IQAir president Frank Hammes trapped inside a smoke chamber with an air purifier
IQAir president Frank Hammes trapped inside a smoke chamber with an air purifier
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Can air purifiers trap gases effectively?

There is a lot of confusion about air purifiers and gas removal. Read more about the science behind air purifiers and removal of airborne gases.

The answer is yes! But not the way you might think.

Typical HEPA filters are meant to capture solid particles, including dust, pollen, and mold spores down to about 0.3 microns (for reference, a human hair is anywhere from 60 to 120 microns across).1 The most efficient particle filters capture particles as small as 0.003 microns, about the size of a virus or combustion particle from vehicle exhaust.2,3

But these filters, primarily composed of synthetic or inorganic fiber materials, can’t filter out gas. 

And despite what many air purifier manufacturers claim, techniques like ozone generation and UV lights do little to remove gases.4,5 In fact, “purifiers” that use these methods can put even more harmful gas into your air. 

Let’s talk about why gas filtration requires special techniques and how to find a purifier that can capture them for you.

Let’s talk about gas

Unlike solid particles, gas atoms and molecules inhabit an entirely different physical state. 

In a gaseous state, atoms move about at much higher speeds than solid atoms do. They’re also typically much smaller than solid particles, with an average diameter less than 0.001 microns.6

Gases are often abundant in indoor spaces, though not always in concentrations high enough to hurt you. And there are many types of indoor gas pollution – the most important ones you should know are:

  • Gaseous pollutants: includes gas generated by combustion from vehicles as well as paints, varnishes, cleaning products, pressed-wood furniture, and even new carpets. Odors are generally gases, too, but can also be transported on particulates.7,8
  • Volatile organic compounds (VOCs): indoor VOCs are largely emitted by paint, furniture, household chemicals, and other similar sources.9,10 Some VOCs cause headaches, skin reactions, eye and respiratory tract irritation, and memory impairment.11 VOCs have also been linked to cancer. Formaldehyde is the most common indoor VOC.12
  • Toxic gases: includes carbon monoxide, sulfur dioxide, and nitrogen dioxide. These gases can be fatal in large amounts.13 Even in small doses, some can cause respiratory problems and fatigue.14 Common indoor sources include heating systems or poorly maintained gas-fired appliances.

So what’s the best way to remove gases?

To remove these gaseous pollutants, you’ll need an entirely different type of technology than what’s used to filter particulates.

But how do you even know where to start? We’re glad you asked. 

First, know that there are two main processes that remove gaseous pollutants: adsorption and chemisorption

Adsorption happens when atoms or molecules stick to the surface of an adsorbent (not to be confused with absorption, the absorbing of molecules by a liquid or a gas) and become physically bound together.15 The amount of gas that the adsorbent can hold is a certain percentage of the adsorbent’s weight. 

Chemisorption happens when gas or vapor molecules chemically react with a sorbent or with reactive substances in the sorbent.16 This happens on the surface of the sorbent – no adsorption takes place. Chemisorption leaves water and oxygen as a byproduct in the air. 

To accomplish the most effective gas removal, you’ll need a couple of different types of materials that encourage both adsorption and chemisorption.

Materials for adsorption

For adsorption, you just need to remember two words: activated carbon.

Activated carbon (also called activated charcoal) is the most common adsorbent material used in air filtration. It can be made from coal, coconut shells, wood, and much more. Granular activated carbon is most effective because its large surface area allows it to adsorb many different compounds.17

Carbon is “activated” by a steam activation process that creates an extremely porous structure. Like a tiny sponge, activated carbon contains thousands of tiny cracks and pores that give the carbon its large internal surface area. The surface area then attracts many gas molecules and binds them to the carbon surface – the process of adsorption.18,19

For an activated carbon filter to be effective, there needs to be enough carbon so that air passing through the filter can deposit its pollutant molecules within the activated carbon. That’s why good air purifiers for gas removal tend to have pounds of carbon in their filters - the more carbon, the more surface area available for trapping gas molecules from the airflow.

But which type of activated carbon is most effective?

