Exploring the concepts of concentration addition and independent action using a linear low-effect mixture model

Chemicals emitted into the environment are typically present at low concentrations but may act together in mixtures. Concentration‐response curves of in vitro bioassays were often linear <30% effect and the predictions for concentration addition (CA) of similarly acting chemicals and for independent action (IA) of dissimilarly acting chemicals overlapped. We derived a joint CA/IA mixture model for the low‐effect level portion of concentration‐response curves. In a first case study, we evaluated the cytotoxicity of over 200 mixtures of up to 17 components that were mixed in concentration ratios as they occurred in river water. The predictions of the full IA model were indistinguishable from the predictions of the full CA model up to the 10% effect level confirming the applicability of the joint CA/IA mixture model at low effect levels. In a second case study, we evaluated if environmental concentrations trigger effects at levels low enough for the joint CA/IA mixture model to apply. The detected concentrations were scaled by their toxic potencies to estimate the mixture effect of the detected chemicals in a complex mixture. In 86% of 156 samples the effects fell in the validity range of the joint CA/IA mixture model (<10% effect level) confirming the CA assumption for toxic unit summation. The joint CA/IA mixture model is not suitable for testing specific mixture hypotheses and interactions of chemicals in mixtures, where more refined models are required, but it is helpful for the interpretation of effects of complex (multicomponent) environmental mixtures, especially for water samples with relatively low effect level.