A Glossary of Toxicology Terms

Accuracy: The degree of agreement between an observed value and an accepted reference value. Accuracy includes a combination of random error (precision) and systematic error (bias), components which are due to sampling and analytical operations (NELAC, 2001). Note: Because the true toxicity (unlike, for example, mass) of a material cannot be determined this term is generally not considered applicable to toxicity testing. Consequently “true” values for laboratory evaluation studies are typically mean or median values of participating labs.

Acute test: A comparative study in which organisms, that are subjected to different treatments, are observed for a short period usually not constituting a substantial portion of their life span. Acute tests often utilize mortality as the only measured effect; chronic tests usually include additional measures of effect such as growth or reproduction (ASTM, 2002).

Acute-to-chronic ratio (ACR): The ratio of the acute toxicity to chronic toxicity of a chemical or sample which can be used to predict acute toxicity from chronic data and vice-versa. Example: LC50 = 100 ug/l and Chronic Value = 5 ug/l, ACR = 20.

ANOVA: Analysis of variance. A parametric method used for hypothesis testing (i.e. to determine if statistically significant differences in a response occur among two or more groups).

Average Monthly Limit (AML): The calculated average monthly limit of wasteload allocation assigned by a State of EPA for a particular facility (EPA, 2000).

Bioaccumulation: The net accumulation of a substance by an organism from all environmental sources (ASTM, 2002).

Bioaccumulation Factor (BAF): The quotient obtained by dividing the concentration of a substance in an organism (or specified tissue) by its concentration in a specified exposure medium, for example, air, food, sediment, soil, water, when several media are possible sources (ASTM, 2002).

Bioassay: An experiment that uses living whole organisms, tissues or cells to measure the presence, the concentration or the relative potency of one or more chemicals (ASTM, 2002).

Bioconcentration: The net accumulation of a substance by an aquatic organism as a result of uptake directly from aqueous solution (ASTM, 2002).

Biomonitoring: The routine (e.g. monthly, quarterly) assessment of water, air, sediment or soil quality using the response(s) of living organisms as the measurement system.

Chronic test: A comparative study in which organisms, that are subjected to different treatments, are observed for a long period or a substantial portion of their life span. Acute tests often utilize mortality as the only measured effect; chronic tests usually include additional measures of effect such as growth or reproduction (ASTM, 2002). Note: Cost-effective short-term chronic tests have been designed which involve exposure of sensitive life stages (e.g. larval fish, juvenile invertebrates) for a relatively brief period (e.g. seven days).

Chronic Value (ChrV): The geometric mean of the NOEC and LOEC. For example: NOEC = 25 mg/l, LOEC = 50 mg/l ChrV = antilog[(log 25 = log 50)/2] = 35.3 mg/l.

Coefficient of Variation (CV): A standard statistical measure of the relative variation of a distribution or set of data, defined as the standard deviation divided by the mean. It is also called the relative standard deviation (RSD). The CV can be used as a measure of the precision within and between laboratories, or among replicates for each treatment concentration (EPA, 2000).

Confidence limits (or intervals): These are probability limits, based on the data set and statistical model employed, that the "true value" lies within the limits specified. Typically limits are expressed at the 95% or 99% probability levels.

Control, negative: Measures taken to insure that a test, its components, or the environment do not cause undesired effects, or produce incorrect test results (NELAC, 2001). In toxicity tests a negative control typically consists of clean water or sediment that is essentially free of contaminants or the test material. The response of organisms in the negative control is used to determine test acceptability and as a baseline for gauging adverse effects among animals exposed to treatments.

Control, positive: Measures taken to insure that a test and/or its components are working properly and producing correct or expected results from positive test subjects (NELAC, 2001). In toxicity tests positive controls typically consist of reference toxicants.

Control chart: A cumulative summary chart of results from QA tests with reference materials (e.g. reference toxicants). The results of a given QA test are compared to the control chart mean value and acceptance limits (typically 95% confidence limits, i.e. mean + 2 standard deviations) or warning limits (typically 99% confidence limits, i.e. mean + 3 standard deviations).

Criteria Continuous Concentration (CCC): Water quality criteria for chronic exposure (EPA, 2000).

Criteria Maximum Concentration (CMC): Water quality criteria for acute exposure (EPA, 2000).

Effect Concentration (EC): A point estimate of the toxicant concentration that would cause an observable adverse effect (e.g., death, immobilization, or serious incapacitation) in a given percent of the test organisms, calculated from a continuous model (e.g., Probit Model). EC25 is a point estimate of the toxicant concentration that would cause an observable adverse effect in 25 percent of the organisms (EPA, 2000). The lower the EC25, the more toxic the chemical or effluent sample. Other EC values, e.g. the EC50 or EC10 may also be calculated to determine concentrations causing effect in a greater or lesser proportion of the population.

