A simple mathematical model was developed to grade the results of the fraction of inspiratory CO2 levels in respiratory protective devices (RPDs). Using this model, the level of FiCO2 of two chemical warfare RPDs was monitored during the examination of 54 young male volunteers at rest and exercise. No set of results distributed in a normal Gaussian mode; therefore, parametric statistics could not be used to analyze the group's results. Yet the model allowed simple and insightful evaluation of the results, stressing differences between the similar RPDs. We suggest continued use of the analysis model as a tool for quantitative evaluation of results and further understanding of the respiratory patterns of RPDs.