liability of members of the general public to come in contact with substances arising from operations or processes carried out by other individuals in their vicinity.

Calibration - Operation that, under specified conditions, in a first step establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.

Cancer - Disease resulting from the development of a malignant tumor.

Carcinogen - An agent, chemical, physical or biological, that can act on living tissue in such a way as to cause a malignant neoplasm, thus causing cancer.

Carcinogenesis - Development of carcinoma; or, in more recent usage, producing any kind of malignancy.

Carcinogenic - Cancer causing.

Carcinogenic potency - The gradient of the dose-response curve for a carcinogen.

Carcinogenicity - Process of induction of malignant neoplasms, and thus cancer, by chemical, physical, or biological agents.

Carcinogenicity test - Long-term (chronic) test designed to detect any possible carcinogenic effect of a test substance.

Carcinoma - Malignant new growth made up of epithelial cells tending to infiltrate the surrounding tissues and give rise to metastases.

Case-Control Study - An analytical design in epidemiology involving the selection of diseased cases and non-diseased controls, followed by assessment of prior exposures in the case group and control group.

Case-fatality rate - A ratio of the number of deaths due to a disease to the number of cases of that disease in a specified period of time. It expresses the frequency with which affected individuals die of the disease.

Categorical Data - Specify the form or severity of adverse effects as a function of dose rate without reference to the number of animals- affected-or to a continuous measure of one parameter.  Graded data often are presented as categories (liver necrosis, lung lesions) or as judgments of severity. Fatty infiltration of the liver, single-cell liver necrosis, and liver necrosis are examples of sequence of severity judgments.  Graded data can be used to construct a dose severity curve.

Causality, Causal Relationship - The relationship between an environmental exposure and a health effect where the exposure is found to cause the observed effect. Where possible, causality can be best established directly by experimental studies, but in environmental health causality is often determined from observational studies (e.g. epidemiology) using a combination of causality criteria that constitute a weight-of-evidence.

Ceiling value, CV - Airborne concentration of a potentially toxic substance which should never be exceeded in a worker’s breathing zone.

Chemical safety - Practical certainty that there will be no exposure of organisms to toxic amounts of any substance or group of substances: This implies attaining an acceptably low risk of exposure to potentially toxic substances.

Chemical species (of an element) - Specific form of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure.

Chronic - Having a persistent, recurring or long-term nature. As distinguished from acute.

Chronic effect - Consequence that develops slowly and/or has a long lasting course: may be applied to an effect that develops rapidly and is long lasting.

Chronic Exposure – A continuous or intermittent long-term contact between an agent and a target.

Chronic Toxicity - Another term used to describe delayed toxicity.  However, the term "chronic toxicity" also refers to effects that persist over a long period of time whether or not they occur immediate­ly or are delays.  The term "chronic toxicity" is often confused with that of chronic exposure.

Chronic Toxicity Study - A toxicity study designed to measure the (toxic) effects of chronic exposure to a chemical

Chronic toxicity test - Study in which organisms are observed during the greater part of the life span and in which exposure to the test agent takes place over the whole observation time or a substantial part thereof.

Clearance (in toxicology) - Volume of blood or plasma or mass of an organ effectively cleared of a substance by elimination (metabolism and excretion) divided by time of elimination.

Clinical toxicology - Scientific study involving research, education, prevention, and treatment of diseases caused by substances such as drugs and toxins.

Co-Carcinogenic Agent - A chemical, physical or biological factor which acts synergistically with another carcinogen to induce a cancerous state.

Cohort - Component of the population born during a particular period and identified by period of birth so that its characteristics (such as causes of death and numbers still living) can be ascertained as it enters successive time and age periods. The term “cohort” has broadened to describe any designated group of persons followed or traced over a period of time, as in the term cohort study (prospective study).

Cohort analysis - Tabulation and analysis of morbidity or mortality rates in relationship to the ages of a specific group of people (cohort), identified by their birth period, and followed as they pass through different ages during part or all of their life span. In certain circumstances such as studies of migrant populations, cohort analysis may be performed according to duration of residence in a country rather than year of birth, in order to relate health or mortality experience to duration of exposure.

Cohort Study - An epidemiologic study that observes subjects in differently exposed groups and compares the incidence of symptoms.  Although ordinarily prospective in nature, such a study is sometimes carried out retrospectively, using historical data.

Comparable Hazard - A hazard that bears sufficient similarity in its harmful effects and societal consequences to be readily compared by decision-makers when considering the nature and degree of health risks for different environmental hazards.

Comparative risk - An expression of the risks associated with two (or more) actions leading to the same goal; may be expressed quantitatively (a ratio of 1.5) or qualitatively (one risk greater than another risk).

Compartment - Conceptualized part of the body (organs, tissues, cells, or fluids) considered as an independent system for purposes of modeling and assessment of distribution and clearance of a substance.

Complete Carcinogen - An agent that, by itself, can cause cancer.  This agent does not require separate promoters and initiators; it is all these things in itself.

Compounded Conservatism - The use of a series of conservative (pessimistic) assumptions is a risk analysis, where each conservative assumption taken alone is not unreasonable, but where the overall effect of many conservative assumptions taken together results in an extremely pessimistic prediction of possible risk.

Computational toxicology - Application of mathematical and computer models to predict adverse effects and to better understand the mechanism(s) through which a given chemical causes harm.

Concentration – Amount of a material or agent dissolved or contained in unit quantity in a given medium or system.

Concentration-effect relationship – Relationship between the exposure, expressed in concentration, of a given organism, system, or (sub)population to an agent in a specific pattern during a given time and the magnitude of a continuously graded effect to that organism, system, or (sub)population.

Confidence interval - A range of values (a1 < a < a2) determined from a sample of definite rules so chosen that, in repeated random samples from the hypothesized population, an arbitrarily fixed proportion of that range will include the true value, x, of an estimated parameter. The limits, a1 and a2, are called confidence limits; the relative frequency with which these limits include a is called the confidence coefficient; and the complementary probability is called the confidence level. As with significance levels, confidence levels are commonly chosen as 0.05 or 0.01, the corresponding confidence coefficients being 0.95 or 0.99. Confidence intervals should not be interpreted as implying that the parameter itself has a range of values; it has only one value, a. On the other hand, the confidence limits (a1, a2) being derived from a sample, are random variables, the values of which on a particular sample either do or do not include the true value a of the parameter. However, in repeated samples, a certain proportion of these intervals will include a provided that the actual population satisfied the initial hypothesis.

Confounder - A condition or variable that may be a factor in producing the same response as the agent under study.  The effects of such factors may be discerned through careful design and analysis.

Confounding (in data analysis)
1. Situation in which the effects of two processes are not distinguishable from one another: The distortion of the apparent effect of an exposure on risk brought about by the association of other factors which can influence the outcome.
2. Relationship between the effects of two or more causal factors as observed in a set of data, such that it is not logically possible to separate the contribution which any single causal factor has made to an effect.
3. Situation in which a measure of the effect of an exposure on risk is distorted because of the association of exposure with other factor(s) which influence the outcome under study.

Confounding Variable - A variable that can cause or prevent the outcome of interest, is not an intermediate variable, and is not associated with the factor under investigation.  Such a variable must be controlled in order to obtain an undistorted estimate of the effect of the study factor on risk.

Conservatism - 1.  In empirical science: A tendency to avoid making premature judgments about a scientific issue until the uncertainties are reduced by further study.  2.  In public health: One or more reasonable worst-case assumptions in environmental health risk assessment, which are intended to ensure that potential risks are seldom underestimated, but at the cost of over-estimating some risks.  Often applied to risk identification and risk estimation procedures to ensure adherence with a Precautionary Principle in public health. 

Conservative assessment of risk - Assessment of risk that assumes the worst possible case scenario and therefore gives the highest possible value for risk: Risk management decisions based on this value will maximize safety.

Construct validity - Extent to which a measurement corresponds to theoretical concepts (constructs) concerning the phenomenon under study; for example, if on theoretical grounds, the phenomenon should change with age, a measurement with construct validity would reflect such a change.

Contact volume - A volume containing the mass of agent that contacts the exposure surface.

Containment - Process by which possible release, discharge, or spill of a toxic substance during normal use or after an accident is prevented by appropriate action.

