Is it safe to fly while the COVID-19 pandemic rages? According to a recent article published in the Journal of the American Medical Association (JAMA), Risk of COVID-19 During Air Travel, you are safer in flight than you are in your office, class, supermarket, or commuter train.
The October 1, 2020 JAMA article, published as a "Patient Page," should not to be confused with the American Medical Association's peer-reviewed papers. It has garnered over 220,000 views by the time this article was written. It has not been cited by peers and has elicited skeptical comments and some have openly questioned the paper's research and conclusions.
Faced with a lack of customers on a scale unseen since 9/11, airlines are once again in an existential crisis. To survive, they need to present air travel as safe, to counter the public perception of the crowded cabins of passenger aircraft as a surefire place to contract contagion. They also need to bend public safety guidelines. Airlines cannot maintain a safe distance for each passenger (six ft or two meters apart) the same way high rise building managers cannot for their elevator passengers. In both cases, distances are shortened to meet practical necessity without regard to the physics of airborne transmission, as we will show here.
The authors disclose a financial interest and employment with the airline industry (Air Portugal, Qantas, and airline association IAMA). As journalists, it would be only too easy to dismiss the article as self-serving and the authors as biased. But as engineers, we felt we needed to study the facts.
An Inconvenient Truth – Air Flow Between Rows
The paper contends that airborne spread of the SARS-CoV-2 virus, the cause of COVID-19, is dealt with by airflow and filtration1.
"Modern airplanes maintain clean air by circulating a mix of fresh air and air recycled through HEPA filters, the same type of air filters used in hospital operating rooms," say the researchers.
The grossly simplified diagram shows a neat, unobstructed airflow across a row of seats, neglecting obstructions like entertainment systems (found under the seats in some configurations), the passengers themselves, and whatever they have crammed under the seats such as knapsacks or handbags, service animals or shoes.
"As a rule of thumb, the air that you're typically breathing and exposed to is usually anywhere from two to five rows surrounding your seat," says Dr. Mark Gendreau, an expert on the in-flight spread of infectious diseases.
Several simulations contradict the 2D, smoothly flowing, passenger-less depiction of airflow that states "there is relatively little airflow forward and backward between rows."
Julian Toumey, a senior research engineer at Converge, a CFD vendor, published his study of a private jet fleet (ECS) on his company blog in 2017. He found the plume behind a person walking through the aisle of a passenger plane would find a way to seated passengers even with forced air directed downward into the aisle. The simulation, made before the pandemic, could be considered the worst case, as it does not include mask wear.
Researchers from the faculty of engineering at Cairo University, Hassan Kotb, and ASHRAE Fellow Essam Khalil, conclude:
- The transmission of the sneezed droplets that were exhaled from the standing passenger could reach the seated passengers in the first and second rows and still travel to more passengers. The coughed droplets could travel up to 1.1 meters without any movement from the passenger.
- During the movement of the passenger at a constant speed, the droplets spread widely inside the aircraft cabin and manage to attack many passengers inside it.
In a popular CFD study, researchers at Purdue simulated a middle seat passenger sneezing in a twin-aisle cabin configuration to find that droplets were disbursed through the air to several rows behind.
Purdue researchers found the airflow in a Boeing 767 aircraft cabin to resemble more of a swirling, chaotic mess using smoke trails, hardly the neat row-contained 2D diagram of airflow of the JAMA article.
Do HEPA Filters Help?
The sophisticated air circulation system in our Airbus aircraft enables a complete exchange of air in about 3 minutes. Our systems engineer Sebastian Wehrmann explains how it works. #MeetTheExpert pic.twitter.com/kBHZ3zsnwL
— Lufthansa USA (@Lufthansa_USA) May 22, 2020
Lufthansa released this video of Sebastian Wehrman, A/C system engineer, showing a HEPA filter in an Airbus A320. (Video courtesy of Lufthansa)
The JAMA paper states "the airflow in current jet airliners is much faster than normal indoor buildings. Half of it is fresh air from outside, the other half is recycled through HEPA filters of the same type used in operating rooms."
