IUPAC Glossary of Terms Used in Toxicology, 2nd Edition
IUPAC Recommendations, 2007

IUPAC Glossary of Terms Used in Toxicology - Terms Starting with R

radiant power
Power emitted, transferred or received as radiation.

radiation toxicology
Scientific study involving research, education, prevention and treatment of diseases caused by ionizing or nonionizing radiation.

See crepitations

random (probability) sample
antonym biased sample
Subset of units of a population that is arrived at by selecting units such that each possible unit has a fixed and known probability of selection.

rate (in epidemiology)
Measure of the frequency with which an event occurs in a defined population in a specified period of time.
Note 1: Most such rates are ratios, calculated by dividing a numerator, e.g. the number of deaths, or newly occurring cases of a disease in a given period, by a denominator, e.g. the average population during that period.
Note 2: Some rates are proportions, i.e. the numerator is contained within the denominator (as when a number of patients with a given disease is divided by the total population from which they come).

rate constant, k
rate coefficient
Numerical constant in a rate-of-reaction (v) equation; for example, v = k [A]a[B]b.......where [A], [B] etc. are reactant concentrations, k is the rate constant, and a, b, etc. are corresponding empirical constants.
Note: For further consideration of the relevant mathematics, see [3]

rate-controlling step
rate-determining step
rate-limiting step
In a reaction occurring by a composite reaction sequence, an elementary reaction, the rate constant for which exerts a strong effect, - stronger than that of any other rate constant - on the overall rate.
Note 1: It is recommended that the expressions rate-controlling, rate-determining and rate-limiting be regarded as synonymous, but some special meanings sometimes given to the last two expressions are considered under a separate heading.
Note 2: For further consideration of this term, see [3].

rate-determining step
See rate-controlling step

rate difference (RD)
Absolute difference between two rates.
Note 1: 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.
Note 2: In comparisons of exposed and unexposed groups, the term excess rate may be used as a synonym for rate difference.

rate-limiting step
See rate controlling step

rate ratio (in epidemiology) (RR)
Value obtained by dividing the rate in an exposed population by the rate in an unexposed population.

Substance intended to kill rats.

re-absorption (in biology)
Absorption by a living organism of a substance which it has previously absorbed and then released, e.g., the uptake of a substance from the proximal renal tubule following glomerular filtration.

reactive nitrogen species (RNS)
Radical nitrogen-based molecules that can act to facilitate nitrosylation reactions; reactive nitrogen species include dioxidonitrogen(•) (nitrogen dioxide, nitryl radical) NO2•, oxidonitrogen(•) (nitrogen monoxide, nitrosyl radical) NO• oxidonitrogen(1+) (nitrosyl cation) NO+, hydroxyoxidonitrogen (nitrous acid) HNO2 and oxidonitrate(1-) NO-.

reactive oxygen species (ROS)
Intermediates in the reduction of molecular dioxygen O2 to water.
Note: Examples are superoxide anion O2-•, hydrogen peroxide H2O2, and hydroxyl radical HO•.

readily biodegradable
Arbitrary classification of substances that have passed certain specified screening tests for ultimate biodegradability; these tests are so stringent that such compounds will be rapidly and completely biodegraded in a wide variety of aerobic environments.
See also biodegradation

reasonable maximum exposure (RME)
Highest exposure that is reasonably expected to occur.
Note: Typically the 95% upper confidence limit of the toxicant distribution is used: if only a few data points (6-10) are available, the maximum detected concentration is used.

Ability of a substance to remain in a particular environment in an unchanged form.

Molecular structure in or on a cell which specifically recognizes and binds to a compound and acts as a physiological signal transducer or mediator of an effect.

receptor-mediated endocytosis
Endocytosis of a substance and its receptor following receptor binding.

recessive gene
Allele which in the heterozygous state is expected to have no effect on the phenotype of the organism which carries it.
After [9]

recombinant DNA
DNA made by transplanting or splicing DNA into the DNA of host cells in such a way that the modified DNA can be replicated in the host cells in a normal fashion.

recombinant DNA technology
Methods involving the use of restriction enzymes to cleave DNA at specific sites, allowing sections of DNA molecules to be inserted into plasmid or other vectors and cloned in an appropriate host organism (e.g. a bacterial or yeast cell).
After [9]

recommended exposure level (REL) (in toxicology)
Highest allowable regulatory airborne concentration.
Note: This exposure concentration is not expected to injure workers. It may be expressed as a ceiling limit or as a time-weighted average (TWA).