Two primary types of activated carbon are used in air purification: coconut shell and coal-based.20,21 

  • Coconut-shell activated carbon is low-grade, inexpensive, and widely available. It is also very soft and tends to generate dust during transport and sometimes even during usage. When compared with coal-based activated carbon, coconut shell carbon has fewer micropores, which are needed to remove odors and chemicals in concentrations more typical of the home environment. Some people report allergies or respiratory symptoms when they’re exposed to coconut shell carbon dust.22
  • Coal-based activated carbon has an incredibly large internal surface area. It’s a much more effective adsorbent than coconut shell carbon. Of the four major coal types (sub-bituminous, bituminous, lignite, and anthracite), bituminous coal has the widest range of carbon content.23 (That’s why IQAir chooses bituminous coal-based activated carbon for adsorption.)

And another fun fact about activated carbon filters: many contain zeolite, a “filler” that’s much less expensive than conventional activated carbon. But being cheap comes at a cost: there is no evidence to show that zeolite can remove any gaseous compound better than specialty carbons, and the highest efficiency carbon filters are free of this mineral.24

Also, here’s a pro tip: effective pre-filtration helps keep carbon pores from getting clogged by excess particles. Without a pre-filter, a gas-phase filter’s life is significantly reduced.25

Degree of activation

Another factor in the effectiveness of activated carbon for indoor air purification is the degree of activation. Most activated carbon is designed for industrial applications, where carbon is activated for as many pores as possible.26

While high degrees of activation make better at capturing gases at high concentrations, this actually makes it less effective for removing odors and chemicals in your home. This can seem counterintuitive, but higher degrees of activation equal larger pores – and only tiny micropores can remove odors and chemicals in the concentrations typically found in homes.27.28

Materials for chemisorption

Chemisorption involves both adsorption and chemical reactions on the sorbent surface. These additional chemical reactions improve effectiveness against specific contaminants.

Potassium permanganate is an example of a common chemisorbent used in high-performance gas-phase air purifiers. Potassium permanganate permanently breaks down pollutants such as formaldehyde, hydrogen sulfide, and sulfur dioxide into safe byproducts.29

Is there gas removal technology I should avoid?

Not all gas-phase air filtration is created equal. Some purifiers that claim to filter gas do so by dangerous means, sometimes adding more harmful substances to the air than they remove.

The two worst offenders of this kind of gas filtration are ozone generators and purifiers that use photocatalytic oxidation (PCO) with ultraviolet (UV) lights, both of which are astonishingly common in the air purifier market.

Ozone-generators

Ozone generators are air cleaners that deliberately produce ozone as the primary cleaning mechanism.

Ozone (O3) is a reactive gas made of three oxygen atoms, and it’s the main ingredient in smog. At low levels (typically produced by ozone generators), ozone has little potential to remove air pollutants.30 But inhaling even small amounts of ozone can irritate the lining of the respiratory system, causing coughing, chest tightness, and shortness of breath.31 

Long-term exposure can cause or worsen asthma and even lead to premature death.32 Based on the research behind their harmful effects, ozone generators are illegal in California.33 

Photocatalytic oxidation (PCO)

PCO technology uses UV lamps and a catalyst (a substance that causes a reaction) that reacts with the light to destroy gaseous pollutants by changing them into harmless byproducts. 

Titanium oxide is the most common PCO catalyst. When using titanium oxide as the catalyst, PCO devices are supposed to convert harmful gases into carbon dioxide (CO2) and water. But PCO devices are known to produce harmful byproducts, such as formaldehyde.34,35

PCO air purifiers are often marketed as more effective than activated carbon or other solid gas filters. But currently available catalysts are ineffective against harmful gases.36

Thanks, I feel smarter now! But what should I do next?

Don’t fall prey to inflated claims of effectiveness or efficiency that aren’t backed by real science.

If you’re concerned about indoor gases and odors, try IQAir’s GC MultiGas™, an air purifier for gas removal that uses 12 pounds of activated carbon and impregnated alumina for maximum filtration of all types of indoor gases.

Still not sure what purifier is best for your IAQ needs? Download How to Choose the Right Air Purifier, our e-book about the science and technology behind truly effective air purification, brought to you by IQAir’s team of certified Air Quality Experts.

