The method involves determining actual number of passengers and indoor air quality in vehicles (1) by person counting sensors (2) and indoor air quality detection sensors (6, 7), respectively. The determined data are transmitted to controllers (4, 5) of an air conditioning system. A factor is formed from the data of actual number of passengers, and is maintained additional to existing parameters in a control process for determination of necessary fresh air amount. The data about indoor air quality are utilized in defective person detection devices for monitoring purpose.
