The primary purpose of the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL) is to develop guideline levels for short-term exposures to airborne concentrations for approximately 400 to 500 high priority, acutely hazardous substances within the next ten years. These Acute Exposure Guideline Levels (AEGLs) are needed for a wide range of planning, response, and prevention applications. The NAC/AEGL Committee seeks to develop the most scientifically credible, acute (short-term) exposure guideline levels possible within the constraints of data availability, resources and time. This includes a comprehensive effort in data gathering, data evaluation and data summarization; fostering the participation of a large cross-section of the relevant scientific community; and the adoption of procedures and methods that facilitate consensus-building for AEGL values within the Committee. The NAC/AEGL Committee is currently comprised of representatives of federal, state and local agencies, private industry and other organizations in the private sector that will derive programmatic or operational benefits form the existence of the AEGL values. AEGL values are determined for three different health effect end-points. These values are intended for the general public where they are applicable to emergency (accidental) situation. Threshold exposure values are developed for a minimum of 4 exposure periods (30 minutes, 1 hour, 4 hours, 8 hours). In certain instances, AEGL values for a 10-minute exposure period also will be developed. Each threshold value is distinguished by varying degrees of severity of toxic effects, as initially conceived by the AIHA ERP Committee, subsequently defined in the NAS' National Research Council Report, Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances, published by the National Academy of Sciences in 1993, and further refined by the NAC/AEGL Committee. To date, the committee has reviewed over 80 chemicals.