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IRIS Glossary / Glossary Archive / Acronyms & Abbreviations

Disclaimer: This archive contains former IRIS definitions that were active prior to the date listed below each entry.

This glossary contains definitions of terms used frequently in IRIS. It is intended to assist users in understanding terms utilized by the U.S. EPA in hazard and dose-response assessments. These definitions are not all-encompassing, but are useful "working definitions". It is assumed that the user has some familiarity with risk assessment and health science. For terms that are not included in this glossary, the user should refer to standard health science, biostatistics and medical textbooks and dictionaries.

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

A

Acute Exposure: One dose or multiple doses of short duration spanning less than or equal to 24 hours. This definition archived 9/30/03

Acute Reference Concentration (RfC): An estimate of a continuous inhalation exposure for an acute duration (24 hours or less) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMCL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. This definition archived 12/23/05

Acute Reference Dose (RfD): An estimate of a daily oral exposure for an acute duration (24 hours or less) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMDL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used.This definition archived 12/23/05

B

Benchmark Dose (BMD) or Concentration (BMC): A statistical lower confidence limit on the dose that produces a predetermined change in response rate of an adverse effect (called the benchmark response or BMR) compared to background.This definition archived 9/30/03

Biologically Based Dose Response (BBDR) model: A predictive tool used to estimate potential human health risks by describing and quantifying the key steps in the cellular, tissue, and organismal responses as a result of chemical exposure. This definition archived 9/30/03

C

Chronic Exposure: Multiple exposures occurring over an extended period of time, or a significant fraction of the animal's or the individual's lifetime. This definition archived 9/30/03

Chronic Reference Concentration (RfC): An estimate of a continuous inhalation exposure for a chronic duration (up to a lifetime) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMCL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used.This definition archived 12/23/05

Chronic Reference Dose (RfD): An estimate of a daily oral exposure for a chronic duration (up to a lifetime) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMDL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used.This definition archived 12/23/05

D

Dose-Response Assessment: A determination of the relationship between the magnitude of an administered, applied, or internal dose and a specific biological response. Response can be expressed as measured or observed incidence, percent response in groups of subjects (or populations), or as the probability of occurrence within a population. This definition archived 9/30/03

Dose-Response Relationship: The relationship between a quantified exposure (dose), and the proportion of subjects demonstrating specific, biological changes (response). This definition archived 9/30/03

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E

Epidemiology: The study of disease patterns in human populations. This definition archived 9/30/03

L

Linear Dose Response: A pattern of frequency or severity of biological response that varies proportionately with the amount of dose of an agent. This definition archived 9/30/03

Lowest-Observed-Adverse-Effect Level (LOAEL): The lowest exposure level at which there are statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. Also referred to as lowest-effect level (LEL). This definition archived 9/30/03

N

No-Observed-Adverse-Effect Level (NOAEL): An highest exposure level at which there are no statistically or biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered adverse, nor precursors to adverse effects. This definition archived 9/30/03

Non-linear Dose Response: A pattern of frequency or severity of biological response that does not vary proportionately with the amount of dose of an agent. When mode of action information indicates that responses may not follow a linear pattern below the dose range of the observed data, non-linear methods for determining risk at low dose may be justified. This definition archived 9/30/03

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P

Physiologically Based Pharmacokinetic (PBPK) Model: Physiologically based compartmental model used to characterize pharmacokinetic behavior of a chemical. Available data on blood flow rates, and metabolic and other processes which the chemical undergoes within each compartment are used to construct a mass-balance framework for the PBPK model. This definition archived 9/30/03

Point of Departure: The dose-response point that marks the beginning of a low-dose extrapolation. This point is most often the upper bound on an observed incidence or on an estimated incidence from a dose-response model. This definition archived 9/30/03

R

Reference Concentration (RfC): An estimate of a continuous inhalation exposure for a given duration to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMCL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. [Durations include acute, short-term, subchronic, and chronic and are defined individually in this glossary]. This definition archived 12/23/05