Elutriate: An aqueous sediment extract prepared by decanting the overlying water from a sediment-water mixture, presumably representing the labile, bioavailable components of the sediment.

False negative: A determination that a material is nontoxic when it is in fact toxic.

False positive: A determination of toxicity when the material is in fact nontoxic.

Hypothesis Testing: A statistical technique (e.g. Dunnett¡s test) for determining whether a tested concentration is statistically different from the control. Endpoints determined from hypothesis testing are NOEC and LOEC. The two hypotheses commonly tested in WET are:
Null Hypothesis (Ho): The effluent is not toxic. Alternate hypothesis (Ha ): The effluent is toxic. (EPA, 2000).

Indigenous Species: A species that is likely, due to historical presence, to occur at a specified site for some portion of its life span (ASTM, 2002).

Inhibition Concentration (IC): A point estimate of the toxicant concentration that would cause a given percent reduction in a non-lethal biological measurement (e.g., reproduction or growth), calculated from a continuous model (e.g., Interpolation Method). IC25 is a point estimate of the toxicant concentration that would cause a 25-percent reduction in a non-lethal biological measurement (EPA, 2000). The lower the IC25, the more toxic the chemical or effluent sample. Other IC values, e.g. the IC50 or IC10 may also be calculated to determine concentrations causing a more or less severe effect.

Instream Waste Concentration (IWC): The concentration of a toxicant in the receiving water after mixing. The IWC is the inverse of the dilution factor. It is sometimes referred to as the receiving water concentration (RWC) (EPA, 2002). This value is frequently used to determine permit limits for toxicity (e.g., no chronic toxicity at the IWC). For effluents the IWC is typically calculated using critical flow conditions of the receiving stream (e.g., 7-day, 10-year low flow or 7Q10) and the design (maximum) flow of the plant effluent: IWC = 100 x effluent flow/(effluent flow + 7Q10).

Interstitial Water: Water occupying space between sediment or soil particles (syn. Pore water) (ASTM, 2002).

LC50 (lethal concentration, 50 percent): The toxicant or effluent concentration that would cause death in 50% of the test organisms (EPA, 2000). The concentration is calculated from the data set using statistical or graphical models. The lower the LC50, the more toxic the chemical or effluent sample. Other LC values, e.g. the LC90 or LC5 may also be calculated to determine concentrations causing more or less mortality to the population. Note: The LC value must always be associated with the duration of exposure. Thus a 48-h LC50, 96-h LC50, etc. is calculated.

Life-cycle Test: A comparative study in which organisms, that are subjected to different treatments, are observed at least from a life stage in one generation to the same life stage in the next generation (ASTM, 2002).

LOEC (Lowest-observed-effect-concentration): The lowest concentration of an effluent or toxicant that results in adverse effects on the test organisms (i.e., where the values for the observed endpoints are statistically different from the control) (EPA, 2000).

Long-term Averages (LTAs): Average of pollutant concentration or effluent toxicity calculated from waste load allocations (WLAs), typically assuming that the WLA is a 99^th percentile value (or another upper bound value) based on the lognormal distribution. One LTA is calculated for each WLA (typically an acute LTA and a chronic LTA for aquatic life protection). The LTA represents the expected long-term average performance from the permitted facility required to achieve the associated WLA (EPA, 2002).

Maximum Daily Limit (MDL): The calculated maximum WLA assigned by a State or EPA for a particular facility (EPA, 2000).

Minimum Significant Difference (MSD): The minimum difference which can exist between a test treatment and the controls in a particular test and be statistically significant; a measure of test sensitivity. The lower the MSD the more sensitive the test. Related to test power, the MSD is decreased by increasing the number of replicates and/or decreasing the amount of between-replicate variability in the controls and treatments.

Mixing Zone: Area where an effluent discharge undergoes initial dilution and is extended to cover the secondary mixing in the ambient water body. A mixing zone is an allocated impact zone where water quality criteria can be exceeded as long as acutely toxic conditions are prevented (EPA, 2000).

NOAEC (No-observed-acute-effect-concentration): The NOEC for acute effects (typically survival); see NOEC.

NOEC (No-observed-effect-concentration): The highest concentration of an effluent or toxicant that causes no observable adverse effects on the test organisms (i.e., the highest concentration of toxicant at which the values for the observed responses are not statistically different from the control) (EPA, 2000).

Nonparametric hypothesis tests: Hypothesis test methods that are not dependent on data being normally distributed and exhibiting homogeneity of variance. These are also called distribution-free methods. Examples: Steel¡s Many-one and Kruskal-Wallis tests.

