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Coronavirus fact check: Does the ‘recycled’ air on planes really put you at a high risk of infection?

Some airlines have decided to leave middle seats open to help practice social distancing but an industry body says it’s not enough to meet most countries’ requirements.

Airlines are preparing to take to the sky again as South Africa eases its national coronavirus lockdown to allow for domestic business travel. Here’s what you need to know about air travel in the time of COVID-19.

Question: Will breathing in recirculated air on a plane put me at a greater risk of contracting the new coronavirus?

Answer: The risk of contracting the new coronavirus via the air on planes is low if cabin air filters are well-maintained.

Here’s why: Most modern aeroplanes use High Efficiency Particulate Air (Hepa) filters to clean the air inside of them. These filtration systems are largely the same as those used in conventional operating theatres, explains Shaheen Mehtar, who is the chair of the education working group the Infection Control Africa Network, a professional association for specialists.

Hepa filters remove up to 99.9% of even extremely tiny particles between 0.1 to 0.3 microns in size, says the International Air Transport Association (IATA), which sets standards for the airline industry. The filters clean the air, which is then returned to the cabin mixed with 50% clean air from outside, the airline association explains in a 2018 briefing paper.

How small are the particles that Hepa filters capture? For reference, a single strand of hair is typically 50–100 microns in diameter. Sars-CoV-2 is about 0.1 microns in diameter, according to research published in April in the journal eLife.

“Hepa filters are very efficient… therefore these will work for coronaviruses,” Mehtar says.

According to the 2018 IATA briefing paper, most airlines replace cabin air filters at regular intervals as part of routine maintenance.

Q: Are Hepa filters enough to protect me if someone with the virus near me sneezes or coughs?

Answer: Not entirely, warn Ana Rule, the director of the Exposure Assessment Laboratories at the Johns Hopkins School of Public Health and her colleague Gurumurthy Ramachandra.

Here’s why:

“All the air inside an aircraft cabin is filtered using Hepa filters installed in the ventilation systems in planes, and some of this filtered air is recirculated back into the cabin. However, the air in a cabin has to travel some distance and takes some time before it is filtered and recirculated,” Rule and Ramachandra explain.

“If a person next to you coughs or sneezes, some of the smallest particles will stay in the air and will be captured by the ventilation system, but if you are very close you may inhale some particles before they are captured by the ventilation system.”

Additionally, depending on whether the person was close to you or a ventilation vent — for example — larger particles of possibly virus-containing droplets of saliva may land on surfaces such as armrests or tray tables, Rule and Ramachandra say:

“But the good thing is that those smaller particles also easily follow the airflow, so having a ventilation system that is pulling air from the cabin will “pull” those particles out of the air. The air full of particles, and some viruses, then pass through Hepa filters and are cleaned and then returned to the cabin mixed with 50% clean air from outside.

“As a rule of thumb, particles larger than five micrometres in diametre drop to the ground within about two metres depending on the size of the particle  and particles smaller than five micrometres don’t drop to the ground and can stay ‘suspended’ in the air for minutes to hours.

This is important on a plane because ventilation systems cannot ‘pull’ larger particles from the air. Those will drop or impact a surface that can then be touched by you or another person who then touches their face.”

Masks — even homemade ones — help in all of this because they capture larger particles released as people talk or cough, so there are fewer particles released into the air that could come to rest on surfaces, Rule and Ramachandra say. 

In short: Hepa filters are great — if they’re working —  and the strong flow of air from ceiling vents downward in planes may also help reduce the spread of particles, but they still might not be able to prevent infections if you’re sitting close enough to an infected passenger.

Q: Airlines are limiting how close passengers sit to each other, is that enough?

Some airlines have begun leaving middle seats open to try to practice social distancing. As of May, the IATA did not recommend this method because leaving a middle seat open doesn’t provide sufficient social distancing space.

“Most authorities recommend 1m-2m [between passengers] while the average seat width is less than 50 cm,” the airline association wrote in a May press release.

The IATA does, however, support the use of masks by passengers and crew members, along with — for instance — more frequent and thorough cleaning of aircraft as well as limiting movement in the cabin during flight.

And although the IATA doesn’t say how full aeroplanes should be when they fly during the coronavirus pandemic, it does warn that restricting the number of passengers to anything less than 77% of a plane’s capacity threatens to make flights economically unviable.

Meanwhile, as of March, South Africa’s guidelines for tracing the possible close contacts of confirmed COVID-19 cases recommend that anyone sitting within two seats of a person in any direction, as well as travel companions and crew members working in that section of the aircraft, be tested for COVID-19.

But a 2016 study published in the Annals of Global Health warns there may still be a risk of infection for people seated even farther away from confirmed cases. As part of the research, scientists reviewed eight cases of infections of, for instance, other coronaviruses and measles, linked to air travel. The research found that although people within a 2-row radius of patients had about a 6% risk of infection, there was still a roughly 2% risk of infection for people seated beyond this.

Researchers concluded that although public health authorities should continue to prioritise passengers seated within two rows of a confirmed case of infectious disease, they should also follow-up with passengers outside this zone.

Elsabé Brits is a freelance science journalist. Follow her on Twitter @elsabebrits