Buildings are aired to evacuate indoor pollutants, in particular those produced by the occupants. CO2 is a good indicator of occupancy, as long as there is no other significant source. This is the case in most buildings. Cheap, portable analysers and loggers that allow easy recording of the CO2 concentration in a room or in the exhaust duct of a ventilation system are available on the market. The peak value of CO2 concentration during occupancy is an indicator of the minimum air flow rate per person. Analysis of the decays observed when the occupants leave the building provides the nominal time constant of the ventilated space, which is directly dependent on the outdoor air flow rate from the ventilation system and infiltration. Depending on the state of the ventilation system during the decay, this method provides either the total outdoor air flow rate provided by the system, or the infiltration rate. When combined with a simple pressure differential measurement, this method can also be used to check the air tightness of building envelopes. A user-friendly computer program helps with the interpretation of the records. This paper describes the method, its application to several rooms and buildings, and its validation by comparison with SF6 tracer gas measurements.