Carbon Air Filters for VOC and Odor Filtration in Air Purifiers

Baseline Analysis

An introduction to carbon/charcoal filters in air purifiers.

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.

In-Depth Analysis

A more detailed analysis of gas filtration in air purifiers.

Not So Fast

So far, we’ve made buying an air purifier for gas (mostly VOC) filtration look fairly easy: Just buy a top rated 250 CFM unit with a pellet based carbon filter and you’re good to go. Or are you?

The short answer is yes. Something like the Winix 5500-2 will remove many if not most unwanted gases from the air.

The longer answer is yes, but maybe not to the extent that you may think. The fact is that gas filtration is much more difficult to accomplish than particle filtration. So much so that often, the best advice is to simply get rid of the gas (remove the source, ventilate) instead of trying to filter it.

So, what exactly goes on when you try to filter a gas? How much is an air purifier really helping the process of removing harmful VOCs from your home? What can you really expect, in terms harmful gas removal, when you buy an air purifier for this exact purpose? We try to answer these questions and many more in the paragraphs that follow.

Four Warnings Before You Buy An Air Purifier For VOCs

Before you buy an air purifier to remove VOCs you first need to determine if the air you want to purify contains VOCs and which VOCs it contains.

Here’s the problem. It’s not only difficult to determine the presence of VOCs but it’s also difficult to test or predict the removal of VOCs. Let’s take a closer look at those two difficulties now.

1. Difficult to Determine the Presence of VOCs

To determine the presence of airborne particles in your home things couldn’t get easier. A high quality particle meter is relatively inexpensive (relative to the price of VOC meters at least) and can give you complete breakdown of particle counts in different areas of your home.

To determine the presence of VOCs in your home is much more difficult. First of all, an industrial grade PID (photoionization detector) is required if you want to take accurate readings yourself. Such a device can easily cost in the thousands of dollars. A good example of such a PID is the Rae Systems miniRAE 3000. It retails for well over $3,000.

Not only is such a device extremely expensive, but it also requires extensive knowledge and experience to use properly. Furthermore, it requires calibration per individual chemical tested – and this requires that you know which chemical you want to test for.

There are three less expensive alternatives:

Inexpensive air quality testers are mostly inaccurate and imprecise. Many don’t even list the VOC range or accuracy (resolution up to how many ppm, ppb etc.). Many only test for TVOC – total VOC concentration. This measure isn’t particularly helpful for many reasons including

Inexpensive testers may be able to measure formaldehyde levels (HCHO) and while this is a VOC many homeowners are concerned about, it’s still only one VOC. There are many other VOCs that can be problematic for your health.

The second less expensive alternative is a home air quality test kit. These kits are delivered to your home. You take a sample of air using the kit. You then return the kit and receive a report on the chemicals in the sampled air. These test kits are helpful but have a few shortcomings:

Remember, you need to know the specific chemical and the exact concentration to make an accurate assessment regarding your home’s air quality and more importantly, to decide if a carbon filter will be sufficient or if additional filter media are required.

The third and final inexpensive alternative for determining the presence of VOCs in your home is to use your own knowledge and intellect to determine the likelihood of certain chemicals being present in gas form in your home. We’ll show you how to go through this process next.

Throughout this process you’re going to need to use the chemical’s characteristics, especially its BP and MW to determine its behavior in your home.

Boiling Point

How do certain chemicals become airborne in your home? One way to answer this question is by evaluating a chemical’s boiling point.

All chemicals can be put into one of two categories:

• low BP
• high BP

High BP Chemicals

Heat

Things get a little more complicated with high boiling point chemicals. Benzene is a good example with a boiling point well above room temperature - 176.2°F (80.1°C). At room temperature, benzene is in fact a liquid.

How does benzene become a gas that you can breathe in inside your home? The answer is heat.

The tip of a cigarette or the inside of a car engine – one of two common places where benzene can form – is a very hot place. Thus benzene produced from such sources can easily become gaseous – at least until it cools off.

Evaporation

High temperature sources explain why high BP chemicals like benzene can exist as gases in your home but what about high BP chemicals like odors? The answer is evaporation.

Quite simply, boiling is not required for evaporation.

Boiling point has to do with a liquid as a whole – what is the temperature at which a certain quantity of liquid will completely vaporize?

Evaporation has to do with the surface of the liquid and can occur at any temperature.

