HEPA filters do one thing very well: they capture and trap solid airborne particles.

But they cannot capture gases. A different “filter” is necessary to capture gases. The most common? Carbon. But as you’ll see in just a moment, carbon cannot “filter” all gases. Instead, different filters are necessary to filter different gases.

Methods of Evaluation

In order to evaluate solid particle filtration (with a HEPA filter) we look at the SIZE of the particle. This allows us to evaluate:

Solid airborne particles can be very large (>100 μm) or very small (0.001 μm). As we showed above, studying the SIZE of such particles can be very helpful when it comes to evaluating their filtration.

Gas particles – molecules - are always small (<0.001 μm). Looking at the SIZE of a gas particle (molecule) is essentially meaningless when evaluating the filtration of such a particle.

In order to evaluate gas filtration (with a carbon filter, for example) we need to look at the characteristics of the molecule.

Namely, its

These characteristics determine how well certain materials/mediums filter certain gases.

Generally, the higher the MW and BP and lower the VP of the gas, the better activated carbon can bond to it – and thereby filter it out of the air.

Conversely, the lower the MW and BP and higher the VP of the gas, the worse activated carbon can filter it.

Note: as we’ll show later, the most important of these characteristics is molecular weight. Almost all gases that can be filtered by carbon have a high MW and almost all gases that cannot be filtered with carbon have an especially low MW.

What about these low MP/BP and high VP gases? How do you filter them?

A different medium is needed. Below are some popular options:

These mediums can exist independently but more often than not you’ll find carbon “treated” or “impregnated” with them – eg. activated carbon impregnated with potassium permanganate.

A Closer Look

Your goal is to filter gases. But clearly you don’t want to filter all gases. Air is a gas and obviously you don’t want to remove air from itself – that wouldn’t even make any sense.

You only want to filter unwanted gases. The two most common types of unwanted gases are

• Harmful VOCs
• Odors

“VOC” stands for Volatile Organic Compound. Let’s break down this definition

For our purposes, the only real difference between a VOC and an odor is the fact that a VOC will always contain carbon. An odor may or may not contain carbon. Both types of gases have similar chemical compositions and are a similar size (on the molecular scale).

So, let’s take a look at the most common harmful VOCs (in the home) and the most common odors and see how they compare in terms of CHARACTERISTICS. Remember, these characteristics – MW, BP, and VP but especially MW – will determine whether you can filter them using a carbon filter or not.

Note the low MW of all the chemicals that cannot be filtered by carbon.

*note – VP is not included in the tables to keep things more simple

The tables above show that carbon filters out the vast majority of harmful VOCs and odors. These are the notable exceptions:

Harmful VOCs

• Formaldehyde
• Ethanol

Odors

• Acetaldehyde
• Trimethylamine
• Ammonia (lighter than air)

The only real concern on this list is formaldehyde. Gaseous ethanol is very rarely found in sufficient quantities in the home to be a concern. The odors are just that – odors – unpleasant to smell but rarely if ever in high enough concentrations in the home to be a health concern. Ammonia will also naturally want to ventilate up and out of your home because its lighter than air – ammonia has a MW of 17 g/mol  while air has a MW of about 29 g/mol.

That leaves us with formaldehyde. If your home has abnormally high levels of formaldehyde a carbon filter will not remove it from the air. An air purifier equipped with a gas filter that’s impregnated with zeolite is recommended – the Austin HealthMate is one of the better options on the market. For almost all other gases, a carbon filter will work just fine.

Model Recommendations for Filtering Most Gases

CFM

Any air purifier with a carbon filter will remove most odors and most VOCs from the air. However, certain units are simply too small – have too low a CFM and too small a filter - to really have any type of sizable impact on gas concentrations in even the smallest room in the smallest home.

After extensive research and testing, we found that units in the 250 CFM range offer the best combination of value, low noise output, good energy efficiency, and proper particle and gas filtration.

For rooms up to 300 sq. ft. we recommend one top rated 250 CFM unit. For larger rooms we recommend multiple top rated 250 CFM units.

Pellets

Other than CFM, another very important feature you want to look out for if gas filtration is a priority for you is what type of carbon filter the air purifier you’re planning on buying is using.

Two types of carbon filters dominate the market:

If gas filtration is important to you then you definitely want to opt for an air purifier equipped with the second type of filter described above.

Of the two specific models we recommend for most applications, the Coway Mighty air purifier comes equipped with a thin replaceable filter and the Winix 5500-2 comes equipped with a thicker washable pellet based filter. For users concerned about gas filtration we strongly recommend the Winix.