An LC50 vs time model for the aquatic toxicity of reactive and receptor-mediated compounds. Consequences for bioconcentration kinetics and risk assessment

For aquatic toxicants that act by so-called nonpolar narcosis, it is genera lly acknowledged that the Critical Body Residue (CBR) at death, as a surrog ate dose metric for the amount of target that has interacted with the toxic ant, is constant. This constancy is not only maintained across exposure tim es but also across different (narcosis) compounds as well as species. We pr esent here an alternative model, applicable to reactive and receptor-mediat ed toxicants, that implies that for these compounds there is no constant CB R. The model also shows that for each single species-compound combination, the Critical Area Under the Curve (CAUC) is constant and independent of exp osure time. These findings can have profound consequences for the interpret ation of experimental toxicity data (such as 96 h LC50 values) in risk asse ssment. Among other things, it shows us that for compounds other than nonpo lar narcotics, LC50 VS time values may decrease significantly even after bi oconcentration steady state has been achieved. Consequently, it also shows us that the incipient LC50 will be severely overestimated (i.e. toxicity un derestimated) when using the familiar models based on just bioaccumulation kinetics.