Mixture toxicity of PBT-like chemicals: A relevant focus for future regulation?

Even though most chemicals regulation is still conducted on a chemical by chemical basis, mixture toxicity is achieving increasing attention. The scientific understanding has increased substantially in the last decades, and a general consensus now seems to have been acieved that concentration addition is a suitable model for default estimations of mixture effects. One of the major challenges is therefore how to select specific chemicals for actual mixture toxicity assessments. Persistant chemicals are likely to be present in the environment for an extended period of time, thus increasing the likelihood of them being present in environmentally found mixtures. Persistant, bioaccumulative and toxic (PBT) chemicals are therefore a highly relevant group of chemicals to consider for mixture toxicity regulation. The present study evaluates to what extent a number of PBT-like chemicals posess concern beyond that of the individual components. Firstly, the effects of three chemicals with PBT-like properties (acetyl cedrene, pyrene and triclosan) was examined on the freshwater snail, Potamopyrgus antipodarum. Secondly, mixture bioaccumulation of the same three chemicals were assessed experimentally. Thirdly, a review of the existing litterature on mixture effects of 132 PBT-like chemicals were conducted to evaluate the quantity and quality of mixture studies on PBT chemicals in order to guide future research within the field. The experimental data illustrated that no observed effect concentrations (NOECs) of PBT-like chemicals can lead to effects when mixed. Such “something from nothing” effects will be underestimated if mixture effects are not considered for environmental risk assessment. The review illustrated that the scientific knowledge of PBT-like mixtures is still limited, thus demonstrating a need for future focus on such mixtures.