On the use of scaling factors to improve interspecies extrapolation of acute toxicity in birds.

In avian toxicology, it is customary to extrapolate between species on the basis of acute toxicity measurements expressed in mg/kg body weight. Recently, it has been suggested that extrapolations should be on the basis of weight raised to the 0.6-0.7 power because there is good empirical evidence that, for mammals, this produces the best agreement between species. We used an avian LD50 database to derive empirically the appropriate scaling factor for birds. With a subset of 37 pesticides of varying structures but heavily weighted to cholinesterase inhibitors, we found that the appropriate scaling factor in birds is usually higher than 1 and can be as high as 1.55. Extrapolations on the basis of weight alone or, worse, the use of inappropriate mammalian scaling factors could lead to serious underprotection of small-bodied bird species modeled in the course of risk assessment procedures.