This thesaurus presents the specialized terminology and air pollution indexing terms used for the storage of, and search for, information in the Air Pollution Technical Information Center (APTIC) data base file, and illustrates the rules formulated for their use. The meanings of the terms are implied rather than defined, being implicit in the network of conceptual relationships represented among the terms. The thesaurus is divided into three main parts: (1) Thesaurus of Terms--arranged alphabetically; (2) Systematic Microthesaurus--subdivided into four sections, one of which contains the hierarchical air pollution descriptors; and (3) Descriptors--lists descriptors and superseded terms separately. (Author/CWM)