Restoration to original form of a substance previously for preservation and storage


  1. Process leading to partial or complete restoration of a cell, tissue, organ or organism following its damage from exposure to a harmful substance or agent.
  2. Term used in analytical and preparative chemistry to denote the fraction of the total quantity of a substance recoverable following a chemical procedure.

recovery factor
Fraction or percentage of the total quantity of a substance extracted under specified conditions.

recycling (of waste)
Process or method allowing for the recovery of some value from a waste, either as re-usable material or as energy.

reference concentration (RfC)
An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups which include children, asthmatics and the elderly) that is likely to be without an appreciable risk of deleterious effects during a lifetime.
Note: It can be derived from various types of human or animal data such as NOAEL, LOAEL, or benchmark concentration, with uncertainty factors generally applied to reflect limitations of the data used. It is generally used in EPA's noncancer health assessments.

reference distribution
Statistical distribution of reference values.

reference dose (RfD)
An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.
Note: It can be derived from a NOAEL, LOAEL, or benchmark dose, with uncertainty factors generally applied to reflect limitations of the data used. It is generally used in EPA's noncancer health assessments.

reference group
See reference sample group

reference individual
Person selected with the use of defined criteria for comparative purposes in a clinical study.

reference interval
Area between and including two reference limits, for example the percentiles 2.5 and 97.5.

reference limit
Boundary value defined so that a stated fraction of the reference values is less than or exceeds that boundary value with a stated probability.

reference material (RM)
calibration material
standard material
Material, sufficiently homogeneous and stable regarding one or more properties, used in calibration, in assignment of a value to another material, or in quality assurance.

reference population
Group of all reference individuals used to establish criteria against which a population that is being studied can be compared.

reference sample group
Selected reference individuals, statistically adequate numerically to represent the reference population.

reference value
Quantity value, generally accepted as having a suitably small measurement uncertainty, to be used as a basis for comparison with values of quantities of the same kind.

regioselectivity, (regioselective)
Terms referring to a reaction in which one direction of bond making or breaking occurs preferentially over all other possible directions.
Note: Reactions are termed completely (100%) regioselective if the discrimination is complete, or partially (x%), if the product of reaction at one site predominates over the product of reaction at other sites.

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

regulatory dose
Term used by the USEPA to describe the expected dose resulting from human exposure to a substance at the level at which it is regulated in the environment.

regulatory sequence
DNA sequence to which specific proteins bind to activate or repress the expression of a gene.

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 R1is the rate in the population, R2 is the rate in the comparison population, and R0is the rate in the general population.
Note: Rate is used here as in epidemiology.

relative odds
See odds ratio

relative risk (RR)
risk ratio
rate ratio
  1. Ratio of the risk of disease or death among the exposed to the risk among the unexposed.
  2. Ratio of the cumulative incidence rate in the exposed to the cumulative incidence rate in the unexposed.

relative systemic availability
Quantity of metabolizable substance divided by product of quantity of absorbed substance and exposure.

  1. Giving a remedy.
  2. Removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water for the general protection of human health and the environment.

Anything, such as a medicine or therapy, that relieves pain, cures disease, or corrects a disorder.

Pertaining to the kidneys.

renal plasma flow
Volume of plasma passing through the kidneys in unit time.

See nephropathy

measurement repeatability
Measurement precision under repeatability conditions of measurement.

repeatability condition
repeatability condition of measurement
Condition of measurement in a set of conditions including the same measurement procedure, same operator, same measuring system, same operating conditions and same location, and replicated measurements over a short period of time.