In other words, a lot of energy (high temperature) is required to vaporize a liquid completely. Not nearly as much energy (any temperature) is required to vaporize surface molecules.

The evaporation rate does increase with higher temperatures but higher temperatures are not a requirement for evaporation.

Thus, wet clothes can be hung outside to dry. The boiling point of water is 212°F (100°C) yet the water in the clothes evaporates over time at much lower temperatures.

Similarly, you can easily smell isopentyl acetate – the “odor” given off by bananas – at room temperature even though its BP is 285.8°F (141°C).

Molecular Weight

You can also use a chemical’s molecular weight to determine its behavior in the air inside your home.

Consider for a moment, that air itself has a MW of approximately 29 g/mol. Any chemical with a lower MW will naturally be lighter than air.

Note that such chemicals are rare. Neon (20.1797 g/mol), hydrogen (1.00794 g/mol), and helium (4.002602 g/mol) are good examples, but they rarely exist in a typical indoor environment.

Ammonia (17 g/mol) is perhaps the most notable chemical that’s lighter than air.

But much more relevant to this discussion is the fact that most chemicals – and therefore most indoor gaseous pollutants - are heavier than air.

This means that most of the gases we discussed earlier – even something as light as formaldehyde (30.031 g/mol) will naturally sink in air.

How does this information help you? Take, for example, somebody smoking in a bathroom with the window open.

In this scenario we need to account for

Air moves according to pressure differences – specifically, from areas of high pressure to low pressure. If the air outside the window is at a lower pressure than the air inside then the air will move towards the outside. Conversely, if the air outside the window is at a higher pressure the air will move towards the inside of the house.

If there’s a gust of wind towards the window this creates an area of high pressure right beside the window relative to the air inside the house. In this scenario air will move into the house. Conversely, if there’s a gust of wind away from the window, this will create an area of low pressure beside the house which will allow air to move from inside the house (high pressure) to outside the house (low pressure).

This principal is also why fans help with ventilation. The fan creates an area of low pressure around it by moving air away from it. This allows air from outside the house to move into the house – from high pressure outside to the lower pressure inside generated by the fan.

Note that pressure is directly related to temperature. Differences in temperature cause differences in pressure. If it’s colder outside the house than inside the house the air will tend to move from inside the house (higher temperature = higher pressure) to outside the house (lower temperature = lower pressure). Conversely, if it’s warmer outside than inside, the air will move towards the inside of the house.

If there’s no pressure difference the air inside the house will slowly diffuse outside while the air outside will slowly diffuse into the house.

That accounts for the air – it will move according to pressure differences.

The same is true for the gaseous components of the tobacco smoke. Those gases will also move in the direction of low pressure. But there’s another factor to consider with these components. Namely, whether they’re heavier or lighter than the air. If they’re heavier than the air they will naturally want to sink back inside the house. If they’re lighter than air they will naturally want to rise up and out the window.

As we showed up, most gaseous chemicals – and this includes gaseous chemicals in tobacco smoke – are in fact heavier than air. That is to say they have a molecular weight greater than 29 mol/g.

Thus, not accounting for pressure differences, most of the gaseous components of tobacco smoke will eventually sink back down into the house.

Finally, we have to consider the solid and liquid components of the tobacco smoke. These particles are much larger and heavier than gas molecules. Regardless, they are still airborne and therefore will move according to the direction of air.

Of course, if the air is stagnant – if there’s no movement of air – no pressure differences – then the particles will fall to the ground. Smoke particles are about 0.08 to 0.2 microns in diameter which is very small for an airborne particle. These particles could take several days or weeks to fall to the ground but they will eventually do so.

2. Difficult to Test or Predict the Removal of VOCs in General and Specific VOCs

So far, we’ve shown that analyzing an air purifier’s ability to remove unwanted gases from the air is difficult to do because it’s difficult to determine the presence of VOCs in the first place. Next, we’ll show how it’s also  difficult to test or predict the removal of VOCs.

Particle filtration is an exact science.

VOC filtration is much more of an inexact science. So much so that even scientific studies conducted on their removal (via carbon filters and the like) have several obstacles to overcome. We consider those below.

The bottom line

Consider this – if these are difficult obstacles to overcome for scientists working in controlled environments with the best measuring equipment available, how much more difficult would it be for you to test the removal of VOCs in your home? The point is that it’s virtually impossible.

