Why Do HEPA Air Filters Protect Us From COVID-19 On Planes?

We know that people carry the SARS-CoV-2 virus with them from one country to another, one city to another, on planes. However we haven’t traced many outbreaks to contact on planes as a significant vector of spread. One of the reasons that’s usually cited for this is the use of high efficiency particulate (HEPA) air filters.

These filters are essentially sheets of fiberglass fibers, with diameters between half a micrometer and 2 micrometers, randomly arranged. A fan pushes air through the filter, and particles get trappd inside. The very smallest of particles collide with molecules of gas, which slows them from passing through the filter and increasing the chance of their becoming trapped.

HEPA filters aren’t really a kind of filter, though, the name speaks more to their standard of effectiveness – capturing at least 99.97% of airborne particles that are larger than 0.3 micrometers (or 0.00001 of an inch). Bacteria are easily trapped by HEPA filters. Viruses are smaller than a HEPA filter’s pores, but they often travel in clusters, on mucus, or respiratory droplets

This level of air filtration was developed in the 1940s. The Manhattan Project used HEPA air filtration to limit the spread of radioactive material. They ‘spread’ into commercial use in the 1950s to catch viruses, pollen, bacteria, and other particles in the air.

Even with these filters on planes, passengers and flight attendants have historically had higher rates of cold and flu infection, which may be traced to surfaces – and just coming into contact with more people. That’s why I hope current increased cleaning regimens last past the current outbreak.

Research into the spread of tuberculosis has found some ‘drift’ of air between rows – aircraft don’t appear to increase the risk of TB spread compared to other indoor settings, however.

Many smaller planes, including 50 seat regional jets, do not have HEPA air filtration. However DASH-8 turboprops can now be retrofit to have them.

This Delta video shows a HEPA filter being replaced. As filters age airflow reduces, but aren’t necessarily less efficienct capturing virus particles.

About Gary Leff

Gary Leff is one of the foremost experts in the field of miles, points, and frequent business travel - a topic he has covered since 2002. Co-founder of frequent flyer community InsideFlyer.com, emcee of the Freddie Awards, and named one of the "World's Top Travel Experts" by Conde' Nast Traveler (2010-Present) Gary has been a guest on most major news media, profiled in several top print publications, and published broadly on the topic of consumer loyalty. More About Gary »

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  1. There are two other EXTREMELY important reasons why COVID-19 is not easily transmitted on planes.

    (1) Airplanes provide consistent fresh-air replacement, preventing concentration of the virus.

    (2) Airplane air is never static. (That’s why your farts disappear quickly!) Air is sucked downward and out of the cabin.

  2. Last time you posted an article similar to this, I tried to find if there is a commercially/residential version of a Hepa filter that meets these high standards. I could not, I could only use what Consumer reports recommend. Although my home has a good filtration system along with UV light to protect from mold, it would be nice to have something I can place in our offices. Open to recommendations on this topic. I bought a blue air for my basement because of increased dust from cleaning out old boxes and items in storage from an extended time at home. It is expensive and if the best solution, I would get one for the office, I have no way of knowing.

  3. What criteria are you using when you say there haven’t been any “traced outbreaks to contacts on planes anywhere in the world”? My understanding is that IATA has identified a flight from the UK to Vietnam in March where 1 person infected up to 14 others, as well as airlines informally identifying four flights in Q1 with suspected in-flight transmission.

    For the record, I would feel safe flying now if everyone wore a mask and middle seats were blocked. The problem is that without enforcement, the people who refuse to wear masks are also much more likely to have been the ones to engage in other higher risk behaviors.

  4. These facts are all so readily available….which again reinforces the idea that Delta and Southwest block middle seats for personal comfort and good PR instead of actual public safety.

  5. @Sco – IATA concedes that there may have been ONE flight this year on which spread happened, though I’ve also read arguments debunking.

  6. @Tony – I’m not going to speak to the air filtration on JetSuiteX I do not know about their situation specifically. Robert Isom (American Airlines President) reported theirs do not have HEPA filters, and the airline subsequently reported to me that the planes can not be retrofit to offer it.

  7. Gary, in your TB example, proclaiming that ‘it’s not worse than sitting anywhere else’ isn’t the same thing as saying it cannot spread on planes. Especially when you present a ‘burden of proof’ argument… what evidence do you have that it cannot spread? You didn’t find an article about it? Googling something and not finding results does not make you a domain expert.

    I hate sounding like the Trumpers that occasionally occupy your comment section, but this isn’t your domain, and presenting misinformation or unproven assumptions is not going to help us curb the infection. We all want to get back to travel, but it would be better if we all survived too.

