Carbon Air Filters for VOC and Odor Filtration in Air Purifiers
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:
How long a particle will stay airborne
smaller particles stay airborne longer
How easily a particle can become airborne
smaller particles are disturbed more easily
When inhaled, how far into a human being’s respiratory system the particle will travel
smaller particles travel more deeply into the respiratory system
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.
Molecular Weight (MW)
the mass of the molecule
Boiling Point (BP)
the temperature at which the liquid form of the molecule turns to vapor
Vapor Pressure (VP)
the pressure of the vapor above the molecule in its liquid form
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:
for formaldehyde and ammonia.
for mercaptans, hydrogen sulfide.
for a wide variety of chemicals including acidic and corrosive gases and odor causing gases like hydrogen sulfide.
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
“VOC” stands for Volatile Organic Compound. Let’s break down this definition
has the tendency to vaporize (go from liquid to gas state)
comprised of multiple elements (not just carbon)
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.
|Chemical (VOC)||Source||MW (g/mol)||BP (°F)||BP (°C)||Carbon can filter|
|acetone||nail polish remover, paint remover, cleaning products, smoke (vehicle exhaust, tobacco)||58.08||132.8||56||y|
|1,4-Dichlorobenzene||disinfectants, pesticides, deodorant (has replaced naphthalene in mothballs because it's less flammable)||146.998||345.2||174||y|
|formaldehyde||building materials, furniture, insulation, textiles, DIY products like paints and glues, cleaing products, cosmetics, combustion processes (smoking, heating, cooking, candle burning), even certain electronics like computers and photocopiers||30.031||-2.2||-19||n|
|toluene||vehicle exhaust, stored fuel, paints, glues, personal care products||92.14||231.1||110.6||y|
|xylene||same as toluene||106.16||281||138.4||y|
|methylene chloride||solvents (paint strippers/thinners)||84.93||103.3||39.6||y|
|benzene||smoke (vehicle exhaust, tobacco), stored fuel, paints, glues, detergents, pesticides||78.11||176.2||80.1||y|
|ethanol||antiseptics (medical wipes, hand sanitizer), stored fuel, solvents||46.07||173.1||78.37||n|
Note the low MW of all the chemicals that cannot be filtered by carbon.
|Chemical (Odor)||Smell||MW (g/mol)||Carbon can filter|
|random sample of smells|
|2,3-dimethylpyrazine||nutty (roasted peanuts, baked bread)||108.144||y|
|2,6-dimethylpyrazine||nutty (roasted peanuts, baked bread)||108.144||y|
|2-isobutyl-3-methoxypyrazine||green bell pepper||166.224||y|
|methyl salicylate||wintergreen altoids||152.1494||y|
|typical after cooking meat|
|2,3-pentanedione||sweet, caramel, malty, buttery||100.117||y|
|Cigarette Smoke||Quantity in 1 cigarette|
|Polycyclic aromatic hydrocarbons (eg. naphthalene)||28 to 100 milligrams||128.1705+||y|
|Acetaldehyde||980 micrograms to 1.37 milligrams||44.05||n|
|1,3-Butadiene||152 to 400 micrograms||54.0916||y|
|Benzene||5.9 to 75 micrograms||78.11||y|
|Pet Smell (especially wet dog)||Smell|
|urea||urine (contributes indirectly by releasing ammonia as a byproduct)||60.06||y|
|Butyric acid||strong, rancid butter-like||88.11||y|
|Dimethyl disulfide||unpleasant, onion-like||94.19||y|
|2-nonanone||fruity, floral, fatty, herbaceous||142.242||y|
|hydrogen sulfide||rotten eggs||34.1||y|
|dimethyl sulfide||rotten cabbage||62.13||y|
|acetic acid||sour, vinegary||60.05||y|
*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:
- 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
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.
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:
a thin replaceable filter made of fibrous material that’s coated with carbon
This type of filter is by far the most common. It’s cheap to make (for the manufacturer) and therefore cheap to replace (by you). But, it has to be replaced often. The carbon on this type of filter saturates very quickly. Once saturated, the carbon will no longer be effective in adsorbing unwanted gases from the air. This type of filter also does not filter gases as effectively as the second type of filter described below.
a thicker washable filter that contains actual carbon pellets
This type of filter is far less common. It’s more expensive to make and therefore more expensive to replace. In this type of filter carbon pellets are held inside a plastic honeycomb type structure with a metal mesh holding in the pellets on each side. This type of filter is washable which is only one of the reasons why it doesn’t need to be replaced as often. The primary reason is this: this type of filter provides much greater surface area for unwanted gases to bond to. This is also the reason why it is far more effective in removing unwanted gases from the air than the thin fiber filter described above.
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.
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
- Home air quality test kits
- Deductive reasoning
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
TVOC includes all VOCs
Many VOCs are not harmful and these are included in this measurement.
There’s no apparent regulatory standard for TVOC
you may be able to measure TVOC but there’s no way of knowing with certainty if it’s too high or too low unless you measure extremely high levels – in which case it’s likely you have a problem. But even in this scenario, you still won’t know the exact VOC that may be causing such high levels. If you don’t know the exact VOC (and thereby its MW, BP, and VP) you don’t know how to filter it.
