The fan permanently records the odour present (VOC) and forms a moving average. This forms the background reference. Only an increase above the reference value by at least 20% leads to activation of the demand ventilation stage with activated VOC control. This prevents, for example, a permanently present room odour from leading to permanent ventilation. Odour peaks are effectively detected and removed. 5 minutes after detection and elimination of an odour peak, the fan is automatically lowered again to the basic ventilation level.