Contaminant - A substance foreign to a natural system of present at unnatural concentrations.  A contaminant is an unwanted agent in the environment which has the potential under certain circumstances to cause harmful health effects or ecological damage, depending on the degree of exposure and the susceptibility of exposed individuals.  An environmental contaminant may originate both from natural processes and from anthropogenic (>man-made=) activities, although for the purposes of risk management the principal focus is usually on anthropogenic contaminants.

Content validity - Extent to which the measurement incorporates the domain of the phenomenon under study; for example, a measurement of functional health status should embrace activities of daily living, occupational, family, and social functioning, etc.

Contingent valuation - Willingness to pay for something that is non-marketed (e.g. improved health status), as derived from people=s responses to questions about preferences for various combinations of situations and/or controlled discussion groups.

Continuous Data - represent the change in some measured value of a biological indicator (e.g., organ weights, triglyceride levels in the liver, and serum enzyme measurements) as a function of dose rate.  Continuous data can be used to construct a dose-effect curve.

Control Group - A group of subjects observed in the absence of agent exposure or, in the instance of a case/control study, in the absence of an adverse response.

Control, matched - Control (individual or group or case) selected to be similar to a study individual or group, or case, in specific characteristics: some commonly used matching variables are age, sex, race, and socioeconomic status.

1. Causing a surface-destructive effect on contact; in toxicology, this normally means causing visible destruction of the skin, eyes, or the lining of the respiratory tract or the gastrointestinal tract.

Cost-Benefit Analysis (CBA) - It is measured by difference between the sum of all costs and the sum of all benefits to give a net benefit figure. A positive CBA implies that the program of health improvement is predicted to be worthwhile. This term can be modified by looking only at certain costs and certain benefits to fit the needs of the particular decision-maker. Clearly, the effectiveness of such an analysis depends on how the benefits are quantified.

Cost-Effectiveness - The relationship describing the cost of control for reducing the level of a risk by a specified amount.  For example, cost-effectiveness is often measured as the number of dollars expended per life saved for a series of alternative control options. 

Cost-Effectiveness Analysis (CEA) - A CEA assumes that money will be spent on improving health and examines the best or most effective way to effect improvements.  A standard way of measuring this is by a cost-effectiveness ratio (CE = cost/gains) where the lowest CE will represent the most effective program; the CE measurement is a useful way of comparing the costs of different strategies for effecting health improvements.

Cost-Utility Analysis (CUA) - QALYs are a standardized measure of mortality and morbidity rates used to describe the quality of health. Use of identical units in differing CUAs allows for simultaneous comparison among varying programs.

Criterion - Validated set of data used as a basis for judgment.

Criterion validity - Extent to which the measurement correlates with an external criterion of the phenomenon under study.

Critical concentration (for a cell or an organ) - Concentration of a substance at and above which adverse functional changes, reversible or irreversible, occur in a cell or an organ.

Critical dose - Dose of a substance at and above which adverse functional changes, reversible or irreversible, occur in a cell or an organ.

Critical effect - For deterministic effects, the first adverse effect which appears when the threshold (critical) concentration or dose is reached in the critical organ: adverse effects with no defined threshold concentration are regarded as critical.

Critical organ (in toxicology) - Organ that attains the critical concentration of a substance and exhibits the critical effect under specified circumstances of exposure and for a given population.

Critical period (of development) - Stage of development of an organism that is of particular importance in the life cycle if the normal full development of some anatomical, physiological, metabolic, or psychological structure or function is to be attained.

Critical toxic effect - The most sensitive and specific biological change which is outside of acceptable physiological variation.

Cross-Sectional Study - A study that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defined population oat one particular time.  The presence or absence of disease and the presence or absence of the other variables (or, if they are quantitative, their level) are determined in each member of the study population or in a representative sample at one particular time.  The relationship between a variable and the disease can be examined 1.  In terms of the prevalence of disease in different population subgroups defined according to the presence or absence (or level) of the variables 2.  In terms of the presence or absence (or level) of the variables in the diseased versus the nondiseased.  Note that disease prevalence rather than incidence is normally recorded in a cross-sectional study.  The temporal sequence of cause and effect cannot necessarily be determined in a cross-sectional study.

Cumulative effect - Overall change that occurs after repeated doses of a substance or radiation.

Cumulative risk
1. Probability of a common harmful effect associated with concurrent exposure by all relevant pathways and routes of exposure to a group of substances that share a common chemical mechanism of toxicity.
2. Total probability of a harmful effect over time.

Damage - Damage is the severity of injury or the physical, functional, or monetary loss that could result if control of a hazard is lost.

Danger - Expresses a relative exposure to a hazard. A hazard may be present, but there may be little danger because of the precautions taken.

Decision Criteria - Critical factors of a subjective non-scientific nature that help to identify and select an optimal control strategy from a variety of possible control options.

Decision Maker - A person or group with the power or authority to make decisions.

>de manifestis= risk - A predicted health risk that is sufficiently large and serious to be considered for immediate risk control, usually set by convention at a lifetime risk level of 10-4.{McColl, Hicks, et al. 2000 206 /id}

>de minimus= risk - A predicted risk that is so small or its expected health consequences so slight that society is deemed willing to accept or be subjected to that risk, as an >insignificant risk= or >trivial risk=.  Based on a traditional legal principle that trivial risks are not subject to legal proceedings.  Often held by convention to be a default risk value of 1 in a million (10-6) or one in a hundred thousand (10-5) risk of severe illness or death over an individual lifetime.{McColl, Hicks, et al. 2000 206 /id}

Deterministic effect, deterministic process - Phenomenon committed to a particular outcome determined by fundamental physical principles.

Deterministic Process - A biological disease process that occurs through the gradual accumulation of cell damage or harmful physiological changes, so that greater exposure to a toxic substance typically leads to greater expression of the disease along a continuous measure of severity.

1. Process, or processes, of chemical modification which make a toxic molecule less toxic.
2. Treatment of patients suffering from poisoning in such a way as to promote physiological processes which reduce the probability or severity of harmful effects.

Developmental Toxicity - The study of adverse effects on the develop­ing organism (including death, structural abnormality, altered growth, or functional deficiency) resulting from exposure prior to conception (in either parent), during prenatal development, or postnatally up to the time of sexual maturation.

Diffusion - Spontaneous differential movement of components in a system.

Disability Adjusted Life Year (DALY) – Disability Adjusted Life Year (DALY) is the sum of Years of Life Lost due to premature mortality (YLL) and the Years Lost due Disability (YLD).

Disabling injury - An injury causing death, permanent disability, or any degree of temporary total disability beyond the day of the accident.

Disease - A general term describing a morbid condition which can be defined by objective, physical signs (e.g. hypertension), subjective symptoms or mental phobias, disorder of function (e.g. biochemical abnormality), or disorders of structure (anatomic or pathological change). Existence of disease may be questioned in disorder of structure without associated disorder of function.

Dispersion (in environmental chemistry) - Dilution of a pollutant by spreading in the atmosphere or water due to diffusion or turbulent action.

1. Natural tendency shown by an individual or group of individuals, including any tendency to acquisition of specific diseases, often due to hereditary factors.
2. Total of the processes of absorption of a chemical into the circulatory systems, distribution throughout the body, biotransformation, and excretion.

1. Apportionment of a solute between two phases. The terms “partition” or “extraction” may also be used in this sense where appropriate.
2. Dispersal of a substance and its derivatives throughout the natural environment or throughout an organism.
3. Final location(s) of a substance within an organism after dispersal.

Distribution volume - Theoretical volume of a body compartment throughout which a substance is calculated to be distributed.

Dominant half life - Half life of a fraction of a substance in a specific organ or compartment if it defines approximately the overall clearance rate for that substance at a specific time point.

Dosage - Dose divided by product of mass of organism and duration of dose. Often expressed mg (kg body weight)–1 day–1 and may be used as a synonym for dose.

Dose - Total amount of an agent administered to, taken up by, or absorbed by an organism, system, or (sub)population.

Dose–effect - Relation between dose and the magnitude of a measured biological change

Dose–effect curve - Graph of the relation between dose and the magnitude of the biological change produced measured in appropriate units.