HEPA filters are not installed on older planes like old McDonnell Douglas aircraft and smaller modern planes, responds a ventilation engineer in the comments. Also, tailies (aft cabin passengers) may not rate a HEPA filter.
"The plane is almost a clean room," says Liam Bates, CEO, and co-founder of Kaiterra, an air-quality monitor manufacturer quoted by National Geographic.
However, a comparison to a cleanroom may not be deserved. Clean rooms are clean not only because of the air filtration but also because everyone in them has on a cleanroom suit, commonly called bunny suits, a big step up in safety from a leaky cloth mask.
Besides, clean, fresh, or filtered air is not the case for the entirety you are on the airplane. The forced air filtration systems work to full effect after the airplane is in flight and scooping up the cold, fresh air. Passengers boarding and exiting the aircraft, taxiing on the runway, or stranded on the tarmac have to depend on the crew keeping the air conditioning on.
But All Passengers Have Masks, Right?
Despite much reporting of passengers not wearing masks earlier in the year, recent reports show masks in widespread use – most of the time. Airlines continue to serve food or drinks, which means that people must lower or remove their masks to replenish their system. One person took to a frequent flyer website to question, does having a can of Coke in front of you for 30 minutes qualify as drinking?
By the latest consensus of sound medical advice on COVID-19 spread, the masks should reduce the far airborne emission of the SARS-CoV-2 virus that would be expelled by coughing and sneezing. Masks reduce the risk of catching the virus by up to 70 percent, according to Dr. Trish Perl, the chief of the Division of Infectious Diseases at the University of Texas Southwestern Medical Center.
However, the risk with masks of any kind is not zero. Fabric masks still allow much of your exhalation, which includes small droplets, to leak out – a fact easily verified by anyone wearing a mask with fogged up glasses. The same leaks can allow passage for the small droplets that can float in the air after leaving an infected passenger near you.
While masks can only block big droplets (with a high viral count from an infected passenger) and reduce small droplet ingestion (with a proportionally small viral count), they cannot be 100% effective against COVID-19. However, recent studies have shown that the infection as well as the severity of the symptoms are proportional to the number of viruses that enter the victim.
In a recent Southern Hemisphere ocean cruise that left from Argentina to duplicate the voyage of the South Pole, mask use and isolation were effective after the infection broke out on board and only 20% suffered from COVID-19 symptoms from the 128 passengers who were infected.
Viral Load by Activity
Indeed, much anecdotal evidence provides proof of high viral counts contributing to the severity of the disease. The meat processing plant workers that stood close to other workers for entire shifts provided a megadose of viruses to each other. Another factor to consider is the type of activity being done during exhalation, each expelling a different viral load.
On the low end is normal resting breathing, which would be the case of seated passengers in a flight that expels the least number of small droplets, all of them small enough to be invisible. According to Dr. Erin Bromage, comparative immunologist and professor of biology at the University of Massachusetts, Dartmouth, whose plain-speak blog posts on COVID -19 have gone viral (pun intended), just breathing releases 50 to 5000 droplets with each breath. Since breathing requires little force, viruses from the lower respiratory tract are not expelled.
Bromage estimates that it takes up to a thousand virus particles for someone to get infected. If each breath releases 20 viruses into the air every minute, and let's say in the worst case, every one of them ended up in your respiratory tract, it would take 50 minutes for that person to catch COVID-19 if they are close to an infected person.
Speaking releases 10 times the amount of droplets that breathing does or about 200 virus particles per minute. A face to face meeting of 10 minutes can lead to an infection. As speaking generates small droplets that float in the air, anyone in the room with an infected person is at risk. Think of call centers, open offices, and packed conference rooms, as well as being on a flight stranded on the tarmac with no air conditioning.
A cough can release 3,000 droplets. A sneeze, ten-times that amount. Big droplets fall to the ground in less than the nominal safe distance or are blocked with a mask. It is the small droplets, with coughs and sneezes, you have to worry about.
