Extremely hazardous substances (EHSs) can be released intentionally through terrorist activities or accidentally as a result of chemical spills, industrial explosions, fires, or accidents involving railroad cars or trucks transporting EHSs. EHSs can also be released because of improper storage or handling. Workers and residents in communities surrounding industrial facilities where EHSs are manufactured, used, or stored and in communities along the nation's railways and highways are potentially at risk for exposure to airborne EHSs during accidental or intentional releases. Pursuant to the Superfund Amendments and Reauthorization Act of 1986, the U.S. Environmental Protection Agency (EPA) has identified some 400 EHSs on the basis of data on acute lethality in rodents. As part of its efforts to develop acute exposure guideline levels for EHSs, EPA and the Agency for Toxic Substances and Disease Registry (ATSDR) in 1991 requested that the National Research Council develop guidelines for establishing such levels. In response to that request, the National Research Council published Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances in 1993. Using the 1993 and 2001 National Research Council guidelines reports, the NAC-consisting of members in EPA, the Department of Defense (DOD), the Department of Energy (DOE), the Department of Transportation (DOT), other federal and state governments, the chemical industry, academe, and other organizations in the private sector-has developed AEGLs for about 200 EHSs. At its meetings, the committee hears presentations from NAC staff and its contractors on draft AEGL documents. At some meetings, the committee also hears presentations from the NAC's collaborators in other countries, such Germany and the Netherlands. The committee provides comments and recommendations on those documents to the NAC in its interim reports, and the NAC uses the comments to make revisions. The present report is the committee's 17th interim report. It summarizes the committee's conclusions and recommendations for improving the NAC's AEGL documents for 17 chemicals. The report also summarizes the committee's conclusions and recommendations for improving the Standing Operating Procedures for Developing Acute Exposure Guideline Levels for Hazardous Substances, published in 2001.