An equation describing the time course and variability in uptake and toxicity of narcotic chemicals to fish

Abstract

AbstractAn equation is proposed to express the time course of uptake of organic chemicals by fish. The chemicals' octanol/water partition coefficients are used to describe equilibrium partitioning, whereas the clearance rate constant, estimated as a function of water‐ and organic‐phase series resistances to transport, is used to describe the time course of uptake. The probability of the concentration of chemical in the fish causing a defined toxic end point is expressed by using a modified Weibull distribution function that contains an adjustable parameter describing variability in organism response. The effect of metabolism can be included if desired. The combined uptake and probability equations are tested by using mortality data for fathead minnows exposed to 18 narcotic chemicals. A single set of parameters is obtained to fit the entire data set. The equation highlights the importance of considering the kinetics of toxicant accumulation when interpreting toxicity results, especially for hydrophobic chemicals, and reinforces the fact that a common organism residue level (in either molar volume or molarity) is associated with 50% mortality in acute bioassays with narcotic organic chemicals. Assorted features of the equations and their applicability to toxicants with specific modes of action are discussed.