Why air cleaning devices are incapable of removing gas pollutants/ odour?

Case Study - Activated Carbon

Sometimes, relatively small quantities of activated carbon will reduce odors in a residence to imperceptible levels. However, because many chemicals produce health effects at levels below those where odors are perceived, removal of odors alone is not an indicator of a healthful environment.

Tests of gaseous pollutant removal by activated carbon have generally been performed using only high concentrations of pollutants, so little information is available on the effectiveness of carbon in removing chemicals present at the low (part per billion, or ppb) concentrations normally found in indoor air.

The ability of carbon to reemit pollutants it has trapped from indoor air is also of concern. The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), is currently developing a standard method to be used in evaluating the effectiveness of media used for gaseous pollutant removal. They have reported the results of a study using activated carbon, in which the concentration of toluene in the air flowing into the carbon was varied during the test (from 150 to 0 to 340 to 26 to 0 ppm). The experiment simulates the variations in pollutant levels which would be expected in indoor air situations. They found that toluene initially absorbed by the media was slowly reemitted each time the pollutant level entering the media dropped. The amount of toluene emitted by the media during the 45-hour experiment was approximately equal to that adsorbed.