With many air cleaning technologies on the market, a frequent question we hear is, what is a carbon filter and how does it work? First, let’s review…

What is carbon and charcoal? 

Carbon and charcoal filters are frequently used when it comes to air purification amenities. They are interchangeable and refer to leftovers of incompletely burning something (combustion). 

Imagine a piece of wood in a bonfire that has been incompletely burned. The black char on the wood is mostly carbon. Charcoal is made through an industrial process that frequently uses wood and heats it in a vacuum chamber resulting in carbon as an end byproduct.

What is activated carbon?

Activated carbon or charcoal has had additional processing applied to help it effectively trap gas molecules. The process includes injecting hot air, carbon dioxide, or steam into the carbon which creates a porous lattice within the carbon. This results in an exponential increase in the carbon’s surface area. By increasing the surface area, there is far more trapping space for molecules to get caught, thus making activated carbon an enhanced filter medium. In fact, a single gram of activated carbon has a surface area that exceeds 32,000 sq. ft (3,000 m2).

How do carbon filters trap gases?

Activated carbon traps gaseous molecules from the air via adsorption. This is different from absorption. A good example would be water being absorbed by a sponge, where the water fills the spaces inside of the sponge. Adsorption, on the other hand, is when a substance sticks to the outside surface of the structure. Remember that activated carbon has extra surface area, thus providing more exterior surface for gaseous contaminants to adhere.

Adsorption allows activated carbon filters to remove gases from the air. However, the problem with activated carbon is that, over time, there is limited space available for the gas contaminants to bind. Once the carbon filter is covered with gas molecules, it can no longer trap more. 

Interestingly, some gaseous chemicals have a much higher affinity for adsorption than others, and the ability for adsorption to occur for a specific gas is dependent on the ambient conditions (ex. room temperature) of its surroundings. The important takeaway is that if temperature or relative humidity changes, some chemicals can actually be released from the carbon filter because the adsorption is ineffective. When a carbon filter can no longer trap gases, an odor may occur, which is a good indication that it is time to change the filter. 

Although carbon filters can improve air purification, they require frequent replacement. If contaminants saturate the filter, it will no longer effectively clean the air, and you may not know when the filter reaches its saturation point. Although there is often a smell associated with the saturation of the filter, it does not always occur, and can be undetectable. Follow the manufacturer’s guidelines and replace the filter regularly. However, be aware that following the guidelines may not be accurate. Carbon saturation is based on the indoor environment and the amount of air contaminants the filter is exposed to. Costs of frequent carbon filter replacement can quickly add up.