Substance used mainly to repel blood sucking insects in order to protect man and animals.
Note: This term may also be used for substances used to repel mammals, birds, rodents, mites, plant pests, etc.

replicate sampling
Act of taking multiple samples concurrently under comparable conditions.
Note: Replicate sampling may be accomplished by taking samples adjacent in time or space.


  1. Duplicated or repeated performance of an experiment under similar (controlled) conditions to reduce to a minimum the error, and to estimate the variations and thus obtain a more precise result: each determination, including the first is called a replicate.
  2. Process whereby the genetic material is duplicated.

measurement reproducibility
Measurement precision under reproducibility conditions of measurement.

reproducibility condition
reproducibility condition of measurement
Condition of measurement in a set of conditions including different locations, operators, and measuring systems.
Note 1: The different measuring systems may use different measurement procedures.
Note 2: A specification should give the conditions changed and unchanged, to the extent practical.

reproductive toxicant
Substance or preparation that produces non-heritable adverse effects on male and female reproductive function or capacity and on resultant progeny.

reproductive toxicology
Study of the nonheritable adverse effects of substances on male and female reproductive function or capacity and on resultant progeny.

reserve capacity
Physiological or biochemical capacity that may be available to maintain homeostasis when the body or an organism is exposed to an environmental change.

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

residence time
See mean residence time

residual risk
Health risk remaining after risk reduction actions are implemented.

residual time
See mean residence time

Contaminant remaining in an organism or in other material such as food or packaging, following exposure.

resistance (in toxicology)
Ability to withstand the effect of various factors including potentially toxic substances.

resorption (in biology)
Process in which the components of some differentiated structure that has been produced by the body undergo lysis and assimilation.
Note: Specifically in developmental toxicology, term applied to the lysis and assimilation of the fetus caused by chemical or biological stress of the pregnant mother.

resorptive effect
Action of a substance after its re-absorption from the gut into the blood.

respirable dust
respirable particles
Mass fraction of dust (particles) that penetrates to the unciliated airways of the lung (the alveolar region).
Note: This fraction is represented by a cumulative log-normal curve having a median aerodynamic diameter of 4 μm, standard deviation 2 μm (values for humans).

Proportion of an exposed population with a defined effect or the proportion of a group of individuals that demonstrates a defined effect in a given time at a given dose rate.

restriction enzymes
Endonucleases which recognize specific base sequences within a DNA helix, creating a double-strand break of DNA.
Note: Type I restriction enzymes bind to these recognition sites but subsequently cut the DNA at different sites. Type II restriction enzymes both bind and cut within their recognition or target sites.


  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.

retrospective study
Research design used to test etiological hypotheses in which inferences about exposure to the putative causal factor(s) are derived from data relating to characteristics of the persons or organisms under study or to events or experiences in their past.
Note: The essential feature is that some of the persons under study have the disease or other outcome condition of interest, and their characteristics and past experiences are compared with those of other, unaffected persons. Persons who differ in the severity of the disease may also be compared.

returned effect of poisons
Enhancement of the dose-effect relationship for a poison following repeated exposure to decreasing doses.

reverse mutation (back mutation)
Mutation in a mutant allele which makes it capable of producing the nonmutant phenotype; this may result from restoration of the original DNA sequence of the gene or from production of a new DNA sequence which has the same effect.

reverse transcription
Process by which an RNA molecule is used as a template to make a single-stranded DNA copy.

reversible alteration
Change from normal structure or function, induced by a substance or other agent(s), that returns to normal status or within normal limits after cessation of exposure.

Acute, fulminating, potentially lethal disease of skeletal muscle that causes disintegration of striated muscle fibers as evidenced by myoglobin in the blood and urine.

Inflammation of the nasal mucosa.

rhonch/us sing., /i pl.)
Harsh crepitation in the throat, often resembling snoring.

ribonucleic acid (RNA)
Linear, usually single stranded, polymer of ribonucleotides, each containing the sugar ribose in association with a phosphate group and one of 4 nitrogenous bases: adenine, guanine, cytosine, or uracil.
Note: RNA encodes the information for the sequence of amino-acids in proteins synthesized using it as a template.


