Cabin air filters separate noxious gases, e. g., nitrogen dioxide, from ambient air onto activated carbon by adsorption. The efficiency of the filters is tested comparatively according to the German standard DIN 71460 part 2. The breakthrough of the noxious gas NO2 is measured. Nitrogen dioxide is either supplied directly from a canister or indirectly by oxidation of NO with air or ozone. The influence of dosing on the break-through is examined.