As airlines look to start flying again in earnest amid the Covid-19 pandemic, a key question for travellers is whether it remains safe to travel.
Passengers may be worried about everything from being infected by seats or tray tables, through to catching the coronavirus from aircraft ventilation systems that recycle air.
Evidence about the on-board infection risk from the new coronavirus is limited, but scientists better understand how other pathogens – including the coronavirus that caused Sars – spread on aircraft.
“Clearly if people are close together, that’s how viruses transmit, particularly in indoor environments," said Dr Andrew Freedman, an infectious diseases specialist at Cardiff University in the UK.
"If you have a full plane and they’re close together, there will be a risk. It’s not really airborne, it’s droplets spreading – if someone has a cough or is sneezing or breathing out viruses on to surfaces.”
Echoing this, Mary Wilson, an epidemiology professor at the University of California, wrote in the academic journal Travel Medicine and Infectious Diseases late last month that aircraft cabins could act as "a congenial environment for the transmission of viruses".
“As aircraft grow in size and duration of flights lengthen, exposures and transmission events inside these metal or composite-materials tubes in the sky will increase,” she said.
Research from 2018, part funded by Boeing, modelled virus spread on aircraft. Scientists looked at the coronavirus that caused Sars and found that it could potentially spread person to person by close contact or from the surrounding air. Touching contaminated surfaces created as much risk as these two factors combined.
With the new coronavirus, concerns over aerial contamination may be lower because the pathogen appears to be less transmissible than Sars.
Modern aircraft are also equipped with high-efficiency particulate air (Hepa) filters, which remove bacteria and virus-carrying particles from the cabin, something likely to reduce the risk.
In addition, air in cabins is constantly refreshed, with ventilation systems mixing about half recycled air with half fresh air taken from outside.
“Cabin air circulation is continuous. Air is always flowing into and out of the cabin,” the International Air Transport Association said in a 2018 briefing document, adding that cabin air filter plates can remove “virtually all viruses and bacteria”.
Also, the 2018 modelling suggested that, particularly with influenza, people were significantly more likely to be infected by someone sitting within two rows rather than further away.
These results suggest that comments this month by Jonathan Hinkles, chief executive of a Scottish airline, Loganair, that social distancing on aircraft was pointless because of the on-board recycling of air may be wide of the mark.
While certain airlines, such as the Irish low-cost carrier Ryanair, have been resistant to the idea of reducing passenger numbers per aircraft, saying it would destroy their economic model, some operators currently flying keep the middle seat empty to minimise spread.
Although this ensures people are further apart, the 2018 research showed that the greatest risk of infection from surfaces was in the aisle seat, which are most heavily contaminated because people brush against them or place their hands on them as they walk past.
Prof John Oxford, co-author of the textbook Human Virology, said "it makes sense for people to be nervous" about flying but that, if he were catching a flight, he would take measures to minimise the risk.
“I would try to get a window seat, as you’re away a bit more from people,” said Prof Oxford, emeritus professor of virology at the University of London.
Other measures being adopted by passengers and airlines include wearing masks. While these have little impact on the wearer’s infection risk, they may reduce spread to others.
Expert advice also includes wiping tables with hand sanitiser to destroy infectious virus particles.
“People are trying to go about their business and do things and they must, to some extent. We have to each weigh up what the risks, what the benefits are,” said Prof Oxford.