Dose–effect relationship – Relationship between the total amount of an agent administered to, taken up by, or absorbed by an organism, system, or (sub)population and the magnitude of a continuously graded effect to that organism, system, or (sub)population.

Dose-Rate - dose per unit time.

Dose-related effect – Any effect to an organism, system, or (sub)population as a result of the quantity of an agent administered to, taken up by, or absorbed by that organism, system, or (sub)population.

Dose-response Relationship between the amount of an agent administered to, taken up by, or absorbed by an organism, system, or (sub)population and the change developed in that organism, system or (sub)population in reaction to the agent.

Dose-Response Assessment – Analysis of the relationship between the total amount of an agent administered to, taken up by, or absorbed by an organism, system, or (sub)population in reaction to that agent, and inferences derived from such an analysis with respect to the entire population.

Dose–response curve – Graphical presentation of a dose-response relationship.

Dose-Response Relationship – Relationship between the amount of an agent administered to, taken up by, or absorbed by an organism, system, or (sub)population and the change developed in that organism, system, or (sub)population in reaction to the agent.

Ecological impact - The total effect of an environmental change, natural or man-made, on the community of living things.

Ecology - The science dealing with the relationship of all living things with each other and with their environment.

Ecosystem - The interacting system of a biological community and its nonliving surroundings.

Effect Change in the state or dynamics of an organism, system, or (sub)population caused by the exposure to an agent.

Effect assessment – Combination of analysis and inference of possible consequences of the exposure to a particular agent based on knowledge of the dose-effect relationship associated with that agent in a specific target organism, system, or (sub)population.

Efficacy - A measure of the probability and intensity of beneficial effects.

Elimination (in toxicology) - Disappearance of a substance from an organism or a part thereof, by processes of metabolism, secretion, or excretion.

Elimination rate - Differential with respect to time of the concentration or amount of a substance in the body, or a part thereof, resulting from elimination.

Endpoint - A response measure in a toxicity study.

Emission - Like effluent but used in regard to air pollution.

Emission rate - The amount of pollutant emitted per unit of time.

Environment - Water, air, land, and all plants and man and other animals living therein, and the interrelationships which exist among them.

Environmental monitoring - Continuous or repeated measurement of agents in the environment to evaluate environmental exposure and possible damage by comparison with appropriate reference values based on knowledge of the probable relationship between ambient exposure and resultant adverse effects.

Environmental pathway - All routes of transport by which a toxicant can travel from its release site to human populations including air, food chain, and water.

Epidemiology - The study of the distribution and determinants of health-related states or events in populations, and the application of this study to control of health problems.

Equilibrium - State of a system in which the defining variables (temperature, pressure, chemical potential) have constant values.

Estimated Exposure Dose (LEO) - The measured or calculated dose to which humans are likely to be exposed considering exposure by all sources and routes.

Event - Occurrence of a particular set of circumstances 1.  The event may be certain or uncertain 2.  The event can be singular or multiple 3.  The probability associated with the event can be estimated for a given period of time.

Excess deaths - The excess over statistically expected deaths in a population within a given time interval. Attempts are made to relate excess deaths to specific causes. Note that since every person can (and must) die only once; there can be no excess deaths over all time.

Expected deaths - The number of deaths statistically expected in a population in a given time interval obtained by summing the product of age-, sex-, and race-specific mortality rates from a standard population and person-years in each age, sex, and race category in the study population.

Expected loss - The quantity obtained by multiplying the magnitude of health or environmental effect loss by the probability (or risk) of that loss and adding the products. The expected loss is the average loss over a large number of trials; one must reflect on the appropriateness of its use in cases for which there will be only one, or a few, trials.

Extrapolation - In risk assessment, this process entails postulating a biologic reality based on observable responses and developing a mathematical model to describe this reality. The model may then be used to extrapolate to response levels which cannot be directly observed.

Excretion- Discharge or elimination of an absorbed or endogenous substance, or of a waste product, and/or its metabolites, through some tissue of the body and its appearance in urine, feces, or other products normally leaving the body.

Excretion rate - Amount of substance and/or its metabolites that is excreted divided by time of excretion.

Exposure - Concentration or amount of a particular agent that reaches a target organism, system, or (sub)population in a specific frequency for a defined duration.

Exposure Assessment - Evaluation of the exposure of an organism, system, or (sub)population to an agent (and its derivatives). Exposure assessment is the third step in the process of risk assessment.

Exposure Estimation - Estimation of the amount and duration of contact between a receptor and an agent.  May consider such factors as concentration, route, receptor, population, and timescale.

Exposure Identification - Identification of the conditions of contact between a receptor and an agent. May involve identification of concentration, route, receptor population, and timescale.

Exposure scenario - A set of conditions or assumptions about sources, exposure pathways, amounts or concentrations of agent(s) involved, and exposed organism, system, or (sub)population (i.e., numbers, characteristics, habits) used to aid in the evaluation and quantification of exposure(s) in a given situation.

Extrapolation - The calculation, based on quantitative observations in exposed test species, of predicted dose-effect and dose-response relationships for a chemical in humans and other environmental biota.

False negative results - Results which show no effect when one is there.

False positive results - Results which show an effect when one is not there.

Fate - Pattern of distribution of an agent, its derivatives, or metabolites in an organism, system, compartment, or (sub)population of concern as a result of transport, partitioning, transformation, or degredation.

First-order process
1. Chemical reaction where the rate is directly proportional to the concentration of reactant.
2. Any reaction changing at a constant fractional rate.
First-pass effect -   Biotransformation and, in some cases, elimination of a substance in the liver after absorption from the intestine and before it reaches the systemic circulation.

Food chain - Dependence of a series of organisms, one upon the other, for food. The chain begins with plants and ends with the largest carnivores.

Frank-Effect Level (FEL) - The exposure level which produces unmistak­able adverse effects, such as irreversible functional impairment or mortality, at a statistically or biologically significant increase in frequency or severity between an exposed population and its appropri­ate control.

Genetic effects - Effects that are inheritable and appear in the descendants of those exposed.

Genotoxic - Capable of causing a heritable change to the structure of DNA thereby producing a mutation.

Guideline - A guideline is a recommended exposure limit for a substance or an agent intended to protect human health or the environment, that is not legally enforceable.

Guidance value - Value, such as concentration in air or water, that is derived after allocation of the reference dose among the different possible media (routes) of exposure. The aim of the guidance value is to provide quantitative information from risk assessment to the risk managers to enable them to make decisions.

Half-life - The time in which half the molecules of a chemical substance disappear as a result of chemical or biochemical transformation.

Half-life, biological - The time required for a living organism to eliminate, by natural processes, half the amount of a substance that has entered it.

Half-life, effective - The time required for a radionuclide contained in a biological system to reduce its activity by half due to the combined result of radioactive decay and biological.

Harmonization - Harmonization represents the attempts by regulatory bodies at the provincial/state, federal, and international level to adopt identical or similar regulatory limits, including definitions of allowable limits, monitoring and measurement methods, and legal penalties.

Hazard – Inherent property of an agent or situation having the potential to cause adverse effects when an organism, system, or (sub)population is exposed to that agent.

Hazard assessment A process designed to determine the possible adverse effects of an agent or situation to which an organism, system, or (sub)population could be exposed. The process focuses on the hazard identification and hazard characterization. The process focuses on the hazard, in contrast to risk assessment, where exposure assessment is a distinct additional step.

Hazard characterization – The qualitative and, wherever possible, quantitative description of the inherent property of an agent or situation having the potential to cause adverse effects. This should, where possible, include a dose-response assessment and its attendant uncertainties. Hazard characterization is the second stage in the process of hazard assessment and the second of four steps in risk assessment.

Hazard Identification - The identification of the type and nature of adverse effects that an agent has an inherent capacity to cause in an organism, system, or (sub)population. Hazard identification is the first stage in hazard assessment and the first of four steps in risk assessment.

Hazardous waste - Any waste or combination of wastes which pose a substantial present or potential hazard to human health or living organisms because such wastes are nondegradable or persistent in nature or because they can be biologically magnified, or because they can be lethal, or because they may otherwise cause or tend to cause detrimental cumulative effects; also, a waste or combination of wastes of a solid, liquid, contained gaseous, or semisolid form which may cause, or contribute to, an increase in mortality or an increase in serious irreversible, or incapacitating reversible illness, taking into account the toxicity of such waste, its persistence and degradability in nature, its potential for accumulation or concentration in tissue, and other factors that may otherwise cause or contribute to adverse acute or chronic effects on the health of persons or other organisms.

