Activated carbon adsorption is the oldest and widest recognized form of molecular filtration. Its ability to capture a broad range of organic chemicals makes it the largest applied technology in North America. Indoor air quality is the largest application of activated carbon filtration. The surface area of activated carbon allows for significant loading of gas molecules; it also acts as a catalyst for the destruction or change of some gas molecules.

Typically manufactured from coconut shells, wood or bituminous coal, an additional step in processing creates an extensive internal pore structure of the carbon. The pore structure is classified in three segments:

• macropore -captures very large molecules such as fats or oils

• mesopore - captures most volatile organic compounds (VOC's)

typical in IAQ problems

• micropore - captures small, lower molecular weight contaminants.

The adsorption capacity of activated carbon averages around 25 percent by weight of the carbon for typical HVAC organic removal applications. The filters are typically formatted in some form of V configuration to maximize surface area for lower pressure drop. The V trays are usually between 5/8 inch and two inches thick in HVAC systems, although new disposable filters are 12 inches deep.

Activated carbon media is totally recyclable through reactivation, a thermal process by which the captured pollutants are driven from the carbon and destroyed, leaving a cleaned activated carbon for reuse. Some recent innovations for this filtration process are the use of smaller meshed activated carbons which are either enmeshed or layered with nonwoven filters, or as a powder adhered to nonwoven fiber. About 35 percent of activated carbon sales are from water treatment use.