was enhanced in applicable data bases by using Chemical Abstracts Service Registry Numbers, which uniquely identify each individual chemical. Individual searches were customized to reflect the structure of each data base. For MEDLARS data bases, searching was done on the standardized terminology and alphanumeric designators for each chemical found in NLM's Medical Subject Headings (MeSH) and the MeSH tree structures.

Although there is subject and content overlap, each data base serves a unique function, has a distinct subject emphasis, and indexes literature not available elsewhere. To serve the comprehensive goals of this study, it was decided to search all relevant data bases in their entirety for epidemiologic studies, case reports, and secondary literature (reviews, letters, news articles, etc.). The animal toxicology literature on this subject is extensive, and these studies build on and reference previous work. The committee decided to retrieve all animal toxicology citations from 1980 through the present.

Throughout the study, searches were updated, and targeted searches were done on specific topics such as porphyria cutanea tarda.