How Purex fume extractors filter gas  
Gas / Vapour Filtration
Purex filters capture contaminant gases from the process by using some or a mixture of the following. 

• Activated Carbon - Alkali or Acid Impregnated
• Alumina Permanganate

A gram of activated carbon can have a surface area in excess of 500 m², with 1500 m² being readily achievable. For comparison, a tennis court is about 260 m².
Under an electron microscope, the structure of activated carbon shows there are a great number of nooks and crannies, and many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously.
Alumina Permanganate is aluminium oxide impregnated with potassium permanganate, used as an oxidizing agent when mixed with carbon for MEK applications.