Health and safety study - Any study of any effect of a chemical substance or mixture on health or the environment or on both, including underlying data and epidemiological studies, studies of occupational exposure to a chemical substance or mixture, toxicological, clinical, and ecological studies of a chemical substance or mixture, and any test performed pursuant to this [TSCA] Act.

Health effect - A deviation in the normal function of the human body.

Health risk - Risk in which an adverse event affects human health.

Healthy worker effect - The difference in mortality risk due to selection forces between a population of active workers healthy enough to have been (and remain) employed and the general population which includes sick and disabled persons. If working in a safe environment, such a population of active workers has been variously estimated to have a mortality risk 60-90% that of the general population.

Human Equivalent Dose - The human dose of an agent that is believed to induce the same magnitude of toxic effect as that which the known animal dose has induced.

Hypersensitivity - A hyper susceptible individual is one who will experience an adverse health effect significantly before the general population, because of one or more factors which predispose the individual to the harmful effects of pollutant exposure.

Idiosyncratic Reaction - A genetically-determined abnormal reactivity to a chemical.  

Idiosyncrasy - The increased individual sensitivity of an organism to the effect of certain substances.

Immediate versus Delayed Toxicity - Immediate effects occur or develop rapidly after a single administration of a substance, while delayed effects are those that occur after the lapse of some time. These effects have also been referred to as acute and chronic, respectively.

Impact - The force of impression of one thing on another.

Incidence - The number of instances of illness commencing, or of persons falling ill, during a given period in a specific population.  Incidence is usually expressed as a rate, the denominator being the average number of persons in the specified population during a defined period or the estimated number of persons at the mid-point of that period.  The basic distinction between incidence and prevalence is that whereas incidence refers only to new cases, prevalence refers to all cases, irrespective of whether they are new or old.  When the terms incidence and prevalence are used, it should be stated clearly whether the data represent the numbers of instances of the disease recorded or the numbers of persons ill.

Incidence Rate - The rate at which new events occur in a population.  The numerator is the number of new events that occur in a defined period; the denominator is the population at risk of experiencing the event during this period, sometimes expressed as person-time.  The incidence rate most often used in public health practice is calculated by the formula
Number of new events in a specified period       X   10n
    Number of persons exposed to risk during the period                      

In a dynamic population, the denominator is the average size of the population, often the estimated population at the mid-period.  If the period is a year, this is the annual incidence rate.  This rate is an estimate of the person-time incidence rate, i.e., the rate per 10n person-years.  If the rate is low, as with many chronic diseases, it is also a good estimate of the cumulative incidence rate.  In follow-up studies with no censoring, the incidence rate is calculate by dividing the number of new cases in a specified period by the initials size of the cohort of person being followed; this is equivalent to the cumulative incidence rate during the period.  If the number of new cases during a specified period is divided by the sum of the person-time units at risk for all persons during the period, the result is the person-time incidence rate.

Individual Risk - The probability that an individual person will experience an adverse effect.  This is identical to population risk unless specific population subgroups can be identified that have different (higher or lower) risks.

Individual susceptibility - The marked variability in the manner in which individuals will respond to a given exposure to a toxic agent.

Initiator - An agent that causes a cell to become immortalized. This is the first step in the multi-step process of carcinogenesis in which a cell continues to divide and fails to terminally differentiate. This step may be referred to as anti-differentiation.

Intake – The process by which an agent crosses an outer exposure surface of a target without passing an absorption barrier, i.e., through ingestion or inhalation.

Interspecies Dose Conversion - The process of extrapolating from animal doses to equivalent human doses.

In vitro - Outside the living organism.

In vivo - Within the living organism.

Involuntary Risks - Exposures which are not under individual control, for example, exposure to air or water pollutants.  In general, voluntary risks (such as smoking) are more seen as more acceptable than involuntary risks.

Iterative Process - A process that is repeated periodically in a systematic fashion.  Some or all of the steps in the risk management process can be revisited as new information becomes available, allowing for more informed decisions to be considered as uncertainty is reduced by further study and analysis.

Kinetics (in chemistry) - Branch of chemistry concerned with measuring and studying rates of chemical reactions.

Latent Period - Delay between exposure to a disease-causing agent and the appearance of manifestation of the disease.  After exposure to ionizing radiation, for instance, there is a latent period of five years, on average, before development of leukemia, and more than 20 years before development of certain other malignant conditions.  The term Alatent period@ is often used synonymously with Ainduction period@, that is, the period between exposure to a disease-causing agent and the appearance of manifestations of the disease.  It has also been defined as the period from disease initiation to disease detection.

Lethal concentration, LC - Concentration of a substance in an environmental medium that causes death following a certain period of exposure.

Lethal concentration fifty (LC50)

  • A calculated concentration [in air] which when administered by the respiratory route is expected to kill 50% of a population of experimental animals during an exposure of four hours. Ambient concentration is expressed in milligrams per liter.

Lethal dose, LD - Amount of a substance or physical agent (e.g., radiation) that causes death when taken into the body.

Lethal dose fifty (LD50) - A calculated dose of a chemical substance which is expected to kill 50% of a population of experimental animals exposed through a route other than respiration. Dose is expressed in milligrams per kilogram of body weight.

Lethal synthesis - Metabolic formation of a highly toxic compound often leading to death of affected cells.

Lifetime Risk – Lifetime Risk is the risk of developing a disease during an individual’s lifetime.

Linearized multistage model - Sequence of steps in which (a) a multistage model is fitted to tumor incidence data; (b) the maximum linear term consistent with the data is calculated; (c) the low-dose slope of the doseresponse function is equated to the coefficient of the maximum linear term; and (d) the resulting slope is then equated to the upper bound of potency.

Linear No-Threshold Model (LNT) - The simplest of toxicological dose-response relationships in which a doubling of the original dose would be expected to result in a doubling of the response frequency, and a halving of the original dose would produce a halving of the response frequency and so on down the dose ladder all the way to zero dose.  Used primarily for carcinogenic or mutagenic environmental contaminants.

Local effect - Change occurring at the site of contact between an organism and a toxicant.

Local versus Systemic Toxicity - Local effects refer to those that occur at the site of first contact between the biological system and the toxicant: systemic effects are those that are elicited after absorption and distribution of the toxicant from its entry point to a distant site.

Lowest effective dose, LED - Lowest dose of a chemical inducing a specified effect in a specified fraction of exposed individuals.

Lowest-observed-adverse-effect level, LOAEL - Lowest concentration or amount of a substance (dose), found by experiment or observation, which causes an adverse effect on morphology, functional capacity, growth, development, or life span of a target organism distinguishable from normal (control) organisms of the same species and strain under defined conditions of exposure.

Lowest-observed-effect level, LOEL - Lowest concentration or amount of a substance (dose), found by experiment or observation, that causes any alteration in morphology, functional capacity, growth, development, or life span of target organisms distinguishable from normal (control) organisms of the same species and strain under the same defined conditions of exposure.

Margin of exposure, MOE - Ratio of the no-observed-adverse-effect level (NOAEL) for the critical effect to the theoretical, predicted, or estimated exposure dose or concentration.

Margin of Safety – For some experts, margin of safety has the same meaning as margin of exposure, while for others, margin of safety means the margin between the reference dose and the actual exposure.

Maximum tolerable concentration, MTC - Highest concentration of a substance in an environmental medium that does not cause death of test organisms or species (denoted by LC0).

Maximum tolerable dose, MTD - Highest amount of a substance that, when introduced into the body, does not kill test animals (denoted by LD0).

Maximum tolerable exposure level, MTEL - Maximum amount (dose) or concentration of a substance to which an organism can be exposed without leading to an adverse effect after prolonged exposure time.

Measurement end-point - Measurable (ecological) characteristic that is related to the valued characteristic chosen as an assessment point.

Median effective concentration, EC50 - Statistically derived concentration of a substance in an environmental medium expected to produce a certain effect in test organisms in a given population under a defined set of conditions.
Note: ECn refers to the median concentration that is effective in n % of the test population.

