The National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL) was created to develop guideline levels for short-term exposures to airborne concentrations for approximately 400-500 high priority, acutely hazardous substances. The program should be completed within the next 10 years. These Acute Exposure Guideline Levels (AEGLs) are being applied to a wide range of planning, response, and prevention applications both within the United States and abroad. 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. The program begins with comprehensive data gathering, data evaluation and data summarization. The resulting Technical Support Documents (TSD) are first reviewed by a small review committee; (chemical manager, two chemical reviewers and the author), then by the full AEGL committee. After that review, a summary is published in the Federal Register for Public comment. When these comments have been addressed, the TSDs are sent to the National Research Council's (NRC) Subcommittee on AEGLs for a peer review. Following acceptance by the NRC, they are published by the Academy. The NAC/AEGL Committee currently comprises representatives of federal, state, and local agencies and representatives from France, Germany, and the Netherlands, private industry, medicine, academia and other organizations in the private sector that will derive programmatic or operational benefits from 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) situations. Threshold exposure values are developed for five exposure periods (10 and 30 min, 1 h, 4 h, 8 h). Each threshold value is distinguished by varying degrees of severity of toxic effects, as initially conceived by the American Industrial Hygiene Association's Emergency Response Planning Committee, subsequently defined in the NAS' National Research Council publication of the Guideline for Developing Community Emergency Exposure Levels for Hazardous Substances and further categorized in the Standing Operating Procedures of the NAC/AEGL Committee. To date, the committee has reviewed almost 100 chemicals.