Epidemiologic analysis of acute, reversible respiratory health effects is uncommonly performed, yet these are important health outcomes because acute responses by the respiratory defense system appear to represent one end of the continuum toward progressive, chronic and potentially disabling physiologic changes. Repeatable epidemiologic studies of dose-response relationships necessitate accurate measures of 'dose'. However, in occupational and environmental settings, exposure to a toxin is seldom found at identical concentrations and/or particle sizes among persons with the same activity patterns. In addition, air concentration does not account for factors such as clearance or metabolism which may alter the biologically effective tissue dose. These factors cause the target tissue dose of the toxin to vary greatly despite exposure to similar air concentrations.