  8. If a plane is circulating outside air, I have to imagine a HEPA filter makes little difference in the grand scheme. I agree with the general sentiment that while you are in close proximity to other passengers it is among the safer crowded places to be due to the nature of the air circulation and the fact that masks have been mandated. With that said, it’s not black and while. There are other ways to pick up the virus than just air circulation. If planes get more crowded you increase the number of vectors, the number of touch points and other ways you may be potentially exposed even if the air is blowing in a favorable direction.

  9. I asked JSX last week if their planes now had HEPA filters and they said that they did, if that helps.

  10. It’s much easier to get it in the airport though ask or look at all the TSA agents who tested positive.

  11. Aircraft while inflight are only one part of the traveling experience.

    However, there are several other aspects that must be included when evaluating one’s risk:benefit trade-offs for travel:

    1.) Getting to/from the airport. Obviously, those who have their own private cars can skip this issue and jump to the next one!

    However, for those who live in places like NYC where many of us do not own cars, we must contemplate what mode of transportation we’re willing to take to/from 1 of the area’s 3 major airports:

    – yellow taxi (expensive, especially to/from Newark)

    – Uber or Lyft (also expensive, but especially to/from JFK)

    – Long Island Railroad/LIRR or NJ Transit trains to their respective AirTrain connections

    – NYC Subway to AirTrain for JFK or Select Bus Service to/from LaGuardia

    – NYC Transit bus to/from LGA

    – Private bus company to all 3 airports

    2.) the airport itself at both ends of the flight; navigating the gauntlet from curbside to seat aboard the aircraft and vice versa (security screenings, walks to/from gates, bathrooms, shops & food, gate hold rooms /lounges

    3.) being aboard the aircraft BEFORE the door closes/ pushing back from the gate, when the airplane uses the preconditioned air pumped in from the HVAC unit mounted under the jetway (think: those big, yellow hoses seen below the jetways).

    Simply put, until the aircraft ventilation is switched from the preconditioned air pumped in from the HVAC units mounted below the jetway at the gate (or pumped in from a mobile unit for aircraft parked at gates that use hardstands), the system that’s being discussed in many marketing campaigns that uses the engine bleed and highest rates air refreshing/cycling is NOT even turned on yet, but instead, is more like the same type of ventilation in an ordinary room or restaurant than the magnificent and highly efficient HEPA filtered systems airlines are touting right now.

    Yeah, I know, I hate being such a Debbie Downer on that, but that super clean as an hospital room air doesn’t kick in until the overhead nozzles start blasting away (think: when the mist clouds appear).

    Otherwise, it’s more like garden variety central air conditioning than the state of the air filtration that’s featured prominently in promotional campaigns.

    Sorry. ☹️

  12. That’s **state of the art** air filtration at the end of my reader comment post above.

    With apologies for the omitted word in the original post!

  13. Addendum:

    A “quick & dirty” check of the filters that are standard for the pre-conditioned HVAC units used by aircraft when parked at the gate indicates that the filter grade is “G4”.

    As is clear from the technical description below, and as found on the web site, G4 is hardly the same as the HEPA filters used while inflight when the engine bleed is the source for in-cabin ventilation.

    A brief description of filter classifications for G1 thru G4 are shown below (source: EMW Filtertechnik; the link will be posted separately immediately below since inclusion of links often results in delays before posts are cleared for publication):

    Filter Classes according EN 779 und EN 1822

    Categorization of filter classes and particle sizes

    Particle Size Examples Filter Class Typical Applications
    Coarse dust, particle size > 10 µm
    textile fibres and hairs
    fly ash
    spores, pollen
    cement dust
    G 1
    G 2
    simple applications (e.g. insect screens for compact machinery)
    G 3
    G 4

    Classification of Coarse and Fine Filters according EN 779:2002

    Filter Class Final Test Pressure Drop Average Arrestance(Am)
    of ASHRAE test dust Average Efficiency(Em)
    of 0.4 µm particles
    G1 250 50% ≤ Am < 65% –
    G2 250 65% ≤ Am < 80% –
    G3 250 80% ≤ Am < 90% –
    G4 250 90% ≤ Am

  14. @Leonard

    Look into the IQ Air brand. Specifically the IQ Air GC Multigas which you can get from Wayfair with a 10% off coupon if they have them in stock. Definitely pricey but they are by far the best and I own two of them as do a number of friends. Not in any way affiliated with the company just a very happy customer.

