SREL Reprint #2425

The individual tolerance concept is not the sole explanation for the probit dose-effect model

Michael C. Newman1 and John T. McCloskey2

1College of William and Mary, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062-1346, USA
2U.S. Fish and Wildlife Service, 6669 Short Lane, PO. Box 99, Gloucester, Virginia 23061

Abstract: Predominant methods for analyzing dose- or concentration-effect data (i.e., probit analysis) are based on the concept of individual tolerance or individual effective dose (IED, the smallest characteristic dose needed to kill an individual). An alternative explanation (stochasticity hypothesis) is that individuals do not have unique tolerances: death results from stochastic processes occurring similarly in all individuals. These opposing hypotheses were tested with two types of experiments. First, time to stupefaction (TTS) was measured for zebra fish (Brachydanio rerio) exposed to benzocaine. The same 40 fish were exposed during five trials to test if the same order for TTS was maintained among trials. The IED hypothesis was supported with a minor stochastic component being present. Second, eastern mosquitofish (Gambusia holbrooki) were exposed to sublethal or lethal NaCl concentrations until a large portion of the lethally exposed fish died. After sufficient time for recovery, fish sublethally exposed and fish surviving lethal exposure were exposed simultaneously to lethal NaCl concentrations. No statistically significant effect was found of previous exposure on survival time but a large stochastic component to the survival dynamics was obvious. Repetition of this second type of test with pentachlorophenol also provided no support for the IED hypothesis. We conclude that neither hypothesis alone was the sole or dominant explanation for the lognormal (probit) model. Determination of' the correct explanation (IED or stochastic) or the relative contributions of each is crucial to predicting consequences to populations alter repeated or chronic exposures to any particular toxicant.

Keywords: Toxicity, Dose-effect model, Probit, Lognormal distribution, Tolerance

SREL Reprint #2425

Newman, M.C. and J.T. McCloskey. 2000. The individual tolerance concept is not the sole explanation for the probit dose-effect model. Environmental Toxicology and Chemistry 19:520-526.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).