Activated Charcoal is simply carbon atoms. It is made by burning wood or materials such as coconut shells. Special manufacturing techniques are used to burn off water, minerals (ash), and all other substances, leaving behind nothing but carbon atoms. Activated Charcoal is a highly porous, fine, black powder with the surface area of 300-2,000 square meters per gram, giving it a significant adsorptive capacity which allows the carbon atoms to bind with unwanted substances and toxins. It is odorless, tasteless an non-toxic.

The word adsorb is important. When a material adsorbs something, it attaches to it by chemical bonds. The huge surface area of activated charcoal gives it countless bonding sites. As "active" chemicals pass by the carbon, they attach to the surface and are trapped. This mechanism of action should not be confused with absorption. Absorption occurs when a substance passes into or through a tissue, like water passing into a sponge.

Activated charcoal is good at trapping carbon-based impurities ("organic" chemicals), as well as "reactive" substances such as chlorine and ammonia. Many other chemicals are not attracted to carbon at all -- sodium, nitrates, etc. -- so they pass right through. This means that an activated charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, activated charcoal stops working. At that point it must be replaced.