In the June 1, 1979 draft staff paper presented to the Clean Air Scientific Advisory Committee, EPA included a table depiciting carboxyhemoglobin (COHb) responses to CO exposures as predicted by the Coburn Model. This table presents baseline or nominal estimates of the COHb levels expected to be achieved by non-smokers to constant CO concentrations for 1 to 8 hours. Uncertainties exist, however, in using these estimates to decide which standards provide an adequate margin of safety. First, even among normal person variations exist in the physiological parameters used in the Coburn model. These variations are sufficient to provide noticalbe deviations from the COHb levels that were predicted using normal parameters. Even more substantial variations in these parameters are observed in population groups such as anemics that may be more sensitive to CO exposure. Second, predictions based on exposure to constant CO concentrations inadequately represent the response of individual exposed to widely fluctuating concentrations that typically occur in ambient exposure situations. An attempt was made to represent these uncertainties by performing a sensitivity analysis unsing the Coburn model in a dynamic mode to predict the COHb responses to specific patterns of fluctuating ambient concentrations of CO for individuals with physiological parameters different from the baseline values. The results of this analysis are given.