Challenges in the regulatory use of water-accommodated fractions for assessing complex substances

Abstract

The use of the water-accommodated fraction (WAF) approach for the preparation of exposure systems of complex substances such as petroleum products has been a standard way to perform aquatic toxicity tests on these substances for over 30 years. In this Commentary, we briefly describe the historical development, rationale, and guidance for the use and reporting of the WAF approach to assess complex substances. We then discuss two case studies, with coal tar pitch and kerosene/jet fuel, which illustrate challenges from regulatory authorities in Europe and the United States when using the WAF approach. We describe how the WAF approach is the only currently known method for testing the toxicity of the whole of a complex substance, even when some of its constituents remain unknown; it accounts for differences in the solubility of the constituents within a complex substance; and use of loading rates to describe any toxic effects is a unifying concept that allows direct comparison with releases of readily soluble substances in hazard assessment and chemical classification.

The Next Frontier of Environmental Unknowns: Substances of Unknown or Variable Composition, Complex Reaction Products, or Biological Materials (UVCBs)

“…However, WAF has fundamental drawbacks: it represents only a fraction and not the whole substance (whose chemical identity is subject to uncertainty), mixture composition may be altered compared to the UVCB it is prepared from, and the WAF composition depends on preparation techniques. Issues related to WAF results interpretation for coal tar pitch and kerosene/jet fuel UVCBs within regulatory processes of the U.S. EPA and REACH have been reported . Alternatives to WAF include solvent extraction followed by solvent spiking, generator systems, saturator columns, and passive dosing methods, the last of which has been in active development in recent years with respect to UVCBs. − …”

“…Issues related to WAF results interpretation for coal tar pitch and kerosene/jet fuel UVCBs within regulatory processes of the U.S. EPA and REACH have been reported. 137 Alternatives to WAF include solvent extraction followed by solvent spiking, generator systems, saturator columns, and passive dosing methods, the last of which has been in active development in recent years with respect to UVCBs. 124−126 On balance, results from whole-mixture testing could be integrated into a weight of evidence (WoE) approach for UVCB assessment.…”

Dosing Methods to Enable Cell-Based In Vitro Testing of Complex Substances: A Case Study with a PAH Mixture

“…DMSO is also used for extraction of complex substances and environmental samples before bioactivity testing [ 14 , 15 ]. However, DMSO-soluble fractions of complex substances and mixtures may not represent the entire bioactive fraction of concern [ 16 , 17 ]. This limitation was recently highlighted in a decision from the European Chemicals Agency (ECHA) on a submission of in vitro bioactivity data [ 18 ] for petroleum substances extracted with DMSO in support of a grouping decision, stating that “testing DMSO extracts does not provide a basis for reliably predicting the properties of the substance” [ 19 ].…”

Aquatic Toxicity Calculation of Mixtures: A Chemical Activity Approach Incorporating a Bioavailability Reduction Concept

“…The term WAF is applied to an aqueous test solution containing only the fraction of a multiconstituent substance that is dissolved and/or present as a stable dispersion or emulsion. Despite efforts to limit this methodology, WAF tests are still the best way to test mixture toxicity in ecotoxicology . The problem with the method is that, unlike studies on single substances, it is difficult to validate the results based on measured concentrations as the constituent concentrations in aqueous medium do not necessarily correspond to the concentration of that constituent in the original mixture.…”

“…Despite efforts to limit this methodology, WAF tests are still the best way to test mixture toxicity in ecotoxicology. 3 The problem with the method is that, unlike studies on single substances, it is difficult to validate the results based on measured concentrations as the constituent concentrations in aqueous medium do not necessarily correspond to the concentration of that constituent in the original mixture. Thus, it is not always clear whether the result is due to substance loss (e.g., through volatility or adsorption during the study) or is an accurate representation of the true toxicity of the mixture.…”