What Do Activated Carbon Filters Remove From Air?

Maybe you’ve recently heard about activated carbon filters and wondered just how effective they are.

But just what do activated carbon filters remove from indoor air? Activated carbon filters found in air purifiers or air conditioning air filters are best suited to trap particles less than 0.3 micrometers in size, such as tobacco smoke, cooking oil fumes, bacteria, viruses, and volatile organic compounds (VOCs). Activated carbon filters are able to achieve this by the process of adsorption, thanks to their high surface area.

So, let’s review just what you’ll need to look for to get the activated carbon filter that does really all those things and will actually deliver on its promise.

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How Activated Carbon Air Filters Work

Activated carbon has a porous structure that allows it to have a disproportionately large surface area compared to its weight. All this surface area makes it ideal to trap small pollutants within its pores, a method known as adsorption. When these pores are open (meaning, unoccupied and ready to adsorb pollutants), the carbon is called “activated carbon”.

It’s as simple as that.

Now let’s talk about what’s more important – WHAT activated carbon filters actually remove.

What Pollutants Can Carbon Air Filters Remove?

Activated carbon filters are best for very small particles, usually below 0.3 micrometers (microns) in size. So, what would those small particles be?

Tobacco Smoke

General size: 0.1 to 1 microns (although can also contain smaller pollutants)

An activated carbon filter can remove smaller tobacco smoke particles that other filters can’t. Tobacco smoke is made up of over 7,000 chemicals, so it’s important to remove as many of them as possible to remove the tobacco odor as well.

It’s also the smaller particles that are more likely to get lodged in the lungs and cause respiratory problems. Those can stay in the alveolar cells of the lungs for years as the lungs have no efficient way of removing them.

Many of these small compounds belong to pollutants known as volatile organic compounds (VOCs)…

VOC (Volatile Organic Compounds)

General size: picometers (for comparison, a picometer is 10^–⁶ of a micron)

These are molecular level pollutants and can only realistically be removed with an activated carbon filter, at least when it comes to what’s out there in the consumer market. Volatile organic compounds are gases that are so small, they can stay suspended in the air.

It’s usually the VOCs inside a home that are responsible for lingering chemical smells and odors.

You already know that some of them are found in tobacco smoke. So by extension, VOCs can also be found in other smoke pollutants, such as those that are released from burning a wood or coal in a fireplace, or using a kerosene heater and gas appliances like stoves and laundry dryers.

But beyond that, VOCs also include dangerous chemicals, many of which are considered to be carcinogens:

Formaldehyde – found in “off-gasing” mattresses, furniture, household cleaners, laminate floors and cabinets. Think new home construction building materials and new flooring when you hear this.
Benzene – typically is found in paint thinners, glues, and cleaning products. It has a sweet odor. Sadly, also found in tobacco smoke.
Napthalene – found in moth balls mostly.
Chloroform – a solvent found in chlorinated products (like laundry bleach) or water.
Acetaldehyde – found where there is smoke; again, tobacco products and wood-burning fireplaces and appliances, to name a few. Also given off in the kitchen while cooking oil is burned.
Trichloroethylene – found in paint, varnishes, spot removers, and carpet cleaners.
Styrene – found in plastic packaging and, you guessed it, tobacco smoke.

Unfortunately, those examples are only the tip of the iceberg when it comes to VOCs in a home. I suggest reading this post if you have a new construction home and want to learn more about VOCs.

Bacteria

General size: 0.2 to 10 microns

Bacteria typically are large enough to be removed by other filters, but an activated carbon filter will help close the gap and take care of virtually all kinds of bacteria.

Viruses

General size: 0.004 to 0.1 microns

Viruses are much smaller than bacteria, so a high quality activated carbon filter will probably trap most of them. Emphasis on high quality – we’re talking about slightly more expensive, if not commercial grade here to truly make a difference.

Having this kind of filter also doesn’t mean that the danger the virus poses is gone. You’ll need some other technology like UV light for that. Some air purifiers also use ionizers and other technologies to kill viruses. However, just how safe some of them are is up for debate and not the topic of this post.

Combining Activated Carbon Filters with Other Filters

The most popular combination of filters in air purifiers nowadays is an activated carbon and HEPA (high efficiency particulate air) filter.

These two filters together create a 2-stage filter mechanism. The HEPA filter takes care of pollutants over 0.3 microns in size, while the activated carbon takes care of pollutants smaller than that.

Of course, it’s not as simple as that thanks to just plain Brownian motion, but you get the idea.

The 0.3 micron pollutant size is a blind spot. It’s a gray area between an activated carbon and HEPA filter since neither really works as efficiently in that size range. It’s probably easier for the activated carbon filter to remove smaller particles than those that are 0.3 microns and above.

Bottom line – the two filters together aren’t perfect but get close enough to perfection.

How to Avoid the Wrong Kind of Activated Carbon Filter in Air Purifiers

Activated carbon looks like a great feature to market on an air purifier, so you have to avoid any hype and be careful when choosing the right one.

There are a few types of filters you’ll be able to choose, but the general rule is that heavier filters are best.

How “Activated” Is the Filter?

The more open and available pores that there are in a filter, the better it is at trapping pollutants. This is considered an activated carbon filter versus just a carbon filter. Activation is achieved by exposing the carbon source like coal, bamboo, or other to extremely high heat and then injecting it with steam to make it as porous as possible.

The activated carbon is then assessed for its pore size and surface area to determine just how potent it is.

Spray-On Vs. Granular Filters

When it comes to most air purifiers, you’ll usually have a choice of a spray-on or granular carbon filter. The latter would be more expensive and rightfully so.

There’s nothing wrong with the bonded spray-on kind, and some can be quite good quality. But many of them are too thin to really make a difference. You want one that has loose granules of carbon inside. These filters are more substantial and are great at removing VOCs more quickly.

What’s In Front Matters

Just because you have a high quality, heavy activated carbon filter doesn’t mean that you’re also getting a high quality air purifier.

What filters are in front of the carbon filter to trap the larger particles? Look for an actual True HEPA filter and avoid terms like HEPA-like, HEPA-type, near-HEPA or similar copycat word trickery.

What an Activated Carbon Filter Won’t Do

An activated carbon filter isn’t an end-all be all- solution. It has its limits. It is a gas phase filter after all.

Here are some things to be mindful of:

Carbon filters must stay “activated” to work. In other words, change them often so the pores aren’t clogged.
Activated carbon filters require moderate temperatures and low humidity (unless you’re looking for your air purifier to act as a dehumidifier instead of removing dangerous pollutants like it’s supposed to). If your air conditioning system isn’t able to keep the air in your home cool and dry in the summer, an activated carbon filter just won’t work for you.
Just because a pollutant is small in size doesn’t mean that an activated carbon filter will remove it effectively. Keep that carbon monoxide monitor in your home – an activated carbon filter you found in your local hardware store just won’t work as a replacement for your carbon monoxide monitors. Other pollutants that won’t work as well are those with very low or high pH, as well as some alcohols and dissolved inorganic materials (according to Wikipedia). This is one of the reasons why activated carbon is used to filter drinking water – it traps small pollutants but keeps healthy minerals in the water.
For relatively larger particles, it’s always more efficient to use a pre-filter or another filter like a HEPA filter first and foremost. It’s just more cost-efficient.

Air Quality