Activated Carbon Air Filter: How Does A Carbon Filter Work? (Adsorption)

A gram of activated carbon can have a surface area in excess of 5,400 sq ft, with 32,000 sq ft being readily achievable. (Wikipedia On Activated Charcoal)

Pretty much every air purifier has an activated carbon filter. What does it do and what does “activated” actually mean?

We’ll look into what an activated carbon filter actually is, how does an activated carbon filter work, how it’s made, and why it’s so frequently used for air purification. We’ll also look into which activated charcoal filters last the longest and which ones are the best.

Here the quick insight:

Air pollutants are found in two different states of matter:

• Solid particulates (dust, pollen, smoke particles, dust mites). HEPA filters are used to filter them out mechanically.
• Gaseous air pollutants (odors, smells, VOCs, cigarette smoke smell, kitchen oil smell, etc.). The job of removing this falls on the activated carbon air filter.

HEPA filter and activated carbon filter work in tandem; they are the 2 most important air purifier filters.

Odors, smells, and other volatile organic compounds just go through the HEPA filter. They are not stopped by a mechanical barrier.

Rather, they are stopped by a process of adsorption to an activated carbon material. During this process, the gas molecules (different undesired odors in our kitchen, bedroom, etc.) adhere to a solid surface.

An activated carbon filter is nothing more (or less) than a material with an enormous surface area that makes the adsorption of odors extremely effective.

In essence, just a single gram of activated carbon can have a surface area of up to 32,000 sq ft. That’s almost an entire football field just from 1 gram.

Let’s dig in a little deeper:

What Is Activated Carbon Filter?

Here is a leftover charcoal, a side product of burning wood:

An activated carbon filter is a tightly packed and “activated” charcoal. It can be in a powdered or granular block form.

There are various sources of charcoal, besides wood. For air purifier carbon filters, coconut husk, willow peat, coir, and bamboo are frequently used.

Here’s the true purpose of using activated charcoal to create filters:

Charcoal is, with the “activation” process, an extremely porous material. The small pores in the charcoal create an incredibly large active surface area where the process of adsorption of odors, smell, and VOCs can take place.

Example: Bigger air purifiers have 0.7 lbs activated carbon filters. 0.7 lbs = 317 grams. 317 g * 5,400 sq ft = 1,711,800 sq ft. That filter might be small but, due to the inclusion of the pores, it has an active surface area of almost 30 football fields at a minimum.

Alen BreatheSmart 75i – the best air purifier currently on the market – has an optional 3.7 lbs activated carbon filter.

Based on the form and the source of charcoal, as well as the “activation” process we’ll learn further on, we differentiate between several different activated carbon species. These include powdered activated carbon, granular activated carbon, bead activated carbon, extruded activated carbon, polymer-coated carbon, and even woven carbon.

Charcoal, by itself, is a porous material. However, it’s not nearly as porous as the carbon powder or granules that are found in activated carbon filters. The trick to achieving the extremely porous material is the “activation process”:

What Does “Activated” Mean? How The Activated Carbon Filters Are Made

1 gram of carbon has a surface area of less than 1000 sq ft.

1 gram of “activated” carbon has a surface area between 5,400 sq ft to 32,000 sq ft.

The only difference here is the activation process. There are two ways of how to activate the secret surface potential of carbon:

1. Physical activation via hot gases.
2. Chemical activation via acid-base neutralization reaction.

In most cases, physical activation is used. This involves creating carbon material and creating the pores in two steps:

1. Coconut husk, wood, or bamboo is burned (pyrolyzed) at very high temperatures (up to 1000°C) in absence of oxygen. This 1st step is called ‘carbonization’.
2. When the temperature of carbonized material falls below 300°C, it is exposed to high-temperature gas activation. This activation process creates tiny holes in the hot charcoal; making it extremely porous. This 2nd step is called ‘activation’.

Activated carbon filters used in air purifiers are mostly created via physical activation.

Chemical activation is more commonly used in industrial processes. It involves pore creation via the use of strong acid, a strong base, and certain salts. Essentially, the strong acid (with the help of other chemicals) ‘eats’ through the charcoal, thereby creating a porous material.

Now that we know what activated carbon is and how it’s made, let’s look at how it’s properties are utilized in activated carbon filters:

How Does Active Carbon Air Filter Work?

Indoor air is full of invisible gaseous air pollutants. These include kitchen smells, different odors, volatile organic chemicals, and so on. All of them are comprised of billions upon billions of gas molecules.

When these gas molecules hit the receptors in our noses, our brain will register that as ‘smell’. In most cases with air pollutants, it’s an unpleasant smell.

To remove odors, we only need to make sure these gas molecules don’t hit the receptors in our noses. They have to hit or be adsorbed to, something else.

Here’s how an activated carbon filter works:

Activated carbon filters present an extremely large surface area where undesirable odors can be adhered to.

An air purifier for odor is basically a fan that collects the air (with undesirable odor gas molecules in it) and brings it to the activated charcoal filter. There the odor molecules are adsorbed and eliminated from the indoor air circulation.

Activated Carbon Filters In Air Purifiers

You’ll rarely see an air purifier without an activated carbon filter. The reason for that is simple:

If the intention of an air purifier is to increase indoor air quality, that does include reducing the odors. Activated carbon, tightly packed in between two layers to create a filter, is the best natural tool to adsorb undesired odors.

Does that mean that all air purifiers have the same activated carbon filters?

Not all at. Not all activated filters are the same. As we have seen, the surface area per gram of activated carbon has quite a big interval: from 5,400 sq ft to 32,000 sq ft.

The surface area per gram of any particular activated carbon filter is usually not measured by air purifier producers per se.

Here are the 3 most common differences between different activated carbon filters:

1. Surface area per gram.
2. Total weight of the activated carbon filter (depends on dimensions, density).
3. “Enrichment”.

Heavier activated carbon filters will, by nature, have a larger capacity for adsorbing gaseous airborne pollutants. They will be more thorough at removing odors and they will last longer.

“Enrichment” refers to the addition of salts like magnesium dioxide and copper oxide to activated carbon filters. These enriched filters, employed by Blueair air purifiers, for example, are designed to remove the smallest gaseous air pollutants like ozone and carbon monoxide.

How Often Do You Need To Replace Activated Carbon Filters?

When a significant part of the activated carbon filter’s surface area has been done its job of adsorbing smells, it will become saturated.

Saturated activated carbon filters need to be replaced. Most common air purifiers will have an indicator of when you need to insert an activated carbon filter.

Depending on the device, usage, and the filter itself, activated carbon filters need to be replaced every 3-9 months.

Essentially, activated carbon filters are filters that need to be replaced very often.

If you don’t have a replacement filter on hand, you can run an air purifier without an activated carbon filter. The odors will remain in the indoor air but an air purifier with just a pre-filter and HEPA filter will work as intended to capture large and small particulates.