Consider if you really need to remove VOCs

At this point, we’ve shown that it’s not only difficult to determine the presence of VOCs but it’s also difficult to test for their removal.

But, what if you don’t really need to worry about VOC removal to begin with? The last two warnings we cover next explain this idea further.

3. VOCS May Not Be As Bad As You Think They Are

First of all, consider the fact that not all VOCs are bad for you. VOCs can be

• human made or
• naturally occurring

The fact is that most VOCs are naturally occurring on the earth and don’t cause any harm. Biological sources – including plants, animals and microbes - emit about 1150 teragrams of VOCs every year. (VOC wiki) These VOCs are mostly helpful – they help plants communicate with each other or with animals, for example.

Man-made sources – paints, coatings, fossil fuels, etc. – account for only 12% as many VOCs as biological sources - 142 teragrams total per year. Under the right circumstances, even these VOCs are not necessarily harmful to your health.

For example, consider the presence of formaldehyde in outdoor air.

The CDC tells us that formaldehyde occurs in these concentrations:

• 0.0002-0.006 parts per million (ppm) in rural and suburban outdoor air
• 0.0015-0.047 ppm in urban outdoor air

Formaldehyde is only potentially hazardous to your health at concentrations higher than 0.1 ppm. At low outdoor concentrations it has not been shown to be harmful.

Indeed, at lower concentrations, inhaled formaldehyde is “quickly broken down in the cells lining your respiratory tract and breathed out”. Only at high concentration levels can it enter your blood stream.

Thus, the primary concern here – as it relates to human health – is abnormally high indoor concentrations of certain VOCs – not all VOCs and not all VOCs at all concentrations.

The bottom line - not all VOCs are bad and VOCs aren’t bad all the time.

4. VOC Air Purification Is Not A Priority In Hospitals

You may be fearful of VOCs. You feel like you need to remove them at all costs. And you may think that you need an air purifier to do that.

But consider for a moment that VOC removal from air is not a priority in hospitals. If it’s not a priority for a hospital – a place that houses the most sick – the most susceptible to VOC – in our population – then why should you be concerned about it for your home?

Hospitals, for the most part, use regular air filters (with a lower MERV rating) and HEPA filters for air purification. Are hospitals tested for the presence of VOCs? Do the individuals involved in hospital construction take into account for VOC off gassing? Do the companies and individuals making and bringing in different tools and materials into a hospital take into account VOC off gassing? Yes, absolutely.

But there is little to no effort in any hospital to remove VOCs with air purification. Why? Because in almost all scenarios VOCs can effectively be reduced in two ways

• removing the source
• ventilating

More on these in a minute…

Our Final Recommendations

We showed above that
1. It’s difficult to determine the presence of VOCs
2. It’s difficult to test for or predict the removal of VOCs

These first two reasons should make it clear that VOC removal is a very complex problem to solve. So much so that it’s almost unreasonable to say that you can have any certainty at all that an air purifier equipped with a carbon filter can solve such a problem. And that’s if it’s even a problem to begin with:

We also showed
3. That not all VOCs are bad and VOCs aren’t bad all the time
4. And that VOC air purification isn’t a priority in hospitals

These last two reasons should give you pause to evaluate if you truly need VOC air purification in your home to begin with.

If you’re absolutely sure that you do have a VOC problem in your home – whether you can smell it, feel its effects (sickness), or have tested for it – we recommend the following resolutions before you consider buying an air purifier:

Maybe these resolutions won’t work for you:

If any of the above scenarios describe your situation then an air purifier can certainly help. See the general model recommendations we made earlier for specific models that will filter most unwanted gases well.

If you are especially concerned about gas filtration and don’t mind spending quite a bit more money to get the absolute best air purifier for gas filtration on the market, our recommendation is the Austin HealthMate. With 15 lb. of activated carbon and zeolite, it will be able to remove even a greater number of gases more effectively than our general recommendations, albeit at a much higher price (approx. $500).

**Note that you don’t need to do both – remove the source AND ventilate – to remove VOCs. You only need to do one or the other. For example, if you have off gassing vinyl flooring it could be sufficient for you to simply open your windows every day (to ventilate) without removing the source (the flooring). Or if you cannot ventilate, removing the source will obviously reduce the presence of VOCs in the room.

Additional Recommendations

If you plan on using the air purifier in an environment with high gas concentrations often – eg. you plan on using it in the kitchen to reduce cooking odors – we recommend the following: