Passengers are exposed to roadway pollutants due to entrainment of outside air into the vehicle cabin. Previous works found cabin air-recirculation can reduce pollutant particle concentrations significantly. However simultaneous increase of CO₂ concentrations in the cabin prevented wide use of recirculation mode for such purpose. A mathematical model was developed to predict CO₂ concentrations in vehicle cabin air during air-recirculation mode. The model predicts temporal CO₂ concentration changes as a function of cabin volume, vehicle body leakage, and number of passengers. This model can be used to design and control air-recirculation mode for a variety of vehicle conditions.