Article Resources

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[2] Nikiforov VN, et al. (2016). Application of laser correlation spectroscopy for measuring virus size.
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[3] Kumar P, et al. (2014). Ultrafine particles in cities.
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[4] Beware of ozone-generating indoor “air purifiers.” (2006).
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[5] Reed NG. (2010). The history of ultraviolet germicidal irradiation for air disinfection.
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[6] Animated gas lab. (2015).
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[7] Nicole W. (2014). Cooking up indoor air pollution: Emissions from natural gas stoves.
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[8] Lee I. (2015). Interaction between fine particles and colloidal gas aphrons (CGA).
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[9] Adeniran JA, et al. (2017). Exposure to total volatile organic compounds from household spray products.
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[10] Volatile organic compounds. (2017).
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[11] Introduction to VOCs and health. (2018).
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[12] Salonen HJ, et al. (2009). Airborne concentrations of volatile organic compounds, formaldehyde and ammonia in Finnish office buildings with suspected indoor air problems.
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[13] Penney D, et al. (2010). Carbon monoxide.
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[14] Jarvis DL, et al. (2006). Indoor nitrous acid and respiratory symptoms and lung function in adults.
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[15] Myers AL, et al. (2002). Adsorption in porous materials at high pressure: Theory and experiment.
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[16] Hellsing B. (2008). Chemisorption.
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[17] Bolourani G, et al. (2008). Evaluation of granular activated carbon filters for removal of VOCs in indoor environments.
https://www.isiaq.org/docs/papers/940.pdf 

[18] Mohammad-Khah, et al. (2009). Activated charcoal: Preparation, characterization and applications: A review article.
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[19] Fabrizi G, et al. (2012). Occupational exposure to complex mixtures of volatile organic compounds in ambient air: Desorption from activated charcoal using accelerated solvent extraction can replace carbon disulfide?
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[20] Gratuito MKB, et al. (2008). Production of activated carbon from coconut shell: Optimization using response surface methodology.
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[21] Athappan A, et al. (2013). A comparison of bituminous coal-based and coconut shell-based activated carbon for removal of trace hazardous air pollutants in landfill gas.
https://uta-ir.tdl.org/uta-ir/bitstream/handle/10106/26224/J.%20Civil%20Eng.%20Urban.%2c49-331-337.pdf?sequence=1&isAllowed=y 

[22] Safety data sheet: Coconut shell activated carbon. (2015).
http://www.californiacarbon.com/pdfs/SDS%20VCAC.pdf 

[23] Subbituminous and bituminous coal dominate U.S. coal production. (2011).
https://www.eia.gov/todayinenergy/detail.php?id=2670  

[24] Choosing an adsorption system for VOC: Carbon, zeolite, or polymers? (1999).
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[25] Guidance for filtration and air-cleaning systems to protect building environments from airborne chemical, biological, or radiological attacks. (2003).
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[26] Teng H, et al. (2004). Activated carbon production from low ash subbituminous coal with CO2 activation.
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[27] Huang Z, et al. (2002). Adsorption characteristics of trace volatile organic compounds on activated carbon fibres at room temperature.
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[28] Hu S, et al. (2016). Removal of carbon dioxide in the indoor environment with sorption-type air filters.
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[29] In situ chemical oxidation using potassium permanganate. (1999).
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[30] Ozone generators that are sold as air cleaners. (2017).
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[31] Health effects of ozone in the general population. (2016).
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[32] Jerrett M, et al. (2009). Long-term ozone exposure and mortality.
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[33] Hazardous ozone-generating “air purifiers.” (2015).
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[34] Farhanian D, et al. (2012). Investigation of ultraviolet photocatalytic oxidation by-products.
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[35] Hodgson AT, et al. (2007). Performance of ultraviolet photocatalytic oxidation for indoor air cleaning applications.
DOI: 10.1111/j.1600-0668.2007.00479.x 

[36] Hay SO, et al. (2015). The viability of photocatalysis for air purification.
DOI: 10.3390/molecules20011319 

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