Assessing the impact of low level chemicals on development: behavioral and latent effects.

There is growing evidence that nervous tissue, especially the brain, is more sensitive to many foreign chemical substances than has previously been suspected, and that toxic effects may be manifested as subtle disturbances of behavior long before any classical symptoms of poisoning become apparent. Early detection of an insidious toxic process (behavioral toxicology) may enable the prevention or attenuation of harm to humans and other organisms. Adding to both the sensitivity and complexity of behavioral toxicologic testing is the increasing evidence that individuals are more vulnerable to adverse factors during the period of development (conception yields puberty) than at any other time in life. Subtle functional disturbances in organisms exposed while immature (behavioral teratology) may be one of the most sensitive indicators of chemical toxicity. Furthermore, defects in a developmental process may have only delayed effects. A morphological or biochemical lesion can be dormant and not manifest itself until later in life as a behavioral disorder, mental deficiency, or overt functional impairment. Longitudinal evaluation is required to detect long-term or delayed effects of a particular developmental influence on biological and behavioral functions. Examples from research on the subtle and latent consequences of prenatal and early postnatal exposure to methylmercury that illustrate the above principles are presented. It is concluded that behavioral and long-term evaluation of organisms exposed during development are essential for a thorough assessment of the impact of certain low level chemicals on human health.