Combined filters ally the effect of the particulate filter and activated carbon filter. Besides dust and pollen this type of filter remove from the air also the odors and gaseous air pollutants such as nitrogen oxides, sulfur dioxide, hydrogen sulfide, ammonia, solvent components or ozone. The open pores of the activated carbon store these air pollutants or convert them completely. The inner surface of the activated carbon is very important to fix the gas molecules. One teaspoon of activated carbon has an internal surface area roughly the size of a football field.