  15. https://www.emw.de/en/filter-campus/filter-classes.html

    That’s the url for the air filter classifications referencing what a “G4” filter’s specs are as provided in my post immediately above re preconditioned air units typically used while an aircraft is parked at a gate.

    Clearly, it’s so NOT the state of the art “Hospital ER Cleanroom” HEPA grade filtration standard touted in airline marketing campaigns.

    Like I said earlier, sorry to disappoint – but facts are very important, especially when it comes to something that includes decisions regarding potential life and death impacts ☹️

  16. Addendum 2:

    By contrast, specs for state of the art HEPA filters are shown below (same source EMW Filtertechnik; url was uploaded, but had not cleared for publication at the time this was posted):

    Classification of High Efficiency Filters EPA and HEPA according EN 1822:2009

    Filter Class Overall Value Local Value
    Efficiency Penetration Efficiency Penetration
    E 10 ≥85% ≤15% – –
    E 11 ≥95% ≤5% – –
    E 12 ≥99.5% ≤0.5% – –
    H 13 ≥99.95% ≤0.05% ≥99.75% ≤0.25%
    H 14 ≥99.995% ≤0.005% ≥99.975% ≤0.025%
    U 15 ≥99.999 5% ≤0.000 5% ≥99.997 5% ≤0.002 5%
    U 16 ≥99.999 95% ≤0.000 05% ≥99.999 75% ≤0.000 25%
    U 17 ≥99.999 995% ≤0.000 005% ≥99.999 9% ≤0.000 1%

    Again, it’s abundantly clear that the “G4” filters in use when the aircraft is at the gate and using preconditioned air is VASTLY DIFFERENT than the standards being touted in marketing campaigns for the state of the art HEPA filtration used inflight.

    Sorry ☹️

    I know this information sucks. But, for decisions that have a life and death dimension included, this is a relevant factor that’s being glossed over (to put it charitably).

    Once the url clears for publication, the presentation format will be much easier to follow – and the gigantic canyon between “G4” filtration standards and the “H” (or “U”) for HEPA and higher (“E” is High Efficiency) will be readily apparent, especially since there are other classifications in between “G4” and the “E” gradations, including “F5” thru “F9“ and “M5” and “M6”.

  17. What you have to remember is that many people are asymptomatic and the incubation period is usually around 7 days. It is therefore very difficult to trace where individuals picked up the virus from even with decent tracing methods in place.

    Would soon become obvious if a whole plane got it but when it’s just the 4 people in close proximity to one particular infected passenger who then go on to do other things over the next week…

    Almost certainly there will be outbreaks from air travel purely just because of the proximity to other pax but determining the exact point of infection is difficult, especially in a country like US that doesn’t have strict lockdown measures (people doing other activities after arrival in destination).

    Interesting discussion on airplane airflow however, I’m sure there is something to it and there may be a lower risk than sitting directly next to someone on a train for example.

  18. There is one EXTREMELY important reasons why COVID-19 is easily transmitted on planes:

    (1) You get it from the person next to you.


    There are two EXTREMELY important reasons why very few instances of COVID-19 spread has been traced to planes:

    (1) Globally most planes are practically empty

    (2) In the only country where planes are packed full, the US, there’s hardly any contract tracing and testing is very delayed so they literally don’t know where COVID-19 is spreading!

  19. @howard miller You posted a lot of material here. Is there a short form summary?

  20. @cr,

    Key Takeaways:

    1.) Cabin air quality featuring filtration comparable to hospital surgical suites using HEPA filters, plus high rates of top (of cabin)/down (floor venting) refreshing/recycling every 2-3 mins as airlines are touting) is guaranteed to take place only after the HVAC is provided by the aircraft’s internal equipment – and NOT by the preconditioned air units (PCU’s; typically seen mounted below the jetway/jetbridge – look for the big yellow and black ringed hoses) when the aircraft is at the gate during the boarding and deplaning periods.

    As noted in my comments above, the PCU’s used while the plane is at the gate is NOT the same as what’s used after the cutover you the aircraft’s internal HVAC, which most flyers will be able to detect when that cutover occurs since that’s when the overhead nozzles begin blasting air – or of course, sometimes expel mist briefly.

    Until then, the PCU’s, which are basically commercial grade HVAC units, typically use filtration rated as “G4”, which is not nearly the same as standards comparable to an hospital’s surgical suite, as airlines are touting.

    For more about the filtration classifications, it’s best to click the link provided in my reader comment post above (since the web site where the original source of information can be found presents the information better than the cut and pasted version in my comments above).

    2.) The air quality for aircraft while at the gate using PCU’s can be upgraded to the same standard airlines (and their trade associations) are describing for inflight (high rates of top/down refreshment/recirculation using HEPA filters).

