The vehicle's AC system should not be operated in recirculation mode for extended periods of time due to build up of CO2 inside the vehicle cabin. This is the CO2 that is exhaled by the occupants of the vehicle. This CO2 is then inhaled by the occupants that goes into their blood streams which results in a negative impact on health. This becomes critical when a number of people are sitting inside the vehicle. Field tests were conducted on a MY 2003 vehicle in recirculation mode to monitor the build-up of the CO2 concentration inside the cabin as a function of number of occupants, vehicle speed and ambient temperatures. The vehicle was driven in Detroit Metro area in city and highway traffic conditions. Based on this investigation it is determined that the cabin concentration levels reaches ASHRAE (Standard 62-1999) specified magnitudes in first 5 minutes of driving with only one occupant in the vehicle. Hence, operation of the vehicle beyond 5 minutes will result in much higher levels of cabin carbon dioxide concentration then specified by ASHRAE standard. This situation would be much severe if a number of people are sitting inside the vehicle and are going on a long drive. Additional tests were conducted by imposing ASHRAE standard 62 limits (700 ppm over the back ground limit on a continuous basis) that resulted in cycling of blower unit's mode door between recirculation and OSA and vice versa. Details of the investigation are presented in this paper.