An important element in considering possible alternative air quality standards is the (uncertain) population exposure to pollutant concentrations that would result if a given standard were just attained. As part of the current review of the National Ambient Air Quality Standards for carbon monoxide (CO), estimates have been made of exposures of the populations of nine U.S. cities to selected CO levels under existing conditions and assuming attainment of various alternative standards. Making use of some ideas that have been develped by other investigators (see discussion in (1)), two models have been developed for estimating exposures to ambient carbon monoxide levels. In the present study, bounds were estimated for the quantities whose uncertainty had the largest impact on the exposure estimates, and lower bound and upper bound estimates are calculated on this basis along.