The activated carbon is treated with oxygen, opening up millions of small pores in the carbon. This allows a huge surface area to attract and neutralize the offending odors, gases, and some chemicals or fumes. A chemical reaction occurs actually adsorbing the airborne material in question.
A One pound activated carbon filter can have a surface area of over 60 acres. The heavier the carbon filter, the more it will be able to adsorb and the longer it keeps on working. When it is full the filter will not adsorb any more and will need to be replaced. As with all filters, the air does need pass through the filter to be adsorbed.
"absorb" - think of a solid being dissolved in a liquid
"adsorb" - the physical attraction and adherence of a gas or liquid molecules to the surface of a solid or a little like absorb in reverse.
A special version carbon filter with impregnated carbon contains added chemical adsorbents that reduce additional chemical particles like VOCs (volatile organic compounds). This is usually an additional or a special add-on filter and would be listed in the specifications for the particular purifier.