The typical situation found in ecosystems is characterised by a multitude of different toxicants that act simultaneously on exposed organisms. However, traditional ecotoxicological studies as well as risk assessment approaches typically simplify this situation by using a compound-by-compound approach that assumes that the momentarily investigated chemical is the only toxicant present in the environment.

Obviously, this is a gross simplification of the actual situation. What makes it absolutely vital to finally consider the joint action of all chemicals that make up a certain pollution situation is that the toxicity of a mixture is usually  higher than the toxicity of each compound alone, often considerably so:


The figure is from a study that we first published in 2000 (Backhaus T, Aquatic Toxicology, 49(1-2), 49-61, 2000).  On the left hand side you see the effects that the individual compounds provoke singly and on the right hand side you see the resulting mixture effects. Even though all compounds (quinolone antibiotics in this case) are present in only very minute amounts – in concentrations that could be considered ”safe” – the toxicity of a mixture is rather substantial: more than half of the exposed bacteria are affected. This shows that any environmental quality standard, e.g. for protecting a river ecosystem, but also for drinking water protection, has to take mixture effects into account. Analyzing the toxicity of individual substances is a necessary first step, but is insufficient alone.

On the far right of the figure I have plotted two mixture toxicity expectations, according to the classical concepts of Concentration Addition (CA) and Independent Action (IA, sometimes also called Response Addition). As can be seen, Concentration Addition was quite good at estimating the joint toxicity of the mixture. Independent Action underestimated the actually observed joint toxicity. You can find more details on this study in

In view of this characteristics of chemical mixtures the aims of my research are

  • to explore concepts and models that describe the joint action of chemicals. The goal is to validate such models for certaing groups of chemicals and to explore the limits of their applicability. The idea is to come up with approaches that ultimately should help us to predict the toxicity of chemical mixtures in the environment, and
  • to analyse the options of including our scientific knowledge on the ecotoxicity of chemical mixtures into environmental regulation. How do we apply our knowledge, what are the hurdles and how can we overcome them?
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