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The Safest Air in the Skies

by Tori Bottomley and Joji Waites

Many passengers are under the impression that flying is unsafe when it comes to COVID-19 transmission, thinking they are just as likely to catch the virus on an aircraft as they are on the London Underground. But this is not the case. In fact, the air circulating around an aircraft cabin is among the safest that you can breathe indoors. This is largely thanks to HEPA filters and environmental control systems (ECS) on board.

What are HEPA filters?

High Efficiency Particulate Air (HEPA) filters are high-quality air filters made from randomly arranged fibres, typically fibreglass or plastic. To have this label, the filter must be able to remove a minimum 99.97% of airborne particles and filter particles as small as 0.1 microns. This means it can trap viruses, bacteria and allergens – including the SARS-COV-2 virus, which causes COVID-19. It’s almost as if these filters were designed for the COVID era, but, in fact, they were designed long before.
Used by the US Army Chemical Corps in the 1940s to filter out radioactive fallout, the HEPA filtration technology has been around for some time. Declassified in the 1950s, HEPA filters have since been used to reduce pollution and fumes-related incidents, and have even been put inside vacuum cleaners.

How do they work?

HEPA filters implement three stages of filtration to achieve their high standard of air purification. This consists of impaction, interception and diffusion. The majority of particles are caught in the first stage of filtration; impaction. Too big to fit through the gaps in the filter, these larger particles are caught in its net. But particles smaller than 0.1 microns are not so easily caught. At this miniscule size, the particles follow the movement of air – or at least they try to. In the same way that you might smash into a wall if you tried to run around a tight corner too fast, these escaping particles are too heavy to follow the air through the maze of the filter. Unable to make these tight turns, they end up getting stuck in the fibrous wall. The remaining particles are caught thanks to diffusion. These escaping particles move in zig-zag patterns instead of straight lines , so they, too, get stuck on the filter walls.
Many media outlets incorrectly state that HEPA filters can only filter particles down to 0.3 microns. The truth is that 0.3 microns is the most penetrating particle size. Particles bigger and smaller than this are easily captured by HEPA filtration; in fact, the HEPA filters are most efficient at capturing smaller particles. COVID-19 particles fall within this bracket at 0.12 microns, meaning they can be easily removed by the HEPA process.


The way the cabin is designed makes the transmission risk much lower than in other forms of transport. All passengers face the same way and seat backs act as a barrier between passengers – but this would be pointless if the air flowed over people’s heads before recirculation. Thankfully, air is circulated top to bottom within the cabin and replaced regularly, so you will never be breathing in the same air as ‘sneezing Sally’ sitting in 21B. The cabin air is also replenished once every two to three minutes, with a 50/50 mix of filtered and fresh air. To put this in perspective, hospitals replenish the air every 10 minutes, and offices only replace air once every 20 minutes.

The combination of HEPA filtration and air circulation is extremely effective at eliminating airborne contaminants. Experiments using aerosols show that tracer particles only remained detectable in the air for a maximum of six minutes. This, according to US Transportation Command, makes cabin air of a higher quality than the design standards for a patient isolation room or a hospital operating room – and, quite possibly, the safest air you can breathe indoors.

Airbus, Boeing and Embraer have all conducted extensive research since the outbreak of the virus, with the aid of computational fluid dynamics, to see just how true this is. Their results show that masked passengers sitting side by side on a plane are less likely to catch/transmit the virus than people standing two metres apart in a supermarket or classroom.

What other measures are in place to keep passengers and crew safe?

1.    Middle seat empty policy

The middle seat empty policy is not used by every airline; however, operators that do leave the middle seat empty reduce the risk of transmission even further. Arnold Barnett’s 2020 paper COVID-19 risk among airline passengers claims that your chance of catching COVID-19 on a two-hour domestic flight is one in 3,900 on a completely full flight. This figure dwindles to one in 6,400 when the middle seat is left empty. Although this measure may not be practical on busy routes, it is an easy solution on flights with few passengers – as is being seated in a more spacious section of the cabin, such as business class.

2.    Masks

As I’m sure every reader knows by now, masks are not just for superheroes. A simple piece of fabric reduces the chance of transmission significantly; however, three-layer masks are considered to be the safest kind of face covering. But no matter the kind, masks should be replaced every few hours of continuous wear. While researchers have not agreed on a hard time limit, it has been shown that, after four hours of continuous wear, the bacterial filtration efficacy of a surgical facemask becomes compromised. By this time, you will probably also have condensation on the inside of the mask, leaving you with little more than a soggy piece of cloth on your face. So, if you’re travelling for more than four hours, it is always prudent to make sure you have a spare mask handy. They are now sold in every supermarket, or incredibly easy to make at home. Wearing a mask is the easiest way to stop the spread of the virus and, potentially, save someone’s life.

3.    Regular cleaning

EASA has now mandated that all aircraft should be disinfected at minimum intervals of 24 hours, unless the aircraft has not been used since the previous cleaning. Additionally, aircraft should be disinfected before and after each long-haul flight, with chemicals approved by the national authority or the European Centre for Disease Prevention and Control.