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Ecotoxicity testing with ‘difficult’ substances

December 21st, 2018

A reliable understanding of the aquatic hazard of a test substance is vital for risk assessment of chemicals, because assessment of both chemical hazard and risk arising from expected use rely on this in most circumstances. The use of standardised toxicity testing guidelines is important for the reliability and acceptability of the result, and to ensure reproducibility and comparability between studies. It is particularly important to apply an understanding of the data needs and test material characteristics when designing test protocols that will satisfy these needs. The standardised methods are thorough, and stipulate specific details regarding test conditions and analytical methods.

It is however also important to have test methods appropriate for the specific physical and chemical properties of the chemical. In some cases, a basic study according to the standard guideline could potentially give a misleading result caused by the characteristics or behaviour of the test material in the test system. It can be particularly frustrating and expensive for a sponsor to find that a standard test protocol does not match the specific properties of the test substance and consequently does not deliver fit-for-purpose results.

The OECD has recently issued a new edition of its important technical guidance on testing and assessment of difficult test chemicals (Guidance document on aqueous-phase aquatic toxicity testing of difficult test chemicals, No. 23 (Second edition), 6 July 2018, ENV/JM/MONO(2000)6/REV1). First published in 2000, this is a vital supplement to the well-established OECD TG 200 series of standard ecotoxicity testing guidelines. OECD No.23 sets out key adaptations appropriate for studying substances that are, for example, poorly/sparingly soluble, highly volatile, degrading in the test system, ionisable, highly adsorbing, have complex composition, or can cause effects due to nutrient complexation or cause changes to the test medium colour.

As well as some rearrangements in the structure of the different sections and concepts discussed, to improve usability, the new edition of OECD No. 23 includes new guidance on various areas such as:

  • New techniques for test media preparation for highly hydrophobic test chemicals: Liquid/liquid saturator units and passive dosing.
  • New specifications regarding the approach to studying UVCBs in general, such as verification of initial composition and demonstration of equilibrium and stability of the UVCB during the test.
  • Expanded discussion of good practice in Water Accommodated Fraction (WAF) preparation, maintenance and how results from such studies should be reported and interpreted.
  • For highly hydrophobic test chemicals, it is pointed out that aqueous concentrations may not be quantifiable, and an aqueous exposure might not be the most environmentally relevant exposure route for such substances. In these cases, sediment or dietary exposure tests may be considered, in consultation with the relevant regulatory authorities.

The availability of this important testing guidance will be well received by all stakeholders in the generation of hazard data which is of high-quality and fit-for-purpose.

In our capacity as consultants, PFA scientists utilise the specifications in the OECD guidance No.23 in designing, placing and monitoring studies in contract testing laboratories on behalf of our clients, and we welcome this new version. We have well-established links with some of the most highly regarded testing laboratories in the UK and Europe.