The invention relates to a controlling system for controlling an air handling system that is arranged to handle air in an enclosure such as a cabin of a vehicle. The controlling system comprises a first sensing unit, a second sensing unit, and a controller. The first sensing unit is arranged to generate a first output signal based on a measurement of the 5 concentration of certain airborne pollutants at the upstream side of an air cleaning unit that is comprised in the air handling system. The second sensing unit is arranged to generate a second output signal based on a measurement of the concentration of certain airborne pollutants at the downstream side of the air cleaning unit. The first output signal and the second output signal are used for specific purposes in a decision-making process that can be 10 performed by the controller. The controller is arranged to switch the air handling system into recirculat ion mode when the first output signal exceeds a predetermined threshold value. Furthermore, the controller is arranged to fine-tune settings of the air handling system, such as the mixing ratio of air admitted from outside the enclosure and recirculating air from the enclosure in the total air flow passing through the air handling system when the air handling 15 system operates in an outside-air-inlet mode. The controlling system according to the invent ion has an increased functionality and can maintain an acceptable air quality inside an enclosed space under a large variety of circumstances.