mode fig3 schematically shows a flow chart for the sequence of an exemplary method according to the present invention an air conditioning system is assumed here for the sake of simplicity however the exemplary method is suited without limitation also for a vehicle without an air conditioning system which is only equipped with a ventilation system block 300 the recirculating flap is closed i e the air conditioning system or the ventilation system is operating in fresh air mode block 301 the recirculating flap is open i e the air conditioning system or the ventilation system is operating in recirculating mode switchover queries take place in blocks 302 and 303 if the conditions queried there are met the recirculating flap is switched over to the other state for example the carbon dioxide concentration in the air flowing through the mixing box i e the circulating air when the recirculating flap is open and the fresh air when the recirculating flap is closed may be queried there 1 assume that the air conditioning system is in recirculating mode 300 2 a query 303 is made regularly to see whether the carbon dioxide concentration in the circulating air i e the air in the interior of the vehicle has exceeded a predefined limiting value 3 as soon as this limiting value is exceeded the recirculating flap is switched over to fresh.