  1. Probability of adverse effects caused under specified circumstances by an agent in an organism, a population or an ecological system.
  2. Probability of a hazard causing an adverse effect.
  3. Expected frequency of occurrence of a harmful event arising from such an exposure.
    After [2]

risk assessment
Identification and quantification of the risk resulting from a specific use or occurrence of a chemical or physical agent, taking into account possible harmful effects on individuals or populations exposed to the agent in the amount and manner proposed and all the possible routes of exposure.
Note: Quantification ideally requires the establishment of dose-effect and dose-response relationships in likely target individuals and populations.

risk assessment management process
Global term for the whole process from hazard identification to risk management.

risk associated with a life time exposure
Probability of the occurrence of a specified undesirable event following exposure of an individual person from a given population to a specified substance at a defined level for the expected lifetime of the average member of that population.

risk aversion
Tendency of an individual person to avoid risk.

risk characterization
Outcome of hazard identification and risk estimation applied to a specific use of a substance or occurrence of an environmental health hazard.
Note: Risk characterization requires quantitative data on the exposure of organisms or people at risk in the specific situation. The end product is a quantitative statement about the proportion of organisms or people affected in a target population.

risk communication
Interpretation and communication of risk assessments in terms that are comprehensible to the general public or to others without specialist knowledge.

risk de minimis
negligible risk
risk that is negligible and too small to be of societal concern (usually assumed to be a probability below 10-5 or 10-6).
Note 1: This term can also mean ‘virtually safe’.
Note 2: In the USA, this is a legal term used to mean ‘negligible risk to the individual’.

risk estimation
Assessment, with or without mathematical modeling, of the probability and nature of effects of exposure to a substance based on quantification of dose-effect and dose-response relationships for that substance and the population(s) and environmental components likely to be exposed and on assessment of the levels of potential exposure of people, organisms and environment at risk.

risk evaluation
Establishment of a qualitative or quantitative relationship between risks and benefits, involving the complex process of determining the significance of the identified hazards and estimated risks to those organisms or people concerned with or affected by them.

risk identification
Recognition of a potential hazard and definition of the factors required to assess the probability of exposure of organisms or people to that hazard and of harm resulting from such exposure.

risk indicator
See risk marker

risk management
Decision-making process involving considerations of political, social, economic, and engineering factors with relevant risk assessments relating to a potential hazard so as to develop, analyse, and compare regulatory options and to select the optimal regulatory response for safety from that hazard.
Note: Essentially risk management is the combination of three steps: risk evaluation; emission and exposure control; risk monitoring.

risk marker
risk indicator
Attribute that is associated with an increased probability of occurrence of a disease or other specified outcome and that can be used as an indicator of this increased risk.
Note: A risk marker is not necessarily a causal factor.

risk monitoring
Process of following up the decisions and actions within risk management in order to check whether the aims of reduced exposure and risk are achieved.

risk perception
Subjective perception of the gravity or importance of the risk based on a person's knowledge of different risks and the moral, economic, and political judgment of their implications.

risk phrases
Word groups identifying potential health or environmental hazards required under CPL Directives (European Community); may be incorporated into Safety Data Sheets.

risk quotient
Ratio of predicted exposure concentration to predicted no effect concentration.

predicted environmental concentration (PEC) predicted no effect concentration (PNEC)
Note: The higher this value above one, the greater the risk. If the value is below one, there should be no risk as a result of the predicted exposure.

risk ratio
Value obtained by dividing the probability of occurrence of a specific effect in one group by the probability of occurrence of the same effect in another group, or the value obtained by dividing the probability of occurrence of one potentially hazardous event by the probability of occurrence of another.
Note: Calculation of such ratios is used in choosing between options in risk management.

risk-specific dose
Amount of exposure corresponding to a specified level of risk.

Substance intended to kill rodents.

route of exposure
Means by which a toxic agent gains access to an organism by administration through the gastrointestinal tract (ingestion), lungs (inhalation), skin (topical), or by other routes such as intravenous, subcutaneous, intramuscular or intraperitoneal routes.