Context, toxicity reference values, and risk

A toxicity reference value (TRV) is a benchmark established to represent a concentration or dose of a contaminant that is expected to produce no observable effects. The effects that are included in this derivation are not reported as part of the TRV, nor are the error terms or uncertainties included in the data set. Essentially, to establish a TRV, a substantial amount of information and data need to be collapsed into a point estimate, entailing the loss of considerable information. A TRV is a number taken out of context with the conditions and data used to generate it; it has no environmental or ecological context. The loss of information and unreported uncertainty are not only a feature of TRVs but also apply to other point estimates such as sediment quality guidelines, aquatic life criteria, or an ecological soil screening level (EcoSSL). The environment in which a contaminant is found influences its chemical form and often its uptake and toxicity. A contaminant exists as part of a mixture with many other compounds, which have varied uptake rates, modes of action, different concentration–response curves, and half-lives. The organisms exposed to the mixture are also exposed to pathogens, competitors, and changes in environmental resources. These same organisms have a variety of life-history strategies, can be dramatically transformed in size, shape and metabolism during their life-cycle, and have wide ranges in mobility throughout the landscape. The rest of this brief communication provides examples of the importance of context. The first example describes the finding of synergism between carbamates and organophosphates in acetylcholinesterase inhibition. The second example discusses the potential interaction between a persistent organic contaminant, disease, and the decline of Pacific herring in Puget Sound, Washington, USA.