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:
They can be expensive
one test kit can be anywhere from $100 to $200+ and samples only one room. If your house has multiple rooms, multiple kits are required. The cost goes up accordingly.
The test results may or may not include a list of specific chemicals and/or specific concentrations
Often the companies that sell these kits do not provide reports listing specific chemicals. You may receive a report that lists chemicals in groups – e.g. VOCs from adhesives, VOCs from paints, etc. – no specific chemicals are listed. These reports may also only tell you if the concentration was measured to be below, at, or above average.
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.
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
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.
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).
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
the tobacco smoke consisting of
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.
You can measure initial particle concentration accurately and precisely with a particle meter.
You can then run a HEPA equipped air purifier.
The HEPA filter reduces all particle concentration – no matter the chemical composition of the particle.
You can then measure particle concentration again to verify that particle concentration was indeed reduced by the air purifier.
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.
High number of total VOCs
A typical indoor environment can contain as many as 300 VOCs. Testing has shown that carbon filters can remove certain VOCs while other media like zeolite can remove certain other VOCs but these tested VOCs represent a small fraction of the total VOCs in the air.
Recall how we said that a molecules characteristics – specifically, it’s MW, BP, and VP determine how well it can be filtered by certain media.
Here’s the problem: At the molecular level – the level at which VOCs exist – chemical reactions are abundant. Remember, these VOCs do not exist in a vacuum. They usually exist in air. Clean air is comprised primarily of nitrogen and oxygen. But the air can also contain a myriad of other chemicals, often other VOCs. Not to mention all the chemicals on various surfaces of a room. All these chemicals – in the air and on surfaces - interact to form new chemicals.
VOC testing usually involves doing everything to reduce such interactions - testing usually involves one chemical at a time in a highly controlled environment. But in so doing, the test is nowhere close to being indicative of real life scenarios where VOCs exist in air and easily interact with other chemicals.
Remember, all VOCs exist as gases. Let’s take a moment to really think about what that means.
Perhaps the best example of a gas is air itself. Air is roughly 80% nitrogen and 20% oxygen. The average distance between each molecule of air is about 10 times greater than the distance across the molecules themselves. Air molecules travel at about 1000 mph (460 m/s) at 63 °F (17 °C) bouncing into each other once every 14 billionths of a second.
This is chaos from a scientific study standpoint. So, what do we do? Instead of focusing on individual molecules we can look at the gas as a whole. To do that, we can use the ideal gas law which does a good job of approximating even non-ideal gas behavior. The law given as an equation is
PV = nRT
This equation tells us that four variables determine gas behavior:
The fifth letter in the equation – R – is a constant – not a variable.
Thus, gas behavior – all those gases you may want to filter – behave differently depending on pressure, volume, concentration, and temperature. This means that any testing of VOCs – gases – must take into account pressure, volume, concentration, and temperature to be accurate and precise. Needless, to say, this makes testing extremely difficult.
VOC concentration levels
VOCs do not have to exist at extremely high concentrations to be a health hazard in the home. The problem is that it’s extremely difficult to test for removal of VOCs at these low concentrations. Thus, most testing involves much higher initial concentrations of VOCs than what is indicative of real life scenarios.
For example, formaldehyde is a health hazard at concentrations greater than 0.1 ppm. The independent study that we reference most often as it relates to carbon’s ability to remove VOCs tested formaldehyde at a concentration of approximately 1.6 ppm.
Any filter media that removes VOCs will saturate. In a carbon filter the solid carbon molecules that make up the filter bond to the VOCs to remove them from the air. Once all the carbon molecules are bonded, the filter won’t remove VOCs anymore.
Most tests that involve carbon filters and VOCs use a filter that is brand new – completely unsaturated. For these tests to be more meaningful the filter would need to be tested at varying degrees of saturation – after 1 day of exposure to a certain concentration of a particular VOC, 1 week, 1 month, etc.
Here’s the problem: It’s virtually impossible to know just how saturated a carbon filter may be. Thus, any such testing – at different degrees of saturation – is virtually impossible.
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
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:
removing the source
if a can of open paint is emitting harmful VOCs, close the lid or use a different paint. If the source is vinyl flooring that’s off gassing harmful VOCs – replace the flooring. If you can smell fumes, remove the source. If you find yourself getting sick after changing something in your home – ie installing new flooring – remove the source (in certain cases you can use a sealant to reduce the off gassing).
remember VOCs are gases. Gases move easily, mix easily, etc. Open a window. Open multiple windows. Add some fans to help move air. Do whatever you can to bring in fresh air and ventilate out the bad air. Ventilation is much more effective in removing VOCs than any air purifier will ever be.**
Maybe these resolutions won’t work for you:
if you cannot remove the source. For example,
if you live near a source of perpetual VOCs (e.g. a factory)
if you or someone you live with is perpetually generating VOCs (e.g. smoking)
if you have a source of VOCs in the home that cannot easily be removed (e.g. vinyl flooring)
or if you cannot ventilate. For example,
if you live in an extreme climate that makes ventilation difficult (extreme cold, heat, humidity)
if you live in an area with extreme outdoor particle pollution or other problematic particles (e.g. pollen that gives you allergies) that keep you from wanting to ventilate (note that you can still let these in to ventilate out the VOCs and simply filter the particles with a HEPA filter)
if there is no window in the room you need to ventilate
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.
