The ability to reconstruct employee exposure histories would be a valuable research tool for the evaluation of occupation as a factor in disease. In many cases, however, historical environmental data are available but have not been used to compute past exposures because of differences in sampling methods. This paper describes a quantitative model to convert historical environmental data (from taconite mine and mill operations) into a form consistent with current sampling methods and results and, therefore, will enable past exposure histories to be used. (Past exposure histories are to be determined in an epidemiological study.) In this study, parallel sampling results from the environmental data base were used to obtain a coefficient for the conversion of impinger-particle counts (old sampling method) to filter-respirable mass sampling results (new sampling method). Parameters in the model were estimated using multiple regression techniques. Results show that a consistent ratio exists between impinger-particle counts and filter-respirable mass concentrations for samples collected at the same locations.