Health effects classification and its role in the derivation of minimal risk levels: neurological effects.

The agency for Toxic Substances and Disease Registry (ATSDR) uses substance-specific minimal risk levels (MRLs) to assist in evaluating public health risks associated with exposure to hazardous substances. By definition, "MRLs are estimates of daily human exposure to a chemical that are likely to be without an appreciable risk of adverse noncancer health effects over a specified duration of exposure." MRLs serve as screening levels for health assessors to identify contaminants and potential health effects that may be of concern for population living near hazardous waste sites and chemical releases. MRLs for each substance are derived for acute (1-14 days), intermediate (15-364 days), and chronic (365 days and longer) exposure durations, and for the oral and inhalation routes of exposure. The MRLs are derived from data compiled from a current comprehensive literature search and are presented in ATSDR's toxicological profile for that substance. In this paper we outline ATSDR's guidance for evaluating the neurological end point as discussed in the agency's toxicological profiles. Ranking neurological effects into less serious and serious categories and applying this procedure to the derivation of health guidance values or MRLs are also described. Specific examples of ATSDR MRLs based on neurological effects are presented.