    However, the most important elements are:

    a.) The PCU’s on their own do NOT currently allow for HEPA grade air quality being touted by airlines;

    b.) Aircraft while at the gate during boarding & deplaning typically rely on PCU’s for HVAC (using their own APU’s while at the gate means the air is coming thru the engines, which means fuel is also required for power. This is both expensive [and much more expensive than the jetway mounted PCU’s which use electricity, at that], plus of course, results in much higher carbon emissions vs electricity used to power PCU’s);

    c.) Air quality for planes parked at the gate and using below jetway mounted PCU’s can be upgraded from the garden variety “G4” category used for commercial grade HVAC units to HEPA category filtration levels, but until then, the air quality during boarding and deplaning is NOT nearly as high as airlines are touting in the marketing hype.

    [Sorry, airlines – I hate being such a Debbie Downer ☹️ But, as noted in earlier reader comments, when the decision-making process includes evaluating the potential risk of life and death, consumers are entitled to have the most complete variables provided to them, and NOT just the inflight portion of their journey, which although the information can pass legal muster as being “substantially correct”, omits the portion when one is aboard the aircraft while it’s at the gate using PCU’s, which, of course, without delays can be 15-45 minutes depending on the size of the aircraft and number of passengers aboard, and with delays can be up to 3 hours before tarmac rules take effect allowing passengers to deplane.]

    d.) Again, the air quality while the plane is at the gate can be upgraded to the HEPA standards and refresh/recycling rates used while inflight, but the operative phrase here is “CAN BE UPGRADED” – which means:

    i.) NOT necessarily (universally) applicable at the moment

    ii.) Upgrading” costs $$$ for equipment to be retrofitted/upgraded, then repaired and maintained, and of course, the highest cost component, the labor required for the initial installation, plus repairs and ordinary maintenance thereafter.

    e.) Needless to say, during a pandemic when survival is on the line as traffic remains deeply depressed and revenues are low, the cost to upgrade PCU-provided air for aircraft parked at the gate to use looms large.

    3.) While the inflight portion airlines (and their trade associations) are touting as having hospital surgical suite like quality (and there’s NO reason to doubt that), there’s the periods when the aircraft is at the gate for boarding and deplaning when garden variety HVAC units are used (in airline-/airport-speak they’re called “Pre Conditioned Air Units”, or PCU’s) plus the entire on ground portion to/from one’s home or office to/from the airport, then the airport itself with all of the lines, clearing security, using bathrooms, shopping or eating or lounging, gate area hold rooms – or worst of all, those whose definition of hygiene and/or common courtesies fall far short of the thresholds needed for masking be reliably effective at reducing transmission of Covid19 to the minimal risk levels that would make flying – or pretty much any other activity other than bars, restaurants, theaters, large indoor gatherings, etc., possible.

    Again, I know this sucks – but, unless and until our society universally embraces proven scientific knowledge such that wearing a proper mask is understood as being for EVERYONE’s (including one’s own and one’s own family) best interest instead of being dragged into a “culture war” and instead becomes comparable to actions (and results) seen in countries and regions where infection and mortality rates are substantially lower than what’s seen taking place in the USA, Brazil and Russia – or of course a proven vaccine becomes available – and it pains me to say this, but ALL of the factors involved with travel, and NOT just the time inflight under the optimal scenarios presented by airlines when HEPA filters and high/rapid rates of air refreshing/recirculation are taking place, must be included in one’s evaluation of their own, and their loved ones’ who they’ll come into contact with after flying (with presenting symptoms evident or worse, asymptomatic) risk tolerance and decision-making.

    Personally, it really would be beneficial for business leaders to step up and exercise enlightened self-interest by demanding mandatory use of proper face masks for everyone while in public spaces since that will best allow for offices, schools and other economic activity including travel to become viable, whereas as continued bickering over face masks as political affiliations or worse, a “culture war” accomplishes nothing but allowing Covid19 to continue spreading like wildfire as it’s now doing (and that’s during the summer – if this continues into the fall and winter, the rates seen now will be nothing compared to the carnage yet to come…).

    Scientific facts have now been probably laid bare around the world:

    Countries that have embraced face masks and social distancing have much lower rates of infection and mortality than those that don’t.

    This may be an inconvenient truth to some – but it’s the truth nevertheless.

    Just look at the statistical data as one would for any other ordinary business transaction.

    The data is readily available.

    Anyhow, I know this is a bit longer than “Cliff Notes”, @cr, but it’s a complicated subject that does not exactly lend itself to a few short paragraphs!

