Tadpoles of four species of frogs were exposed for 30 days at early and late developmental stages to low concentrations of the pesticide atrazine (3, 30, or 100 ppb; the U.S. Environmental Protection Agency drinking water standard is 3 ppb). Survival was significantly lower for all animals exposed to 3 ppb, compared with either 30 or 100 ppb, except for the late stages of two species.

Comment: Atrazine is the most widely used pesticide in the United States. The results of the present study indicate that, within a certain dosage range, lower concentrations of this chemical are more toxic than higher concentrations. This pattern is characteristic of many endocrine-disrupting compounds.

Scientists often disregard studies that demonstrate a toxic effect of a chemical at low doses, but no toxic effect at high doses. One should not assume, however, the effects of toxic chemicals always increase with increasing levels of exposure. Many chemicals function as catalysts at low concentrations, but become inhibitors at higher concentrations. Some compounds influence one biochemical pathway when present at a low concentration and a different pathway when present at a higher concentration. The results of the present study demonstrate the importance of evaluating the effect of toxic substances at realistic concentrations and at various developmental stages.

Storrs SI, et al. Survivorship patterns of larval amphibians exposed to low concentrations of atrazine. Environ Health Perspect 2004;112:1054-1057.

See also: Kon SH. Underestimation of chronic toxicities of food additives and chemicals: the bias of a phantom rule. Med Hypotheses 1978;4:324-339.