Monograph 026 – Aquatic Toxicity Testing of Sparingly Soluble, Volatile and Unstable Substances

Mono 026 : Aquatic Toxicity Testing of Sparingly Soluble, Volatile and Unstable Substances | September 1996

The objective of the Task Force was to review the scientific validity for the application of standard aquatic toxicity tests to substances that are difficult to test, and to develop guidance for the performance of tests and interpretation of results from such studies.

The report discusses the types of substances requiring special consideration for testing procedures, namely substances with attributes such as low solubility, adsorption, high volatility, instability, light attenuation (colour) and complexity, i.e. multicomponent substances.

To facilitate the optimal performance of tests and interpretation of the data on difficult-to-test substances, it is particularly important to have information on the composition and identity of the test substance, as well as appropriate physico-chemical data, prior to performing any tests. Thus, the likely behaviour of the substance(s) may be predicted and the appropriate experimental design employed.

The report considers the effects of dispersants and co-solvents as aids to preparation of test media; this review is made in the context of a discussion of the modes of uptake of compounds by test organisms. The driving force for uptake is the dissolved concentration in the aqueous medium. The presence of undissolved substance does not influence the relationship between dissolved concentration and uptake. Hence, there is no advantage in testing above the water solubility limit in order to assess the inherent toxicity. This conclusion contradicts some regulatory guidelines and some reports of dose-response relationships of inherent toxicity above the solubility limit. Possible explanations of such responses are provided.

Since regulatory guidelines and methods for aquatic toxicity testing were developed for essentially pure substances that are soluble and stable in water, they consequently require modification to properly account for substances which are difficult to test. In this report, good practice for testing is reviewed, including sampling and the extent of analytical chemistry that is required, and guidance is provided on the use and interpretation of the results.

The information and arguments developed are brought together in a suggested strategy for testing, centred around a flow scheme which covers all substance types, including those that are not difficult to test. The scheme describes the limits on the interpretation of test results that apply to risk assessment.

Correct interpretation of effects observed in aquatic toxicity tests is dependent upon:

test media being prepared in a manner that is appropriate to the objectives of the study and the nature of the test substance;
correct measurement and expression of exposure levels;
the distinction of true toxicity from indirect physical effects of the substance, which should be avoided.

With regard to EU legislation, the following recommendations are made:

Risk assessment

In an initial risk assessment for substances that are not acutely toxic at their water solubility limit, the following conservative approach is proposed in order to avoid unnecessary chronic testing: The highest measured soluble concentration (or solubility limit) is taken as the acute LC₅₀ and the assessment factor usually applied to the LC₅₀ is assigned to that concentration in order to calculate a safe upper limit for a PNEC.

Classification and labelling

If sparingly water soluble substances are toxic at a concentration below their water solubility then they should be classified according to the Directive. However, for sparingly soluble substances which in acute tests do not display toxicity at the water solubility limit and toxic impurities are not present in significant amounts, the toxicity criterion for classifying a substance as ?dangerous to the environment? is not fulfilled.

Additional specific recommendations and findings discussed and developed by the Task Force are summarised in Section 8.