Carbon air filters are the most common filters used to remove gases. They are designed to filter gases through a bed of activated carbon (also known as activated charcoal), and are often used to combat volatile organic compounds (VOCs) released by common household products. They are also often used to remove odors from the air, such as the smell of tobacco smoke. They cannot remove fine particles from the air, such as mold, dust, or pollen.
Activated carbon air filters remove pollutants from the air through a process called adsorption. Note that this is different from absorption. In absorption, the substance you want to remove (let's say water) is absorbed into the structure of an absorbent (like a sponge), but it does not become part of the absorbent at a molecular level. So when you sponge water, the water does not chemically bond to the sponge. It simply fills up the spaces inside the sponge.
Carbon filters, on the other hand, work by adsorption, not absorption. The main difference between the two is that in adsorption, the pollutants stick to the outside of the carbon. In absorption, the pollutants are absorbed into the structure, just like a sponge.
Carbon is a lattice of carbon atoms connected to each other. The activation process is important because the increased surface area gives gases more area to attach to. When a molecule of a gaseous substance passes through the carbon, it can attach to the surface of the bed as long as there is an open adsorption site.
The adsorption process is what allows carbon air filters to filter organic chemicals (gases) from the air. The problem with activated carbon beds is that over time, gaseous pollutants gradually fill up the activated carbon's adsorption sites. Once the bed is saturated, the filter can no longer capture pollutants. In fact, chemicals with a greater affinity for adsorption sites can displace chemicals with a lesser affinity, and the affinity of a particular chemical for an adsorbent depends greatly on environmental conditions such as temperature and relative humidity. Therefore, as conditions change, the filter may release different chemicals. When a carbon air filter is saturated, you may notice that it emits a strange odor. This is a strong sign that it is time to replace your carbon filter.