Reference Dose (RfD): An estimate of a daily oral exposure for a given duration to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMDL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. [Durations include acute, short-term, subchronic, and chronic and are defined individually in this glossary]. This definition archived 12/23/05

Risk (in the context of human health): The probability of injury, disease, or death from exposure to a chemical agent or a mixture of chemicals. In quantitative terms, risk is expressed in values ranging from zero (representing the certainty that harm will not occur) to one (representing the certainty that harm will occur). The following are examples of how risk is expressed within IRIS: E-4 or 10^-4 = a risk of 1/10,000; E-5 or 10^-5 = 1/100,000; E-6 or 10^-6 = 1/1,000,000. Similarly, 1.3 E-3 or 1.3 x 10^-3 = a risk of 1.3/1,000=1/770; 8 E-3 or 8 x 10^-3 = a risk of 1/125 and 1.2 E-5 or 1.2 x 10^-5 = a risk of 1/83,000. This definition archived 9/30/03

Risk Assessment (in the context of human health): The determination of potential adverse health effects from exposure to chemicals, including both quantitative and qualitative expressions of risk. The process of risk assessment involves four major steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. This definition archived 9/30/03

S

Short-Term Exposure: Multiple or continuous exposure to an agent for a short period of time, usually one week. This definition archived 9/30/03

Short-term Reference Concentration (RfC): An estimate of a continuous inhalation exposure for short-term duration (up to 30 days) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMCL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used.This definition archived 12/23/05

Short-term Reference Dose (RfD): An estimate of a daily oral exposure for a short-term duration (up to 30 days) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMDL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. This definition archived 12/23/05

Statistical Significance: The probability that a result is not likely to be due to chance alone. By convention, a difference between two groups is usually considered statistically significant if chance could explain it only 5% of the time or less. Study design considerations may influence the a priori choice of a different statistical significance level. This definition archived 9/30/03

Subchronic Exposure: Exposure to a substance spanning approximately 10% of the lifetime of an organism. This definition archived 7/14/05

Subchronic Reference Concentration (RfC): An estimate of a continuous inhalation exposure for a subchronic duration (up to 10% of average lifespan) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMCL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. This definition archived 12/23/05

Subchronic Reference Dose (RfD): An estimate of a daily oral exposure for a subchronic duration (up to 10% of average lifespan) to the human population (including susceptible subgroups) that is likely to be without an appreciable risk of adverse health effects over a lifetime. It is derived from a BMDL, a NOAEL, a LOAEL, or another suitable point of departure, with uncertainty/variability factors applied to reflect limitations of the data used. This definition archived 12/23/05

Systemic Effects or Systemic Toxicity: Toxic effects as a result of absorption and distribution of a toxicant to a site distant from its entry point, at which point effects are produced. Not all chemicals that produce systemic effects cause the same degree of toxicity in all organs. This definition archived 9/30/03

T

Target Organ: The biological organ(s) most adversely effected by exposure to a chemical substance. This definition archived 9/30/03

Toxicity: The degree to which a chemical substance elicits a deleterious or adverse effect upon the biological system of an organism exposed to the substance over a designated time period. This definition archived 9/30/03

Toxicology: The study of harmful interactions between chemicals and biological systems. T his definition archived 9/30/03

Toxic Substance: A chemical substance or agent which may cause an adverse effect or effects to biological systems. This definition archived 9/30/03

U

Uncertainty Factor (UF): One of several, generally 10-fold factors, used in operationally deriving the RfD and RfC from experimental data. UFs are intended to account for (1) the variation in sensitivity among the members of the human population, i.e., interhuman or intraspecies variability; (2) the uncertainty in extrapolating animal data to humans, i.e., interspecies variability; (3) the uncertainty in extrapolating from data obtained in a study with less-than-lifetime exposure to lifetime exposure, i.e., extrapolating from subchronic to chronic exposure; (4) the uncertainty in extrapolating from a LOAEL rather than from a NOAEL; and (5) the uncertainty associated with extrapolation from animal data when the database is incomplete. This definition archived 9/30/03

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