Children differ from adults both physiologically and behaviorally. These differences can affect how and when exposures to xenobiotics occur and the resulting responses. Testing using animal models may be used to predict whether children display novel toxicities not observed in adults or whether children are more or less sensitive to known toxicities. Historically, evaluation of developmental toxicity has focused on gestational exposures and morphological changes resulting from this exposure. Functional consequences of gestational exposure and postnatal exposure have not been as well studied. Difficulties with postnatal toxicity evaluations include divergent differentiation of structure, function and physiology across species, lack of understanding of species differences in functional ontogeny, and lack of common end points and milestones across species. Key words: critical periods of development, extrapolation of animal data, hazard identification, regulatory guidelines. Environ Health Perspect 112:266-271 (2004). doi:10.1289/ehp.6014 available via http://dx.doi.org/[Online 25 November 2003]

Children and adults are different physiologically and behaviorally. Children eat and drink more (based on size), play and act differently (e.g., very young children engage in more hand-to-mouth activity), are still undergoing development, and may be less or more able to metabolize and excrete certain substances [reviewed by U.S. EPA (U.S. Environmental Protection Agency) (2001)]. Because of these differences, children and adults may differ qualitatively and/or quantitatively in how they are affected by xenobiotic exposure. Effects of xenobiotics in children may be completely different from effects from the same exposure in adults (qualitative difference). On the other hand, the effect of xenobiotic exposure may be similar between children and adults but may occur to a greater or lesser extent in the child (quantitative difference).