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

The toxic unit concept is commonly applied for determining the effects of a mixture of toxicants. Use of this concept is, however, limited to compound s 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 c oncentration-addition model by judging additivity at different effect level s. To this purpose, two model toxicants with different modes of action and dose-response relationships were selected: copper, and diazinon. Using mort ality of the mayfly (Ephoron virgo) as the endpoint, it was demonstrated th at the two compounds act in a less-than-concentration-additive manner. Appl ication of the proposed calculation method revealed that the less-than-conc entration-addition effect was independent of the effect level on which the mixture was judged.