Toxicity Endpoints & Tests


Last updated: July 5, 2014

Toxicity Endpoint

Ecotoxicity involves the identification of chemical hazards to the environment, and is defined by the US Environmental Protection Agency (EPA) as “the study of toxic effects on nonhuman organisms, populations, or communities.” Chemical and pesticide manufacturers submit ecotoxicity studies to regulatory authorities to support the registration and/or approval of their products. Testing on animals or plants to determine whether environmental samples such as soil, sediment, or effluents contain toxic compounds is also called ecotoxicity testing.

Although ecotoxicity, in general, refers to hazards to both aquatic and terrestrial animals and plants, this brief review will cover only methods for predicting hazards to the aquatic environment.

Three specific properties of a chemical are used to describe its potential hazard to the aquatic environment (UNECE, 2004):

  • Aquatic toxicity: The hazard of a substance to living organisms, based on toxicity tests to aquatic animals and plants
  • Degradability: The persistence of the substance in the environment, based on molecular structure or analytical testing
  • Bioaccumulation/bioconcentration: The accumulation of a substance in living organisms (from water sources for bioconcentration), which may or may not lead to a toxic effect; based on calculations or bioconcentration factor (BCF) studies using fish

An ecological risk assessment “seeks to estimate the effects of environmental contamination on the growth, reproduction, and survival of a variety of ecological receptors (e.g., birds, mammals, fish, plants) that may be exposed to chemicals in contaminated environmental media, now or in the future.”

Most regulatory authorities require acute toxicity testing using a similar minimal base set of organisms: 1) fish, 2) an aquatic invertebrate, and 3) an algal species. The Globally Harmonized System for Classification and Labelling of Chemicals (GHS) describes testing for hazards to the aquatic environment in Part 3, Chapter 3.10. The GHS criteria for determining environmental hazards does not specify test methods but rather indicates the need to use methods considered to be valid such as the ecotoxicity test guidelines of the Organisation for Economic Cooperation and Development (OECD) and US Environmental Protection Agency (EPA). Further GHS guidance is provided in Annex 8, Guidance on Hazards to the Aquatic Environment, which describes the harmonized classification scheme, aquatic toxicity testing, degradation, bioaccumulation, and the use of (quantitative) structure-activity relationships [(Q)SARs] in aquatic toxicology. The purpose of obtaining aquatic toxicity data for chemicals is to use it in the hazard classification of the chemicals.

The Animal Test(s)

The assessment of aquatic toxicity for the classification of chemicals and environmental risk assessments is typically based on toxicity test data for fish, crustacea (daphnids), and algae/aquatic plants (UNECE, 2004). Toxicity data from freshwater and marine species are considered equivalent, although this is not true for all substances. The data showing the highest toxicity (the lowest acute toxicity values) is to be used in classifying a substance.

The OECD provides a number of Test Guidelines (TGs) for aquatic and terrestrial toxicity testing. The aquatic toxicity TGs involve the testing of chemicals on fish and other aquatic organisms; OECD also provides a number of guidance documents on ecotoxicity test methods.

Fish are used to test for both acute and chronic toxic effects. The Fish Acute Toxicity Test, a 96-hour LC50 test, is the standard acute toxicity test (OECD Test Guideline (TG) 203). LC50 (lethal concentration 50%) refers to the concentration of test substance that is lethal to 50% of the fish. Chronic fish tests may start with eggs, embryos, or juveniles, and last from 7 to more than 200 days (OECD TG 210; US EPA OPPTS 850.1500; other equivalent assay). Test endpoints include “hatching success, growth, spawning success, and survival” (UNECE, 2004).

Acute and chronic tests are also conducted using crustacea: daphnids, mysids, or others. A 96-hour test with lethality as the endpoint is used for acute toxicity (OECD TG 202, part 1; US EPA OPPTS 850.1035; other equivalent assay). Longer term testing through maturation and production is used to assess chronic toxic effects (OECD TG 202, part 2; US EPA OPPTS 850.1350; other equivalent assay). The chronic testing endpoints include “time to first brood, number of offspring produced per female, growth, and survival” (UNECE, 2004).

Although the following are not animal tests, they are included here to illustrate the complete set of tests required by many regulatory authorities. The algal growth inhibition test (OECD TG 201) is typically used to determine an acute EC50 for algae. Several Lemna species of aquatic vascular plants can also be used to obtain an acute EC50 (US EPA OPPTS 850.4400). The EC50 (Effective Concentration 50%) is the concentration causing an adverse effect in 50% of the test organisms.

Bioconcentration factor (BCF) studies in fish are conducted using the OECD TG 305. Experimentally-derived BCF values are preferred for classification purposes (UNECE, 2004).

The OECD recently adopted the Fish Embryo Acute Toxicity (FET) Test (TG 236) (2013) as an alternative test method that would provide a reduction in the number of fish used in testing. In the European Union, the FET is considered a non-animal test (fish become animals when they become free-feeding larvae);  US legislation does not weigh in on this issue.  In the FET test, newly fertilized zebrafish eggs are exposed to a chemical for up to 96 hours, and four indicators of lethality to the embryos are evaluated every 24 hours. EURL ECVAM coordinated the validation study of FET for the OECD, and commented that: “TG236 does not indicate whether the fish embryo acute toxicity test can be used as an alternative to the OECD TG203; however, several recently published papers demonstrate that the LC50 values produced with the fish embryo acute toxicity test correlate well with those observed in juvenile or adult fish.”

