Filter media with a very high surface area, like activated carbon, can filter molecules from the air through a process called adsorption. For adsorption to work, the contaminated air must come in contact with the surface of the media. The mechanism which causes the contact is diffusion. The graphic to the right shows how a molecule diffuses into a grain of carbon media.
Physisorption is a reversible process wherein a molecule comes in contact with the surface of the media and adhears through mechanical forces. Energy input, such as heat, can cause the molecules to detach from the media surface. I this way the media can be regenerated and reused.
In contrast, Chemisorption is an irreversible process. Chemisorption is caused by a chemical reaction between the surface of the media and the molecule in the air when it contacts the media surface. To enable the chemical reaction, specific surface treatments are used depending on the type of molecule to be filtered.