Mixture toxicity of copper and diazinon to larvae of the mayfly (Ephoron virgo) judging additivity at different effect levels

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Abstract
The toxic unit concept is commonly applied for determining the effects of a mixture of toxicants. Use of this concept is, however, limited to compounds with similarly shaped dose‐response relationships. In the present study, a method is proposed to gain insight regarding the influence of differences in the shapes of dose‐response relationships on the applicability of the concentration‐addition model by judging additivity at different effect levels. To this purpose, two model toxicants with different modes of action and dose‐response relationships were selected: copper, and diazinon. Using mortality of the mayfly (Ephoron virgo) as the endpoint, it was demonstrated that the two compounds act in a less‐than‐concentration‐additive manner. Application of the proposed calculation method revealed that the less‐than‐concentration‐addition effect was independent of the effect level on which the mixture was judged.

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