Although several air pollution indices have been proposed in the technical literature, there has been no systematic evaluation of the many indices that are presently being used for daily air quality reporting in metropolitan areas. A literature review and user survey were therefore conducted by CEQ and EPA to develop a compendium of air pollution indices. This compendium describes the index-reporting practices of all U.S. and Canadian air pollution control agencies. In the United States, 35 out of 55 agencies were found to be using some kind of air pollution index. For each of the indices, separate analysis records were prepared to summarize index characteristics and code them into a classification system. Through this system, it was determined that 14 basically different types of indices are currently in use. Except for five States which have adopted internally uniform index formats, no two U.S. cities are presently using the same index. The indices differ in the numbers and types of pollutants included, the methods of calculation, and the descriptor words which are reported with the index values. From these findings, it was concluded that there is need to develop a Standard Air Pollution Index which can be recommended for use by State and local agencies.