A sensitivity analysis was undertaken of the Simulation of Human Air Pollution Exposure (SHAPE) model, which incorporates an enhanced version of the Coburn-Forster-Kane (CFK) physiological model for predicting a person's blood carboxyhemoglobin (COHb) as a function of time in response to activities of a sample of people in an urban area, exposing them to pollutant concentrations from appropriate micro-environments as they move through time and space in a 24-hour period. The CFK model dynamically calculates their blood COHb from their CO exposures, and the person's ventilation rate is altered in response to their level of activity. To conduct this sensitivity analysis, the SHAPE program was run many times using different combinations of values for its parameters, thus allowing the contribution to COHb of each of many variables to be examined. The following phenomena were found to have significant effect, over and above the maximum COHb levels of the population: (1) CO exposure contributed by the highway microenvironment; (2) altitude of the city; (3) the CFK physiological parameters (e.g., Haldane constant and endogenous CO production). In contrast, if it is assumed that a person using an indoor parking garage spends less than 10 minutes there, then use of a parking garage has very little effect on the COHb frequency distribution of the population. For low-level CO exposure, use of either the linear or nonlinear form of the CFK model yields essentially the same results.