Median effective dose, ED50 - Statistically derived dose of a chemical or physical agent (radiation) expected to produce a certain effect in test organisms in a given population or to produce a half-maximal effect in a biological system under a defined set of conditions.
Note: EDn refers to the median dose that is effective in n % of the test population.

Median lethal concentration, LC50 - Statistically derived concentration of a substance in an environmental medium expected to kill 50 % of organisms in a given population under a defined set of conditions.

Median lethal dose, LD50 - Statistically derived dose of a chemical or physical agent (radiation) expected to kill 50 % of organisms in a given population under a defined set of conditions.

Median lethal time, TL50 - Statistically derived average time interval during which 50 % of a given population may be expected to die following acute administration of a chemical or physical agent (radiation) at a given concentration under a defined set of conditions.

Medium - Material (e.g., air, water, soil, food, consumer products) surrounding or containing an agent.

Medium intake rate - The rate at which the medium crosses the outer exposure surface of a target during ingestion or inhalation.

Metabolic activation - Biotransformation of a substance to a more biologically active derivative.
Synonym: bioactivation

Metabolic Model - An analysis and theoretical reconstruction of the way in which the body deals with a specific substance, showing the proportion of the intake that is absorbed, the proportion that is stored and in what tissues, the proportion and rate of breakdown in the body and the subsequent fate of the metabolic products, and the proportion of the substance and the rate at which it is eliminated by different organs.

Metabolic transformation - Biotransformation of a substance that takes place within a living organism.

Metabolism - Sum total of all physical and chemical processes that take place within an organism; in a narrower sense, the physical and chemical changes that take place in a substance within an organism.

Metabolite - Intermediate or product resulting from metabolism.

Metabonomics - Evaluation of tissues and biological fluids for changes in metabolite levels that follow exposure to a given substance, in order to determine the metabolic processes involved and to evaluate the disruption in intermediary metabolic processes that results from exposure to that substance.

Microenvironment - Surroundings that can be treated as homogeneous or well characterized in the concentrations of an agent (e.g., home, office, automobile, kitchen, store). This term is generally used for estimating inhalation exposures.

Mitigation - Limitation of any negative consequence of a particular event.

Model - A formalized expression of a theory or the causal situation which is regarded as having generated observed data.  In statistical analyses the model is generally expressed in symbols, that is to say in a mathematical from, but diagrammatic models are also found.

Modifying Factor (MF) - An uncertainty factor that is greater than zero and less than or equal to 10; the magnitude of the MF depends upon the professional assessment of scientific uncertainties of the study and data base not explicitly treated with the standard uncer­tainty factors (e.g., the number of animals tested); the default value for the MF is 1.

Monitoring - Continuous or repeated observation, measurement, and evaluation of health and/or environmental or technical data for defined purposes, according to prearranged schedules in space and time, using comparable methods for sensing and data collection.

Morbidity - A departure from a state of physical or mental well-being, resulting from disease or injury. Frequently used only if the affected individual is aware of the condition. Awareness itself connotes a degree of measurable impact. Frequently, but not always, there is a further restriction that some action has been taken such as restriction of activity, loss of work, seeking of medical advice, etc.

Mortality - Death; the death rate; ratio of number of deaths to a given population.

Mortality rate - The number of deaths that occur in a given population during a given time interval; usually deaths per l03 or l05 people per year. Can be age, sex, race, and cause specific.

Mutagen - Agent that can induce heritable changes (mutations) of the genotype in a cell as a consequence of alterations or loss of genetic material.

Mutation - Any heritable change in genetic material.  This may be a chemical transformation of an individual gene (a gene or point mutation), which alters its function.  On the other hand, this change may involve a rearrangement, or a gain or loss of part of a chromosome, which may be microscopically visible.  This is designated a chromosomal mutation.

Negligible risk
1. Probability of adverse effects occurring that can reasonably be described as trivial.
2. Probability of adverse effects occurring that is so low that it cannot be reduced appreciably by increased regulation or investment of resources.

No-effect level, NEL - Maximum dose (of a substance) that produces no detectable changes under defined conditions of exposure.
Note: This term tends to be substituted by no-observed-adverse-effect level (NOAEL) or no-observed- effect level (NOEL).

No-Observed-Adverse-Effect Level (NOAEL) - The greatest concentration or amount of a chemical, found by experiment or observation, that causes no detectable adverse alteration of morphology, functional capacity, growth, development, or life span of the target.

No Observed Effect Level (NOEL) - The highest dose level, in a toxicological dose-response study, where no detectable biological effect is found (usually in test animals).  Used as an experimental estimate of the threshold dose at which toxic effects begin to appear in the dose-response relationship.

Non-Threshold Toxicity - A class of toxicity mechanisms where the damaging biological processes are thought to occur at any exposure level above zero dose, often in a linear dose-response relationship.  Usually applied to environmental substances (ionizing radiation, genotoxic chemicals) that are thought to act by a carcinogenic or mutagenic mechanism involving genetic damage to body (somatic) cells or reproductive (germ) cells.

Odds Ratio - The statistical odds or probability of disease occurrence in the exposed group of individuals compared with the unexposed group of individuals in case-control studies.

Oncogenic - A substance that causes tumors, whether benign or malignant.

One-compartment model - Kinetic model, where the whole body is thought of as a single compartment in which the substance distributes rapidly, achieving an equilibrium between blood and tissue immediately.

Option Evaluation - The process of developing and analysing options for risk management.   Development of options may involve consideration of program objectives and current institutional policies, and the regulatory environment.  Option analysis may involve consideration of risks and benefits; uncertainties in risk estimations; public awareness and perception; technical feasibility; and economic, social, political, and cultural impacts.  

Organoleptic - Affecting or involving a sense organ as of taste, smell, or sight.

Other Supportive Data - In the weight-of-evidence determination of the Agency's hazard identification process this phrase refers to informa­tion on structure activity relationships (SARs), results of short-term in vitro studies, mechanistic studies and pharmacokinetics.

Ozone (O3) - A pungent, colorless, toxic gas that contributes to photochemical smog.

Particulates - Fine liquid or solid particles such as dust, smoke, mist, fumes, or smog, found in the air or emissions.

Particulate matter (in atmospheric chemistry)
1. General term used to describe airborne solid or liquid particles of all sizes.
   Note: The term aerosol is recommended to describe airborne particulate matter.
2. Particles in air, usually of a defined size and specified as PMn

Persistence, Persistent Pollutant - A persistent pollutant is one that remains for a long time in the environment.  For example, under the Canada/U.S. Great Lakes Water Quality Agreement, a persistent pollutant is one with a half-life of eight weeks or longer.

Person-year  - The sum of the number of years each person in the study population is at risk; a metric used to aggregate the total population at risk assuming that 10 people at risk for one year is equivalent to 1 person at risk for 10 years.

Pharmacodynamics - The study of the actions of drugs on the living organism. It involves the biochemical and physiological effects of drugs and the mechanisms of their actions, including the correlation of actions and effects of drugs with their chemical structure. The pharmacodynamics of a toxic substance is called toxicodynamics.

Pharmacogenetics - Study of the influence of genetic factors on the effects of drugs on individual organisms.

Pharmacokinetics - The study of the movement of drugs within biologi­cal systems. The absorption, distribution, biotransformation and excretion of substances in the organism are examined. The p­harmacokinetics of a toxic substance is called toxicokinetics.

Phenotype - Observable structural and functional characteristics of an organism determined by its genotype and modulated by its environment.

Physiologically based pharmacokinetic modeling, PBPK - Mathematical modeling of kinetic behavior of a substance, based on measured physiological parameters.
Synonym: toxicologically based pharmacokinetic modeling

PMR - Proportionate mortality ratio.

Poison (in toxicology) - Substance that, taken into or formed within the organism, impairs the health of the organism and may kill it.

Point source - A single isolated stationary source of pollution.

Pollutant - Any undesirable solid, liquid, or gaseous matter in a gaseous, liquid, or solid medium. 

