filter 14 the signals supplied by the two transducers are sent to comparison means 19 which supply a differential signal sf which is a measure for the filtering efficiency the control means 18 comprise comparison means for example a comparator 20 followed by a control sub assembly 22 the comparator 20 compares said differential signal sf representing the filtering efficiency with a predetermined threshold value th in fact a good filtering efficiency becomes apparent from a substantial difference between the signals supplied by the upstream and downstream transducers inversely a low differential signal sf points to a low filtering efficiency in the case of an air cleaning device it is necessary to start this device only when said filtering efficiency is of practical value and is justified by an actual pollution of the air to be filtered on the other hand the operation of the air purification device should be stopped if the filter is not efficient either because it has to be changed or because there is no pollution to comply with these demands according to the invention the control sub assembly 22 stops the fan when the differential signal sf is lower than the threshold value th on the other hand the control sub assembly 22 causes the fan to keep running when the differential signal sf is higher than the threshold value th the sub assembly 22 achieves this control by means of a control signal com when the purification device is stopped by the control means the fan and the.