Does Blocking the Middle Seat Help?
Even with passengers in every other row and middle seats empty, a combination no airline has yet adopted, flights will only reduce 1/3 of its normal capacity but still have less than half of the nominal safe distance. Elevators held to a safe distance cannot accommodate more than two passengers at a time. Prisons could only have one prisoner in every 6 ft by 8 ft cell (typical in the US).
In each industry, where the economics do not permit safe distancing, industry representatives will either turn themselves inside out in defending smaller distances they can tolerate (as in the JAMA paper)– or stay silent.
Arnold Barnett of MIT's Sloan School of Management, a big believer in middle seats staying empty, looked at the statistics for short US flights and found that even with a sold-out flight that has every seat filled, the odds of contracting COVID-19 in flight are 1 in 4,300. The odds fall to 1 in 7,700 for flights with empty middle seats. If you believe Dr. Barnett, you will want to avoid United and American Airlines, who still sell middle seat tickets. The odds will be even less in flights that don't have as many COVID-19 cases as the US (since the US is the top country hit by the pandemic), but the risk could increase with the duration of the flight.
Will You Get COVID-19 On a Flight? Probably Not.
The paper states "despite substantial numbers of travelers, the number of suspected and confirmed cases of in-flight COVID-19 transmission between passengers around the world appears small." Indeed, the much-scrutinized airline industry may have just received a bad rep as a superspreader.
Despite several nonsensical justifications of the self-serving JAMA article, the airflow (half the air being fresh and constant circulation) alone may be the biggest factor in the low rate of COVID-19 transmission in flight.
Rather than studying the proximity of an infectious individual in a cramped, indoor environment of coach class, it turns out the air combined with diluted virus particles due to the forced air circulation has effectively provided the same protection against the airborne spread as being outdoors.
Even though warnings against extended family barbecues and crowded beaches abound, there was a growing consensus among medical experts that catching COVID-19 outside is hard to do, says the New York Times. Similarly, a study of over 7,300 cases in China revealed only one case of COVID-19 being contracted in an outdoor setting.
The unmistakable conclusion from research and studies to date is that it is relatively safe to fly even during the pandemic. Do keep in mind we are considering the enclosed cabin only, which is only one part of the journey from point A to B across a sizeable distance. To get a total risk picture, we should consider getting to and from the airport (for example, how safe is your Uber ride or your airport bus?), the massing at the gate during boarding and on the airplane while seating -- as well as the reverse operations when deplaning). Each nonflight component of the journey, without the mask, distancing or HEPA scrubbed ventilation, may involve much more risk than the flight itself.
All things considered, it may be safe to fly. Although COVID-19 is all about distancing, you are relatively safe while in flight. But do consider some common-sense safeguards.
- Keep your distance as best you can, especially at choke points like the gate, boarding and exiting the aircraft.
- Refrain from talking with other passengers.
- Wear a mask as much as possible. Keep drinking and eating to a minimum (because you can't do that with a mask on).
- Keep the overhead airflow on full blast.
- It doesn't hurt to wash your hands and sanitize your surfaces.
References and Notes
Will a HEPA Filter Help Us Fight Off COVID-19?, Roopinder Tara, Engineering.com, October 26, 2020
1. The author amends the 2D simplification in response to comments, conceding “some longitudinal flow extending a couple of rows” does occur although justifying the 2D simplification with the “low number of cases appears to indicate [air flow contained in a row] is indeed being achieved.”
Risk of COVID-19 During Air Travel, Rui Pombal, Ian Hosegood, David Powell, JAMA, October 1, 2020.
Visualizing the Effectiveness of Face Masks in Obstructing Respiratory Jets, Phys. Fluids 32, 061708, June 30, 2020
How Clean is the Air on Planes? Johanna Read, National Geographic, Aug 28, 2020
What We Know About Your Chances of Catching the Virus Outdoors, Michael Levenson, Tara Parker-Pope, James Gorman, New York Times, May 15, 2020