Reporting Carbon Dioxide levels in a space

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Does anyone know how to report out CO2 levels in a space in EnergyPlus, I am sure there is a way but I can't find the output variable for it. It is a simple space with only natural ventilation and no mechanical conditioning. The space will have 300 occupants and the client wants to check CO2 levels in the space at various times. Is it enough just to specify the people or do I also need to use the "ZoneAirContaminantBalance" object to report out the CO2 levels?