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:
replace the carbon filter often
a carbon filter will saturate over time – the more gases it has to filter out, the quicker it will saturate
look for air purifiers that note carbon content
earlier we talked about units with filters coated with carbon vs units with filters that contain actual carbon pellets, noting that the latter option is more effective in removing gases from the air.
While it won’t necessarily make them more effective (3x), units with more carbon – ie a greater quantity of carbon - will at the very least take longer to saturate.
Most manufacturers aren’t too proud of the amount of carbon in their filters. Other manufacturers proudly display carbon content as a selling point. For example, the Austin Healthmate’s product description notes 780 cubic inches of activated carbon. You can rest assured that if the amount of carbon content is noted like this, it usually means that the unit’s filter contains a lot of carbon which will take much longer to saturate than the average filter on the market.
Look for air purifiers marketed as specializing in VOCs
This last recommendation can be a hit or a miss. Almost all air purifier manufacturers advertise that their units can remove VOCs. But look out for manufacturers that place an extra level of emphasis on VOC removal. Such manufacturers usually sell units that really are well optimized for gas filtration – as good as an air purifier can be notwithstanding all the warnings we went over earlier.
Here’s the one problem with purchasing this level of air purifier. Doing so isn’t cheap. The least expensive Austin air purifier that’s especially good for gas filtration, the HealthMate, normally retails for more than $500. A fully decked out IQAir GCX series air purifier with a GCX MultiGas filter cartridge set (22 lb. of activated carbon and impregnated alumina) retails for about $2200.
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Fabulous overview of carbon filters. I found this very helpful. I own three BlueAir 121 and have opted for their extra protection filter that uses carbon pellets. It is heavy, but have yet to weigh the difference between the two. I'm hoping you'll review this Blueair121 with the special filter in the future. I'd like to see how it compares with the added pellet filter option. I"m wondering also if this trend is trickling over to other air purifying companies as well. Thanks!
This was an excellent read.
I just want to add that using a laser particle counter to "sanity-check" claims made by air filter manufacturers was very important in my case to identify a good HEPA unit. I took a MetOne industrial laser counter to a walk-in store which sold various major manufacturers, and took measurements of the units at their output, compared with the input. That particular store owner (near Berkeley, CA) was very helpful. I cannot mention which unit I ended up buying, but I will mention that the Austin Air unit recommended here only filtered 50% of the 0.3 micron range particles.
This was 15 years ago, and I am still seeing Austin Air ranked highly. Of course, that might have been a faulty unit etc, etc., but not many of us have access to 6K particle counters to identify the faulty units, either.
Thus, if you are serious about air quality, use critical thinking, and borrow testing equipment if you cannot buy it.
I purchased a replacement activated carbon pellet air filter that has a honey comb structure to hold the pellets for a home carbon+HEPA air filter unit. Whereas some of the 1/2 inch wide hexagons are full of pellets so only small points of light show through, about a third of them are only half full of pellets with the center empty. There's got to be point where empty areas reduce the ability of the filter to remove VOCs from the air. When I complained to the seller that the filters I received are only partially full, they say that this is needed to let air through. Of course the honeycombs can't be packed solid with pellets preventing all air flow, but there must be a point where large open areas reduce the effectiveness of the filter. The manufacturer shows advertising photos with the honeycombs completely black. The ones I received are not. Is there a way I can better argue this so I can get my money back besides common sense? Thank you for any help!
Thank you for the detailed and yet "simple" explanation. We are trying to remover second-hand smoke seeping into our apartment from or upstairs neighbor... I have been reading and trying to understand the difference among the many different option on the market and of course, going back and forth between this one, that one and the other one!.
I had originally been looking at the Austin Healthmate but was turned off by the 'one filter' unit having to replace it all together and also the high decibel measure.
I also compared the Airpura and the AllerAir models - do you have any input in those? The AllerAir gas quite impressive reviews.
One concern with the Airpura (T600 DLX) is the use of Potassium Iodide - have read that it produces a sweet aroma and can be irritant for some.
I'm now searching for a smaller unit and am considering/comparing the Austin HM 400 and the AllerAir 3 Supreme.
Can you comment on these?
A private e-mail will be most helpful! (Or link to wherever you post your answer so that I can find it!)
I have yet to test the other models you mentioned but can assure you that the Austin HM400 (HealthMate) will work very well for your application. Yes, it does have a single filter that's very expensive to replace but that filter contains much more filter media than most filters for most other units on the market and so it will last much longer. The HealthMate is also not any louder than most other units we tested. See here to compare its measured noise level to all of the other air purifiers we've tested.