Technical Report 120


Acute toxicity - The harmful properties of a substance which are demonstrated within a short period of exposure (e.g. hours for algae or days for fish and crustaceans).

Chronic toxicity - The harmful properties of a substance which are demonstrated only after long-term exposure in relation to the life of the organism.

Critical body burden (CBB) - The term is used in this report to encompass the various terms used by different authors, including critical body / tissue residue, residue-based toxicity, internal effects concentration etc. It relates to the highest tissue concentration having no effect as well as the ‘lowest concentration’ causing some significant effect (equating to a LOEC).

EC50 - Median effect concentration (generating an effect response in 50% of the test population). Where the endpoint is lethality, this is known as the LC50.

EC10 - Median effect concentration (generating an effect response in 10% of the test population), regarded in the TGD as being of similar value as the NOEC. The EC10 can be based on a population endpoint that is used to for risk assessment application, such as survival, growth or reproduction (termed the adverseEC10), or on a biomarker response (termed the biomarkerEC10).

Fugacity and fugacity capacity -  The term fugacity was introduced in 1901 by G.N. Lewis and is most often regarded as the ‘escaping tendency’ of a chemical from a particular environmental compartment (e.g. water, soil, air, biota, etc). Fugacity (F) has units of pressure, generally pascals (Pa), and can be related to phase concentrations. For any particular environmental phase (e.g. water, soil, air, or biota) there is a corresponding ‘fugacity capacity’ with units of mol/m3-Pa and is denoted by Z. The relationship between fugacity, fugacity capacity and chemical concentration (C) is defined by the equation:


Environmental compartments in equilibrium with each other have equal fucagity values (i.e. the tendency to leave one compartment and enter a second is equal to the tendency to leave the second and enter the first). High fugacity equals high propensity to migrate.

Lowest-Observed Effect Concentration (LOEC) - This can be based on a population adverse effect measurement such as decreased survival, growth or reproduction (termed the adverseLOEC) or possibly on water biomarker response (termed the biomarkerLOEC).

Mechanism of action - A complete and detailed understanding of each and every step in the sequence of events that leads to a toxic outcome, underlying the MoA.

Mode of action (MoA) - A common set of physiological and behavioural signs that characterise a type of adverse biological response (Escher and Hermens, 2002), where the major (but not all) biochemical steps are understood.

Mode of action (Type 1) - Non polar narcotic substances: Narcosis (or baseline) toxicity is believed to be the result of reversible and non-specific disturbance of membrane integrity and function resulting from the partitioning of the chemical into biological membranes (Escher and Hermens, 2002). Because the effects are not specific to particular chemical structures, this can be considered the minimum (or baseline) toxicity that any chemical will display, if it is not obscured by greater toxicity through other modes of action. This MoA is therefore displayed by chemicals that are ’inert’ in terms of chemical or biological reactivity, and by interactions with specific biological receptors.

Mode of action (Type 2) - Polar narcotic substances: This group consists of more polar but essentially non-reactive substances such as substituted phenols and anilines which ionise to some extent depending on pH and display slightly greater toxicity (external concentration) than would be predicted by ‘baseline’ toxicity QSARs. They are often characterised as possessing hydrogen bond donor acidity.

Mode of action (Type 3) - Reactive substances: Reactive substances are considered as a group that includes diverse modes of action resulting from non-selective reactions with biomolecular structures and consequently displaying enhanced toxicity (lower CBBs) compared with baseline narcotics (Verhaar et al., 1992). This group also includes chemicals that are metabolically activated into reactive substances. Of particular importance are electrophilic substances that react with amino, hydroxyl and sulphydryl groups within proteins and DNA (Hermens, 1990), such as certain carbonyls, epoxides, nitriles, hydrazines, acid anhydrides and aldehydes.

Mode of action (Type 4) - Specifically active receptor-active substances: Specifically acting chemicals can be classified by their interaction with one of four major protein targets i.e. (a) receptors; (b) ion channels; (c) enzymes and (d) transporters (Rang et al, 2003).

No-Observed Effect Concentration (NOEC) - The highest concentration below the LOEC where the stated effect was  not observed. The effect can be based on a population endpoint which is used to for risk assessment application, such as survival, growth or reproduction (termed the adverseNOEC) or possibly on a biomarker response (termed the biomarkerNOEC).

Vapour pressure - The pressure exerted by a chemical in the vapour phase in equilibrium with its solid or liquid form. It provides an indication of the relative tendency of a substance to volatilise from the pure state. Typical units are mm Hg, torr, or in Hg.

Water Solubility (S) - The maximum amount of a chemical that can be dissolved in a given amount of pure water at standard conditions of temperature and pressure. Typical units are mg/L, g/L.