The quality of the external air is determined by means of a pollutant sensor. The quality of the internal air is determined by calculation taking account of the air quantities introduced from outside into the internal space. A decision between air supply operation and air recirculation operation is then made on the basis of a comparison of the air qualities inside and outside. The pollutant sensor is preferably located in a casing whose internal space is accessible to gases through an opening which is preferably sealed by a gas-permeable membrane.