We just published the following policy analysis in ”Environmental Science and Technology”:
Thomas Backhaus, Michael Faust: “Predictive environmental risk assessment of chemical mixtures: a conceptual framework” DOI:10.1021/es2034125.
Abstract: Environmental risks of chemicals are still often assessed substance-by-substance, neglecting mixture effects. This may result in risk underestimations, as the typical exposure is towards multi-component chemical “cocktails”. We use the two well established mixture toxicity concepts (Concentration Addition (CA) and Independent Action (IA)) for providing a tiered outline for environmental hazard and risk assessments of mixtures, focusing on general industrial chemicals and assuming that the “base set” of data (EC50s for algae, crustaceans, fish) is available. As mixture toxicities higher than predicted by CA are rare findings, we suggest applying CA as a precautious first tier – irrespective of the modes/mechanisms of action of the mixture components. In particular, we prove that summing up PEC/PNEC ratios might serve as a justifiable CA-approximation, in order to estimate whether there is a potential risk for an exposed ecosystem in a first tier assessment if only base-set data are available. This makes optimum use of already existing single substance assessments as more demanding mixture investigations are requested only if there are first indications of an environmental risk. Finally we suggest to call for mode-of-action driven analyses only if error estimations indicate the possibility for substantial differences between CA- and IA-based assessments.
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One of the central discussion points of the paper is the comparison between the sum of PEC/PNECs and the sum of Toxic Units. Especially the use of PEC/PNEC sums has been challenged, for example in the recent SCHER opinion which states that “[...] a combination of PNECs may be misleading.” The mathematical analysis of the relationship between the sum of PEC/PNECs and the sum of Toxic Unis (which is a widely accepted implementation of Concentration Addition) is published as supporting information to the paper and is freely available. ES&T doesn’t make it available (yet) on their website, so here’s a direct link.
I always feel a bit awkward doing that during the review process or even in the final manuscript. But I have to say that I was really impressed by the amount of work and critical comments that we received by some of our reviewers (as usual, there was an outlier…). “Critical” in the positive sense of the word – the feedback really helped fine-tuning the text. So, whoever you are: thanks a lot, much appreciated!
Thomas