  21. As a quick follow up:

    While I am NOT an HVAC designer or engineer, the work I did for approximately 2 years for Terminals 1 (Air France, Japan Airlines, Korean Air & Lufthansa) and 7 (British Airways) at JFK Airport, as well as terminals where the food concessionaire HMSHost had operations (BA/T7 & Delta’s T2/3 [It was 1999-2001, before DL moved into T4], pretty much covered every element of terminal facilities’ operations inside and outside, including ground support equipment (GSE).

    A particular focus of the work centered on every last inch of the jetways/jetbridges [literally], and all of the equipment and uses attached to the jetways since they have multiple functions beyond enplaning and deplaning (for reasons that cannot be disclosed as it was legally focused research and analysis), so that’s where I became familiar with Pre Conditioned Air Units (PCU’s)!

  22. Note: there are some important technical differences between a typical commercial grade HVAC unit and a Pre Conditioned Air Unit (PCU) used while aircraft a parked at gates.

    However, the for purposes of illustrating the difference between aircraft ventilation during boarding and deplaning versus after pushback and inflight in this discussion, a PCU better resembles a commercial grade HVAC with filtration standards commonly rated as “G4” for cabin ventilation on the ground before the process used inflight that includes use of the engines while inflight and the HEPA filtration standards touted by airlines (and their trade associations) in their marketing campaigns.

  23. @leonard: Yes, IQ Air. If you think they are pricey wait until you spend $350 to replace the 3 filters!

    I have 3 of these in my house. They are supposed to filter 1100 square feet but the intake is at floor level. So, dust settles on everything just as it did before

  24. @loungeabuser

    Thank you for the feedback. Consumer reports recommend the Blue Air. I have one of the higher-end models that also have an App associated with it. This model pulls from the top, I have it in a large part of my basement, no way to really tell how effective vs the IQ. The product looks impressive but looks like there are few models to choose from.

  25. The filters and airflow might be good if the person with covid is a few rows away from you but what about if they are sitting next to you on a five hour transcon?

  26. Following-up my comments above regarding the use of Pre-Conditioned Air (PCA) units when planes are at the gate during boarding, pre-departure & deplaning (or of course during servicing), on Friday, July 10th, in a reply to a series of comments I posted last week on LinkedIn on the subject of air quality aboard aircraft touting high refresh rates & HEPA grade filtration that’s comparable to industrial clean rooms or hospital surgical suites, the International Air Transport Association (IATA), acknowledged that this applies only while inflight, and NOT when the plane is parked at the gate.

    IATA’s reply is as follows:

    “Hi. Thank you for the comments. For the purposes of the video, we are focusing on the in-flight portion as this is when passengers spend most of their time on board the aircraft.”

    With this acknowledgement, those considering the risks of flying until a Covid19 vaccine is available & widely administered are advised that while aboard aircraft at the gate during the boarding, pre-departure & deplaning periods, the ventilation is likely going to be provided by a PCA, and that the PCA’s refresh rates and filtration levels are NOT nearly as robust as that being touted by airlines & their trade-associations.

    Further, if as the expression goes that any chain is only as strong as its weakest link, then the time aboard an aircraft while you’re at the gate waiting to depart or waiting to deplane, which is when close contact with others is likely to be at its highest, is also when the ventilation system in use is NOT nearly at the strength & capacity airlines are saying they are in their efforts to reassure the general public that flying is safe.

    To be clear, while inflight there is NO REASON to doubt the claims being made airlines (and others) about the rapid rates of refreshing; the top-down circulation provided by diffusers above and exhaust vents below; or the use of HEPA grade filters when the aircraft is taxiing and airborne, when the plane is at the gate, the ventilation system is more like a commercial grade HVAC than the hospital surgical suite caliber that’s being aggressively promoted by airlines, IATA and others.

    For some, that could be problematic.

  27. They should put them in any method of mass transit. They should also offer as an option to have riders wear rubber gloves , yes? That would help reduce infection by contact. I would wear special clothing also, if no more than a lightweight over garment to be cleaned or disguarded immediately after unboarding any mass transit or public transport vehicle. Sounds good to me. Most masks don’t do squat to catch airborne pathogens unless you double or even triple them. If you can still smell things when wearing them, they are useless and give a dangerous sense of security.

  28. One of the PamAm jets I was on had such a strong nausiating smell of engine oil, I think it would have choked any virus to death ! Maybe that’s what the airline had in mind ….toxic anti viral that was also dangerous for passengers. But at least you wouldn’t leave that plane with any new germs , just respitory distress !!!

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