A safety factor for LC 50 values allowing for differences in sensitivity among species

Abstract

A safety factor has been derived that can be applied to the mean LC 50 value of several test species for a particular toxic compound, for the purpose of arriving at what has been called a hazardous concentration for sensitive species. The application of this factor should provide limited protection to a certain number of species in a relevant community. The derivation is based on the assumption that the LC 50 values for both the test species and for the community species can be conceived of as independent random trials from a log-logistic distribution. The proposed hazardous concentration for sensitive species has been chosen such that the LC 50 value of the most sensitive species in a community of a certain number of species exceeds that concentration by a specified probability. It allows for the uncertainty in the mean and variance of the LC 50 values for the test species that is due to the number of test species being limited. It is possible to calculate the optimum number of species to be tested on basis of a cost-benefit analysis. Examples of the application are given.