NPDES: National Pollutant Discharge Elimination System, created under the Clean Water Act. The permitting system under which point source discharges are regulated to eliminate or minimize the discharge of toxicants into surface waters. States frequently oversee their own programs which must comply with (i.e. be equally or more stringent) the national permit program.

Parametric hypothesis tests: Hypothesis test methods that assume data are normally distributed and variance is homogeneous. These tests are considered to be more powerful than nonparametric equivalents. Examples: ANOVA, Dunnett¡s, Student¡s tests.

Pathogen interference: An increase in the incidence of false positive or false negative results caused by organisms pathogenic to the test species and which are present in the test sample(s).

Pore Water: Water occupying space between sediment or soil particles (syn. Interstitial Water) (ASTM, 2002).

Power: The probability of correctly detecting an actual toxic effect (EPA, 2000).

Precision: A measure of the reproducibility within a data set. Precision can be measured both within a laboratory and between laboratories using the same test method and toxicant (EPA, 2000).

Proficiency Test Sample (PT): A sample, the composition of which is unknown to the analyst, and is provided to test whether the analyst/laboratory can produce analytical results within specified acceptance criteria (NELAC, 2001).

Quality Assurance: An integrated system of activities involving planning, quality control, quality assessment, reporting and quality improvement to ensure that a product or service meets defined standards of quality with a stated level of confidence (NELAC, 2001).

Quality Control: The overall system of technical activities whose purpose is to measure and control the quality of a product or service so that it meets the needs of users (NELAC, 2001).

Reference Toxicant: The toxicant used in performing toxicity tests to indicate the sensitivity of a test organism and to demonstrate the laboratory¡s ability to perform the test correctly and obtain consistent results (NELAC, 2001).

Replicate: Each of several experimental units that are tested simultaneously using the same experimental conditions (ASTM, 2002).

Resident Species: A species that is regularly present at a specified site for some portion of its life span (ASTM, 2002).

Sediment: (1) particulate material that usually lies below water, and (2) formulated particulate material that is intended to lie below water in a test (ASTM, 2002).

Surrogate Species: A species that is tested to estimate responses of other species, for which direct testing is impractical (ASTM, 2002).

TAC (Test Acceptability Criteria): Specific criteria for determining whether toxicity test results are acceptable (EPA, 2001). For example, a TAC for minimum control survival is typically specified in toxicity test methods.

Total Maximum Daily Load (TMDL): A determination of the amount of a pollutant, or property of a pollutant, from point, nonpoint, and natural background sources, including a margin of safety, that may be discharged into a water-quality limited waterbody (EPA, 2000).

Toxicity: The property of a chemical, or combination of chemicals, to adversely affect organisms, tissues or cells (ASTM, 2002).

Toxicity Identification Evaluation (TIE): A set of procedures used to identify the specific compounds causing effluent toxicity (EPA, 2000).

Toxicity Reduction Evaluation (TRE): A site-specific study conducted in a step-wise process designed to identify the causative agents of effluent toxicity, isolate the source of toxicity, evaluate the effectiveness of toxicity control options, and then confirm the reduction of effluent toxicity (EPA, 2000).

Toxicity Test: A procedure to determine the toxicity of a chemical or an effluent using living organisms. A toxicity test measures the degree of effect of a specific chemical or effluent on exposed test organisms (EPA, 2000).

T.U.: Toxic units. Expresses the relative toxicity of an effluent in such a manner that the larger the toxic unit value the more toxic the effluent. T.U._Ac = 100/LC50. T.U._Chr = 100/NOEC.
Examples: LC50 = 25%, T.U._Ac = 4.0 and NOEC = 12.5%, T.U._Chr = 8.0. Permits with whole effluent toxicity limits (WET limits) often specify a T.U. value that may not be exceeded.

Type I Error (alpha): The rejection of the null hypothesis (H_o) when it is, in fact, true (i.e., determining that the effluent is toxic when the effluent is not toxic) (EPA, 2000).

Type II Error (beta): The acceptance of the null hypothesis (H_o) when it is not true (i.e., determining that the effluent is not toxic when the effluent is toxic) (EPA, 2000).

Waste Load Allocation (WLA): The portion of a receiving water¡s total maximum daily load that is allocated to one of its existing or future point sources of pollution (EPA, 2000).

Water Effects Ratio (WER): A correction factor used to adjust a state or regional water quality standard to account for differences in the toxicity of a specific pollutant between laboratory water and site water.

Whole Effluent Toxicity (WET): The total toxic effect of an effluent measured directly with a toxicity test (EPA, 2000).

Whole Sediment: Sediment and associated pore water that have had minimal manipulation (ASTM, 2002).