Activated Carbon Air Filters: Everything You Need to Know

If you are looking for an activated carbon air filter to get rid of strong odors or harmful gases in your area, you have come to the right place to get all the details. Learn more about why people use carbon air purifiers, whether they work, and if you should use one to filter the air in your home.

Activated carbon has special properties that allow it to remove volatile organic compounds (VOCs), odors, and other gaseous pollutants from the air. It accomplishes this in a way that is different from other air purifiers like HEPA that only filter particle pollution from the air.  Carbon air filters trap gas molecules on a bed of charcoal, a process that has a surprisingly colorful history. Here you will dive deep into how they work. Then you can consider if using a carbon air filter will meet your needs by examining its advantages and drawbacks.

What are activated carbon air filters?

Carbon air filters are the filters most commonly used to remove gases. They are designed to filter gases through a bed of activated carbon (also called activated charcoal) and are usually used to combat volatile organic compounds (VOCs) released from 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 like mold, dust, or pollen from the air.

What are activated carbon air filters?

Humans have been using charcoal to purify water–sometimes accidentally–for thousands of years.

First, what is charcoal or carbon?

These interchangeable terms refer to the remnants of incomplete combustion. Picture the charred piece of wood that is left over after a campfire. Only the readily combustible material in the wood has burned away, either because of insufficient heat or poor supply of oxygen. The black char that remains is mostly carbon. The industrial process of making charcoal accomplishes it by heating a substance in a vacuum chamber, which releases all the volatile compounds and leaves behind all the carbon. Wood is commonly used to make charcoal, but coconut shells and coal are also used. Each substance creates a slightly different kind of charcoal.

How is carbon activated?

Activated carbon is carbon that has undergone some additional processing to make it better at trapping gas molecules. First, it is injected with hot air, carbon dioxide, or steam, which creates a lattice of tiny pores in the carbon, vastly increasing its surface area. This creates many more places for molecules to become trapped and makes the carbon far more effective as a filter medium. A paper by the Ohio Environmental Protection Agency notes that a single gram of activated carbon can have hundreds of square meters of internal surface area. Most activated carbon is also treated with a chemical that enhances its ability to filter specific pollutants.

The colorful history of using carbon filtration.

How did humans figure out that carbon can be effective at filtering contaminants? It is likely the earliest use was to remove impurities in smelted metal for the manufacture of bronze. The Egyptians appear to have been the first to use it in a medical way, to remove odors associated with infections. We know that sailors in the 16th through 18th centuries often stored their drinking water in barrels that had either been charred or smeared with charcoal on the inside to keep the water fresh on long voyages.

In World War I, gas masks utilized charcoal air filters to remove some of the deadly gases used against the troops, but it was only effective against some of the toxins. The production and use of activated carbon grew dramatically only after World War II, eventually leading to the development of modern activated carbon air filters, as well as water filters.

How do carbon air filters trap gaseous pollutants?

Activated carbon air filters remove pollutants from the air with a process known as 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 the absorbent (like a sponge), but it doesn’t become a part of the absorbent on a molecular level. Therefore, when you absorb water with a sponge, the water does not become chemically bonded to the sponge. It just fills in the spaces inside it.

Carbon filters on the other hand use ad-sorption, not ab-sorption. The key difference here is that during adsorption the pollutants stick to the outside of the carbon. Whereas with absorption, the pollutants are absorbed inside the structure itself–as with the sponge.

Carbon is a lattice of carbon atoms connected to each other. The activation process is so important because the increase in surface area gives gases a greater area to stick to. When a molecule of some gaseous substance comes through the carbon, it can stick to the surface of the bed, provided there is an open adsorption site.

The process of adsorption allows carbon air filters to filter organic chemicals (gases) from the air. The problem with the activated carbon bed is that over time, the gaseous pollutants increasingly fill up the adsorption sites of the activated carbon. Once the bed is saturated, the filter can no longer trap pollutants. In fact, chemicals with a greater affinity for an adsorption site can displace those with lesser affinity, and the affinity of a given chemical for the sorbent is highly dependent on ambient conditions such as temperature and relative humidity. So, as conditions change, different chemicals may be released from the filter. When a carbon air filter is saturated, you might notice it giving off a strange odor. This is a strong indicator that it’s time to change your carbon filter.

How to effectively use an activated carbon air filter

Carbon air filters can be an important part of your area air purification system, but they have to be used correctly.

• Make sure it uses enough carbon. Some filters claim to be activated carbon filters, but they use only a trace of carbon. These will not be effective because they become saturated almost immediately. A rough guideline is that a good activated carbon air filter uses at least five pounds of carbon–of course, not all carbon is created equal, and some may have more binding sites (and saturation capacity) than others. But, generally speaking, the more carbon used, the more effective it can be because there will be more sorption sites to stick to.
• Air flow is vital. To remove the most pollutants possible from the air, the air needs to spend the maximum possible amount of time passing through the carbon. In the air filter industry, this is known as “dwell time.” A filter with a good amount of carbon of sufficient thickness and high dwell time is going to be far more effective than a filter with a thin layer of carbon.
• Change your carbon filter. Once the carbon becomes saturated, it loses all effectiveness as an air filter. It is imperative that you change it out for a fresh carbon filter when this happens. The problem is that there is not a perfect method of knowing when carbon is saturated. Sometimes you will notice the smell; sometimes you will not. You can follow the manufacturer’s guidelines and change it on a regular basis, although this is just a rough estimate. The carbon becomes saturated depending on how heavily it is used and the amount of pollutants it is exposed to. Replacing carbon filters is a costly investment, but a monthly replacement schedule is a good starting point.

What is activated carbon good at?

Activated carbon air filters can be helpful in a few areas.

• Volatile Organic Compounds (VOCs). Carbon air filters can be effective at filtering VOCs from the air. These are gaseous substances that most other mechanical filters like HEPA filters, cannot touch. Some of the gases in cigarette smoke or those given off by drying paint or cleaning products can be removed from the air by a carbon filter. Benzene, toluene, xylene, and some chlorinated compounds are among those that may be removed by carbon filters.
• Odors. Mechanical air purifiers that can only filter particles cannot remove unpleasant odors at all. People therefore often use carbon filters to remove smells, though of course, they cannot remove all of them.

Drawbacks to using activated carbon filters

• Filter replacement. Replacing saturated carbon filters may become inconvenient and expensive. It can also be a hard to determine when your carbon filter needs replacing as there are no visible signs when it is fully saturated. You just have to guess or rely on the manufacturer’s replacement recommendations.
• Cannot remove particle pollution. Carbon air filters remove many organic compounds from the air, but as mentioned above, they cannot address particulate pollutants. These particles may include allergens like dust and pollen, or even those from secondhand or wildfire smoke–the latter of which can be especially dangerous in the near term.