Toxic chemicals in the environment can cause a wide range of neurological disease. High-dose exposures to environmental neurotoxicants have produced encephalopathy in children ingesting chips of lead-based paint, blindness in persons who ingested methanol, blindness and ataxia in persons who consumed organic mercury, spinal cord degeneration and peripheral neuropathy in persons exposed to tri-ortho-cresyl phosphate (TOCP), and Parkinsonism in persons exposed to MPTP or to manganese. Environmental neurotoxicants have also been shown to produce a wide range of subclinical neurotoxic effects, including reduction in intelligence, impairment in reasoning ability, shortening of attention span, and alternation of behavior. The first step in the prevention of environmental neurotoxicity is to test chemicals for their toxic potential. More than 70,000 chemicals are currently in commerce. However, except for pharmaceuticals, fewer than 10% of these chemicals have been tested for neurotoxicity. A logical approach to neurotoxicologic assessment of chemical substances will build on and extend currently available test systems. It will have a tiered structure. The first or screening tier will consist of tests to measure obvious structural and functional changes, often a functional observational battery. Subsequent levels of testing will be guided by the results of initial screening. Toxicologic testing must be supplemented by epidemiologic surveillance of populations exposed to known and suspect neurotoxicants. Screening programs in these populations designed to detect excessive absorption of a neurotoxic agent or subclinical neurological dysfunction can be useful in identifying affected individuals before severe disability occurs.