Pollution - The presence of matter or energy whose nature, location or quantity produces undesired environmental effects

Population at Risk - The number of people who can develop the adverse health effect under study and who are potentially exposed to the risk factor of interest.  For example, all people in a population who have not developed immunity to an infectious disease are at risk of developing the disease, if they are exposed.  Similarly, people already having chronic disease are excluded from the population at risk in studies of the incidence of the disease.

Potency (in toxicology) - Expression of relative toxicity of an agent as compared to a given or implied standard or reference

Potential Years of Life Lost (PYLL) – Potential Years of Life Lost (PYLL) is an indicator that measures the number of years of life lost when a person dies prematurely.

ppm (Parts per million) - A measurement of concentration such as 1 µg per gram.

Precautionary Principle - A decision criterion that impels decision-makers towards action in situations where a health or ecological hazard is believed to exist as a possibility, although the exact probability of the suspected hazard is imperfectly understood.

Precision - A measure of how consistently the result is determined by repeated determinations without reference to any "true" value.

Premature death - A death that occurs before statistical expectation, usually attributable to a specific cause, and usually referring to deaths statistically estimated in a population rather than to individuals.

Prevalence - The proportion of a population that has a specified disease at a particular time.  Both previous and newly occurring cases of the disease are counted, so prevalence for a chronic disease usually exceeds its annual incidence.

Prevalence, annual (an occasionally used index) - the total number of persons with the disease or attribute at any time during a year.  It includes cases of the disease arising before but extending into or through the year as well as those having their inception during the year.

Prevalence, lifetime - The total number of persons known to have had the disease or attribute for at least part of their life.

Prevalence, period - The total number of persons known to have had the disease or attribute at any time during a specified period.

Prevalence, point - The number of persons with a disease or an attribute at a specified point in time.

Prevalence Rate - The total number of individuals who have an attribute or disease at a particular time (or during a particular period) divided by the population at risk of having the attribute or disease at this point in time or midway through the period.  A problem may arise with calculating period prevalence rates because of the difficulty of defining the most appropriate denominator.

Principal Study - The study that contributes most significantly to the qualitative and quantitative risk assessment.

Probability - The likelihood or frequency of occurrence of an event.  

Probable error - The magnitude of error which is estimated to have been made in determination of results.

Procarcinogen - Substance that has to be metabolized before it becomes a carcinogen.

Progressor/Completer - A factor that acts synergistically with carcinogens, initiators, promotors to induce the neoplastic state.

Promoter - An agent, usually chemical, that causes an initiated cell to become malignant. This is one of the final steps in carcinogenesis and leads to the formation of a tumour and may ultimately result in metastasis.

Proportionate mortality ratio (PMR) - The fraction of all deaths from a given cause in the study population divided by the same fraction from a standard population. A tool for investigating cause-specific risks when only data on deaths are available. If data on the population at risk are also available, SMRs are preferred.

Prospective Study - A way in which subjects are followed forward in time from initiation of the study.  This is often called a longitudi­nal or cohort study.

Public accident - Any accident other than motor vehicle that occurs in the public use of any premises. Includes deaths in recreation (swimming, hunting, etc.), transportation except motor vehicle, public buildings, etc., and deaths from widespread natural disasters even though some may have happened on home premises. Excludes accidents to persons in the course of gainful employment.

Pulmonary - Pertaining to the lung(s).

Quality Adjusted Life Year (QALY) – A quality-adjusted life year (QALY) measures both the quantity and the quality of life achieved by an intervention.

Quantal Data - specify the number of animals affected as a function of dose rate (e.g., mg/kg bw/day) for a single type of effect.  The numbers of animals with tumors or that die from a chemical exposure are examples.  Quantal data are often reported as an incidence (percent response) and, thus, can be used to construct a dose-response curve.

Quantal Effect - An effect that can be expressed only as Aoccurring@ or Anot occurring”. Typical examples of quantal effects are death or occurrence of a tumour.

Rate - A measure of the frequency of a phenomenon.  An expression of the frequency with which an event occurs in a defined population.

Rate Difference (RD) - The absolute difference between two rates, for example, the difference in incidence rate between a population group exposed to a causal factor and a population group not exposed to the factor:   RD = Ie - Iu   where Ie = incidence rate among exposed, and Iu = incidence rate among unexposed.  In comparisons of exposed and unexposed groups, the term excess rate may be used as a synonym for rate difference.

Rate Ratio (RR) - The ratio of two rates.  The term is used in epidemiologic research with a precise meaning, i.e., the ratio of the rate in the exposed population to the rate in the unexposed population:  RR=  Ie / Iu where Ie is the incidence rate among exposed and Iu is the incidence rate among unexposed.

Reasonably Consistent - In the weight-of-evidence determination of the Agency's hazard identification process this phrase refers to evidence of an effect in most species adequately tested.

Reference Dose (RfD) - An estimate of the daily exposure dose that is likely to be without deleterious effect even if continued exposure occurs over a lifetime.

Regression analysis - Statistical methods for modeling a set of dependent variables, Y, in terms of combinations of predictors, X.

Relative Risk - i) the ratio of the risk of disease or death among the exposed to the risk among the unexposed; this usage is synonymous with risk ratio; ii) alternatively, the ratio of the cumulative incidence rate in the exposed to the cumulative incidence rate in the unexposed, i.e., the cumulative incidence ratio, and iii) the term Arelative risk@ has also been used synonymously with Aodds ratio@ and, in some biostatistical articles, has been used for the ratio of forces of morbidity.  The use of the term Arelative risk@ for several different quantities arises from the fact that for Arare@ diseases (e.g., most cancers) all the quantities approximate one another.  For common occurrences (e.g., neonatal mortality in infants under 1500 g birth weight), the approximations do not hold.

Relative excess risk, RER - Measure that can be used in comparison of adverse reactions to drugs, or other exposures, based solely on the component of risk due to the exposure or drug under investigation, removing the risk due to background exposure experienced by all in the population. The relative excess risk, R, is given by
R = (R1 – R0)/(R2 – R0)
where R1 is the rate in the population, R2 is the rate in the comparison population, and R0 is the rate in the general population.
Note: Rate is used here as in epidemiology

Reliability - The probability a system performs a specified function or mission under given conditions for a prescribed time.

Reproducibility - The degree of variation obtained when the same measurement is made with similar instruments and many operators.

Reservoir (in biology) - Storage compartment from which a substance may be released with subsequent biological effects.

Residence time - The period of time during which a substance resides in a designated area.

Residual risk - Health risk remaining after risk reduction actions are implemented.

Respirable particle - Particle of the size (<5.0 µm) most likely to be deposited in the pulmonary portion of the respiratory tract.

Response - Change developed in the state or dynamics of an organism, system, or (sub)population in reaction to exposure to an agent.

1. Amount of a substance that is left from the total absorbed after a certain time following exposure.
2. Holding back within the body or within an organ, tissue or cell of matter that is normally eliminated.

Reversible versus Irreversible Toxicity - Reversible toxic effects are those that can be repaired, usually by a specific tissue's ability to regenerate or mend itself after chemical exposure, while irreversible toxic effects are those that cannot be repaired.

Risk - The probability of an adverse effect in an organism, system, or (sub)population caused under specified circumstances by exposure to an agent.

Risk Acceptance - a decision to accept a risk 1.The verb Ato accept@ is chosen to convey the idea that acceptance has its basic dictionary meaning 2. Risk acceptance depends on risk criteria.

Risk analysis - A process for controlling situations where an organism, system, or (sub)population could be exposed to a hazard. The risk analysis process consists of three components: risk assessment, risk management, and risk communication.

Risk Assessment - A process intended to calculate or estimate the risk to a given target organism, system, or (sub)population, including the identification of attendant uncertainties, following exposure to a particular agent, taking into account the inherent characteristics of the agent of concern as well as the characteristics of the specific target system. The risk assessment process includes four steps: hazard identification, hazard characterization, (related term: Dose-response assessment), exposure assessment, and risk characterization. It is the first component in a risk analysis process.

Risk Avoidance - Decision not to become involved in, or action to withdraw from a risk situation 1.  The decision may be taken based on the result of risk evaluation. 

Risk-Benefit Analysis (RBA) - Economic gains less the costs of a particular program. Essentially, the RBA is a CBA minus the qualitat­ive gains in health status.  It ignores aspects, such as human suffering which are difficult to quantify.