Due to the large number of fish tests with protocols that overlap in terms of exposure scenarios, fish species and life-stage, in 2012 the OECD published the Fish Toxicity Testing Framework that describes each OECD test guideline and the conditions under which it may be useful.

The ECVAM Scientific Advisory Committee (ESAC) reviewed and recommended the Upper Threshold Concentration (UTC) Step Down Approach for implementation in 2006 “as a valid strategy to significantly reduce the number of fish used in the assessment of acute aquatic toxicity for hazard classification.” The UTC is a tiered testing strategy with the potential for reducing the number of fish used by at least 65%. The UTC is based on pharmaceutical industry studies showing that algae and daphnid acute EC50 tests were more sensitive than fish LC50 tests about 80% of the time (Hutchinson et al., 2003). The OECD Guidance Document, GD 126 (2010), describes the UTC method.

Legislation in the US spurred the EPA to create a program to evaluate the testing of pesticides and some other chemicals, primarily those likely to be found in drinking water, to determine their effect on the endocrine system of humans and animals (discussions are ongoing regarding proposed legislation in the EU and Japan regarding endocrine active substances). The US EPA established the Endocrine Disruptor Screening Program (EDSP) consisting of two batteries or “tiers” of tests – the first tier is intended as a screen for activity, the second as a collection of tests that measure adverse effects on a variety of species.  Tier 1 contains eleven in vitro and in vivo tests, two of which have relevance to ecotoxicity; the Amphibian Metamorphosis Assay (AMA) and the short-term fish reproduction assay.  There are EPA and OECD adopted TGs for both of these assays.  In addition, OECD has published a series of guidance and review documents covering endocrine assessment. EPA is currently in the process of validating multi-generation tests for fish, birds and amphibians.

The endpoint specific guidance on aquatic toxicity for the implementation of the EU REACH legislation (2012) provides a useful overview of aquatic toxicity testing and some approaches for reducing animal use.

Most regulatory authorities still require the submission of in vivo fish lethality test data. OECD deleted the analogous lethality (LD50) assays for assessing mammalian toxicity in 2002. In addition to the significant ethical concerns over this type of toxicity testing, most scientists also question the technical and scientific merit of fish lethality assays for determining the environmental impact and fate of chemicals, pesticides, and pharmaceuticals released into the aquatic environment (Castaño, et al., 2003). The time and costs of conducting animal experiments for estimating ecotoxicity effects for the many existing and new chemicals are prohibitive. To protect the environment and its inhabitants, faster and cheaper alternative methods are needed. In the near term, reduction strategies using tiered testing schemes and/or tests using fish embryos provide the greatest opportunity for reducing the numbers of fish used for ecotoxicity testing.

Validated Non-animal Methods

Cell-based assays, toxicogenomic microarrays, and (Q)SAR models are being used to predict toxic effects to aquatic organisms. Cronin, et al. (2003), and Comber, et al. (2003) summarized the regulatory uses of (Q)SARs to predict chemicals’ ecological effects. The US EPA has significant experience in using (Q)SAR and structure activity relationship (SAR) models. Cronin, et al. (2003), report that the EPA’s Office of Pollution Prevention and Toxics (OPPT) has been using (Q)SARs for more than two decades for predicting effects such as ecological hazard and fate and assessing new chemical risk and testing needs. The GHS describes how QSARs can be used to predict the acute toxicity for fish, daphnia, and algae for certain classes of chemicals (UNECE, 2004, p. 356 and Chapter A8.6), and to predict bioconcentration (p. 376). Greater testing demands in the EU, due to the cosmetic directive and the REACH initiative, have also stimulated efforts there for the validation and greater use of (Q)SAR prediction models to meet regulatory testing needs.

Current in vitro methods for acute aquatic toxicity are neither standardized nor validated. New strategies for addressing the major limitations of cell-based assays have been proposed; see Emerging Science and Policy.

Current in vitro/in silico approaches to replacing animals in aquatic toxicity testing include:

  • Fish cell-based cytotoxicity assays
  • Fish cell-based assays with other mechanisms of toxicity as endpoints
  • Mammalian cell assays using cytotoxicity or other endpoints
  • Bacterial cell assays, usually based on luminescent reporter genes (primarily used for detection)
  • Fish embryo assays (although not always considered non-animal)
  • In vitro endocrine disruptor assays
  • Genomic microarrays (toxicogenomics)
  • (Q)SAR and other computational programs
  • Test batteries and/or tiered testing schemes incorporating the above types of assays

The Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) has not reviewed any alternatives to the fish or crustacean tests for acute aquatic toxicity testing. ICCVAM has participated since 2002 in international activities to validate several cell-based endocrine disruptor assays. Endocrine disruptor test methods are covered in another section of AltTox: Toxicity Endpoints & Tests: Endocrine Disruptors.

An ECVAM ESAC Statement made the point “that the UTC [Upper Threshold Concentration Step Down] approach should be implemented as a valid strategy to significantly reduce the number of fish used in the assessment of acute aquatic toxicity for hazard classification.” However, like ICCVAM, ECVAM has not validated any non-animal methods for ecotoxicity testing.

The Fish Embryo Acute Toxicity (FET) Test (OECD TG 236) that uses fertilized zebrafish eggs is a test method that could reduce the number of fish used in testing. However, as previously mentioned, “TG236 does not indicate whether the fish embryo acute toxicity test can be used as an alternative to the OECD TG203.”

In the near term, reduction strategies using tiered testing schemes and/or tests using fish embryos provide the greatest opportunity for reducing the numbers of fish used for ecotoxicity testing.

More information on in vitro methods being developed for ecotoxicity testing can be found at: