There are different ideas about the application of QSAR studies also. The different understandings of ''similar action'' consequently lead to different regulatory proposals for water quality objectives. The majority of studies dealt with mixtures of heavy metals. Systematic work has been done with regard to the combined effects of industrial organic chemicals with an anticipated unspecific mode of action. Analysis of combination effects of pesticides and other specific acting agents are rare.
Most information is available on the toxicity of mixtures to fish and daphnids.
Relatively few studies investigated combination effects on other aquatic species or multispecies microcosm test systems. Reported combination effects are often rather contradictory, especially with heavy metals. Reference models for the assessment of combined effects might be discussed with respect to pharmacological threshold limits. With respect to environmental standards this means, that combined effects could not have to be addressed by regulations for dissimilar acting chemicals at low exposure levels.