Risk Characterization - The qualitative and, wherever possible, quantitative determination, including attendant uncertainties, of the probability of occurrence of known and potential adverse effects of an agent in a given organism, system, or (sub)population, under defined exposure conditions. Risk characterization is the fourth step in the risk assessment process

Risk Communication - Interactive exchange of information about (health or environmental) risks among risk assessors, managers, news media, interested groups, and the general public.

Risk Control - the step in the Risk Management Framework which focuses on choosing a particular course of preventive or remedial action from an array of possible control options, all of which are intended to reduce health risks through various strategies identified by the stakeholders and the risk management team.

Risk Criteria - Terms of reference by which the significance of risk is assessed 1.  Risk criteria may include associated cost and benefits, legal and statutory requirements, socio-economic and environmental aspects, concerns of stakeholders, priorities and other inputs to the assessment.

Risk Estimation - Quantification of the probability, including attendant uncertainties, that specific adverse effects will occur in an organism, system, or (sub)population due to actual or predicted exposure.

Risk Evaluation - Establishment of a qualitative or quantitative relationship between risks and benefits of exposure to an agent, involving the complex process of determining the significance of the identified hazards and estimated risks to the system concerned or affected by the exposure, as well as the significance of the benefits brought about the agent. Risk evaluation is an element of risk management. Risk evaluation is synonymous with risk-benefit evaluation.

Risk Financing - Provision of funds to meet the cost of implementing risk treatment and related costs  1.  In some industries risk financing only relates to funding the financial consequences of risk.

Risk identification - Recognizing that a hazard exists and trying to define its characteristics. Often risks exist and are even measured for some time before their adverse consequences are recognized. In other cases, risk identification is a deliberate procedure to review, and it is hoped, anticipate possible hazards

Risk Management - Decision-making process involving considerations of political, social, economic, and technical factors with relevant risk assessment information relating to a hazard so as to develop, analyse, and compare regulatory and non-regulatory options and to select and implement appropriate regulatory response to that hazard. Risk management comprises three elements: risk evaluation; emission and exposure control; and risk monitoring.

Risk Management Framework - A systematic decision-making process for assessing and managing health risks.

Risk Management System - Set of elements of an organisation=s management system concerned with managing risk 1.  Management system elements may include: strategic planning; decision making; and other processes for dealing with risk 2.  Risk management systems reflect the culture of the organisation.

Risk Monitoring - Process of following up the decisions and actions within risk management in order to ascertain that risk containment or reduction with respect to a particular hazard is assured. Risk monitoring is an element of risk management.

Risk Optimization - Process, related to a risk, to minimize the negative and to maximize the positive consequences and their respective probabilities 1. In a safety context risk optimization is focused on reducing the risk 2.  Risk optimization follows risk criteria, including costs and legal requirements 3.  Risks associated with risk control may be considered.

Risk Perception - The significance assigned to risks by stakeholders.  This perception is derived form the stakeholders= expressed needs, issues, and concerns.

Risk Reduction - Actions taken to lessen the probability, negative consequences, or both, associated with a particular risk.

Risk Retention - Acceptance of the burden of loss or benefit of gain from a particular risk.  1.  Risk retention includes the acceptance of risks that have not been identified 2.  Risk retention does not include treatments involving insurance, or transfer by other means 3.  There may be variability in the degree of acceptance and dependence on risk criteria.

Risk Transfer - Share with another party the benefit of gain or burden of loss for a particular risk 1.  Risk transfer may be effected through insurance or other agreements 2.  Risk transfer may create new risks or modify existing risk 3.  Relocation of the source is not risk transfer 4.  Legal or statutory requirements may limit, prohibit, or mandate the transfer of certain risk.

Risk Treatment - Process of selection and implementation of measures to modify risk 1.  The term risk treatment is sometimes used for measures themselves 2.  Risk treatment measures may include avoiding, optimizing, transferring or retaining risk.

Safety - Practical certainty that adverse effects will not result from exposure to an agent under defined circumstances. It is reciprocal of risk.

Safety Factor - Composite (reductive) factor by which an observed or estimated no-observed-adverse-effect level (NOAEL) is divided to arrive at a criterion or standard that is considered safe or without appreciable risk.

Safety Factors (in food additives and contaminants) - A factor applied to the no-observed-effect level to derive acceptable daily intake (ADI) (the no-observed-effect level is divided by the safety factor to calculate the ADI).  The value of the safety factor depends on the nature of the toxic effect, the size and type of population to be protected, and the quality of the toxicological information available.

Sample (in statistics)
1. Group of individuals often taken at random from a population for research purposes.
2. One or more items taken from a population or a process and intended to provide information on the population or process.
3. Portion of material selected from a larger quantity so as to be representative of the whole.

Severity - Connotes the toxicological significance attached to the continuum of effects, including adaptive, compensatory and adverse effects, potentially associated with exposure to xenobiotics.

Short-Term Exposure - Multiple or continuous exposures occurring over a week or so.

Short-term exposure limit, STEL - Fifteen-minute time-weighted average (TWA) exposure recommended by ACGIH which should not be exceeded at any time during a workday, even if the 8-h TWA is within the threshold limit value–timeweighted average, TLVTWA.

SMR - Standardized mortality ratio.

Socioeconomic Impact Analysis (SEIA) - An assessment of both allocati­ve (market efficiency as measured by production and consumption) and non-allocative effects (income distribution, market structure, international trade, inflation etc.) on proposed regulations.

Socioeconomic Analysis - An analysis in monetary values of the costs and benefits of various actions to protect health or the environment.

Stakeholder - Any individual, group, or organization able to affect, be affected by, or believe it might be affected by, a decision or activity.  The decision-maker(s) is often, but not always, considered as a stakeholder.

Standard - A standard is a legally enforceable limit for a substance or an agent intended to protect human health or the environment.  Exceeding the standard could result in unacceptable harm.

Statistical significance - The statistical significance determined by using appropriate standard techniques of statistical analysis with results interpreted at the stated confidence level and based on data relating species which are present in sufficient numbers at control areas to permit a valid statistical comparison with the areas being tested.

Steady state (in toxicology) - State of a system in which the conditions do not change in time.

Stochastic - Pertaining to or arising from chance and hence obeying the laws of probability.

Stochastic Effect - Effect for which the probability of occurrence depends on the absorbed dose.  Hereditary effects and cancer induced by radiation are considered to be stochastic effects {ICRP 1977 434 /id}.  The term Astochastic@ indicates that the occurrence of effects so named would be random.  This means that, even for an individual, there is no threshold of dose below which the effect will not appear, but the chance of experiencing the effect increases with increasing dose.

Stochastic Process - A biological disease process that produces as an all-or-nothing outcome (e.g. cancer) through a series of stepwise cellular changes (e.g. gene mutations).  The likelihood of the disease is determined by the statistical probability of the occurrence of each of the underlying steps in the disease process.

Strength-of-Evidence - A common variant of the weight-of-evidence approach for Risk Identification, in which positive findings of harmful health effects are given greater weight than negative findings.  This may lead to the classification of many environmental agents as harmful, when the evidence from animal and human pollution studies is actually contradictory or inconclusive.    

Stressor - Any entity, stimulus, or condition that can modulate normal functions of the organism or induce an adverse response (e.g. agent, lack of food, drought).

Subacute Toxicity Test - An animal experiment serving to study the effects produced by the test material when administered in repeated doses (or continuously in food, drinking-water) over a period of up to about 90 days.

Subchronic - Repeated over a short period, usually about 10 % of the life span; an imprecise term used to describe exposures of intermediate duration.

Subchronic effect - Biological change resulting from an environmental alteration lasting about 10 % of the lifetime of the test organism.
Note: In practice with experimental animals, such an effect is usually identified as resulting from
multiple or continuous exposures occurring over 3 months (90 days).

Subchronic exposure - A contact between an agent and a target of intermediate duration between acute and chronic. (Other terms, such as “less-than-lifetime exposure,” are also used.)

Subchronic Study - A toxicity study designed to measure effects from subchronic exposure to a chemical.

Subchronic toxicity test - Animal experiment serving to study the effects produced by a test substance when administered in repeated doses (or continually in food, drinking-water, air) over a period of up to about 90 days.

Surrogate - Something that serves as a substitute. In risk analysis, surrogates are often used when data on the item of interest (a chemical, an industry, an exposure, etc.) is lacking. As an example, underground mining of coal and hardrock minerals can be used as a surrogate for underground oil shale mining.

Susceptible - Describing a group of organisms more vulnerable to a given exposure than the majority of the population to which they belong.
Note: Susceptibility may reflect gender, age, physiological status, or genetic constitution of the
organisms at risk.

Synergetic - Working together; an agent that works synergistically with one or more other agents.

Synergism - Co-ordinated or concerted action of two or more agents.  The effect observed from combined exposure is greater than the sum of effects of single exposure.  Also referred to as potentiation.

Synergism (in toxicology) - Pharmacological or toxicological interaction in which the combined biological effect of two or more substances is greater than expected on the basis of the simple summation of the toxicity of each of the individual substances

Systematic error - A reproducible inaccuracy introduced by faulty equipment, calibration, or technique.

Systemic - Relating to the body as a whole.

Systemic effect - Consequence that is either of a generalized nature or that occurs at a site distant from the point of entry of a substance.
Note: A systemic effect requires absorption and distribution of the substance in the body.

Target – Any biological entity that receives an exposure or a dose (e.g. a human, a human population, or a human organ).

Teratogen - An agent or factor that causes production of developmental anomalies and/or abnormalities in the developing embryo ie. in utero.

Teratogenic - Substances that are suspected of causing malformations or serious deviations from the normal type, which can not be inherited in or on animal embryos or fetuses.

Teratology - Science that deals with abnormal development of the fetus and congenital malformation.

Threshold - Dose or exposure concentration of an agent below which a stated effect is not observed or expected to occur.

Threshold Dose - The minimum exposure level which induces a statisti­cally significant response in the organism.

Threshold Level - A theoretical concept for the exposure level of a substance that represents the change from the maximum exposure producing no (adverse) effects to the minimum exposure producing an (adverse) effect under defined conditions.

Threshold Limit Value (TLV) - The concentration of a material to which most workers can be exposed daily without adverse effect - - - These values are time weighted concentrations for a 7-or 8-hour workday and a 40-hour workweek. For most materials, the value may be exceeded to a certain extent, provided there are compensatory periods of exposure below the value during the workday (or in some case the week). For a few materials, (mainly those that produce a rapid response) the limit is given as ceiling concentration (ie. a maximum permissible concen­tration) that should never be exceeded.

Threshold limit value-ceiling, TLV-C - Concentration of a potentially toxic substance that should not be exceeded during any part of the working exposure.

Threshold limit valuetime-weighted average, TLVTWA - Time-weighted average concentration for a conventional 8-h workday and a 40-h workweek, to which it is believed nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Threshold limit valueshort-term exposure limit, TLVSTEL - Concentration to which it is believed that workers can be exposed continuously for a short period of time without suffering from (1) irritation, (2) chronic or irreversible tissue damage, or (3) narcosis of sufficient degree to increase the likelihood of accidental injury, impair self rescue or materially reduce work efficiency, and provided that the daily TLVTWA is not exceeded.
Note: It is not a separate independent exposure guideline; rather, it supplements the TLVTWA
limit where there are recognized acute effects from a substance whose toxic effects are primarily
of a chronic nature. TLVSTELs are recommended only where toxic effects have been reported
from high short-term exposures in either humans or animals.

Time Weighted Average - An expression of permissible levels for occupational exposure.  Time weighted average permits exposure above the level, provided they are compensated by equivalent excursions below the level during the work-day shift. In some cases, the magnitude, duration and frequency of permissible excursions above these averages are specified.

Tolerable daily intake, TDI - Estimate of the amount of a potentially harmful substance (e.g., contaminant) in food or drinking water that can be ingested daily over a lifetime without appreciable health risk.
Note: Acceptable daily intake (ADI) is normally used for substances not known to be harmful, such as food additives.

Tolerable weekly intake, TWI - Estimate of the amount of a potentially harmful substance (e.g., contaminant) in food or drinking water that can be ingested weekly over a lifetime without appreciable health risk.

Tolerance - An adaptive state characterized by diminished responses to the same dose of a chemical.

Toxic - Able to cause injury to living organisms as a result of physicochemical interaction.

Toxicity – Inherent property of an agent to cause an adverse biological effect.
Toxicity test - Experimental study of the adverse effects of exposure of a living organism to a substance for a defined duration under defined conditions.

Toxicodynamics - Process of interaction of potentially toxic substances with target sites, and the biochemical and physiological consequences leading to adverse effects.

Toxicogenetics - Study of the influence of hereditary factors on the effects of potentially toxic substances on individual organisms.

Toxicokinetics - Process of the uptake of potentially toxic substances by the body, the biotransformation they undergo, the distribution of the substances and their metabolites in the tissues, and the elimination of the substances and their metabolites from the body.

Toxicology - Scientific discipline involving the study of the actual or potential danger presented by the harmful effects of substances on living organisms and ecosystems, of the relationship of such harmful effects to exposure, and of the mechanisms of action, diagnosis, prevention, and treatment of intoxications.

Toxin - Poisonous substance produced by a biological organism such as a microbe, animal, or plant.

Trace - A very small amount of a material. Usually used in reference to concentrations which are on the order of or less than 1-10 parts per million.

Tumor - Any abnormal mass of cells resulting from excessive cellular multiplication.

Two-compartment model - Product of compartmental analysis requiring two compartments.

Uncertainty - Imperfect knowledge concerning the present or future state of an organism, system, or (sub)population under consideration.

Uncertainty analysis - A detailed examination of the systematic and random errors of a measurement or estimate; an analytical process to provide information regarding the uncertainty

Uncertainty Factor - Reductive factor by which an observed or estimated no-observed-adverse-effect level (NOAEL) is divided to arrive at a criterion or standard that is considered safe or without appreciable risk.

Uptake (absorption) - The process by which an agent crosses an absorption barrier.
Validation - Process by which the reliability and relevance of a particular approach, method, process, or assessment is established for a defined purpose.

Virtually Safe Dose (VSD) - The VSD is an expression of the daily dose (exposure) over a lifespan that is predicted to produce no more than a pre-defined low level of risk, usually set according to an >acceptable risk level= (e.g. 10-6 cancer risk over a 70 year lifetime).

Volume of distribution - Apparent (hypothetical) volume of fluid required to contain the total amount of a substance in the body at the same concentration as that present in the plasma, assuming equilibrium has been attained.

Weight-of-Evidence - Considerations involved in assessing the reliability of available information about hazard; and the quality of testing methods, the size and power of the study design, the consistency of results across studies, and the biological plausibility of exposure-response relationships and statistical associations.

Willingness-to-Pay - An economic approach used to estimate the monetary value of avoiding an adverse health effect based on an individual=s willingness-to-pay (WTP) for risk reduction.  By summing many individuals= WTP to avoid small increases in risk over a large sample, the value of a statistical premature death avoided can be inferred.  This valuation is expressed as Adollars per mortality avoided@ or Avalue of a statistical life@ (VSL) even though the actual valuation represents small changes on mortality risk experience by a large number of people.  The VSL method estimates the dollar value of a given reduction in risk, in reference to an indivdual=s WTP to reduce that risk.  WTP is often based on wage-risk studies, which derive WTP values from estimates of the additional compensation demanded in the labour market for riskier jobs, or from contingent valuation (CV) studies which directly solicit WTP information from personal interviews.

Xenobiotic - Compound with a chemical structure foreign to a given organism.
Note: The term is frequently restricted to manmade compounds.

Years Lost due to Disability (YLD) – Years Lost due to Disability (YLD) gives the number of healthy years lost to a disability.

Years of Life Lost (YLL) – Years of Life Lost is simply the number of years lost due to premature mortality.

Zero order analysis - ­­The simplest approach to quantification of a risk with a limited treatment of each risk component (e.g. source terms, transport, health effects, etc.).

131I (Iodine – 131) - is a radioactive isotope of the chemical element Iodine with mass number 131. It is a beta-emitter with a half-life of 8 days.

137Cs (Cesium-137) -  is a radioactive isotope of the chemical element Cesium with mass number 137, which has a half life of 30.2 years and decays by beta-decay.

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