How Viruses Spread in Aircraft — and the safest place to sit

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The World Health Organization defines contact with an infected person as being seated within two rows of one another.

But people don’t just sit during flights, particularly ones lasting longer than a few hours. They visit the bathroom, stretch their legs, and grab items from the overhead bins. In fact, during the 2003 coronavirus outbreak of the severe acute respiratory syndrome (SARS), a passenger aboard a flight from Hong Kong to Beijing infected people well outside the WHO’s two-row boundary. The New England Journal of Medicine noted that the WHO criteria “would have missed 45 percent of the patients with SARS.”

Passengers in window seats have the lowest likelihood of coming in contact with an infected person (see figure below).



Inspired in part by that case, a team of public health researchers set out to study how random movements about the airplane cabin might change passengers’ probability of infection.

The “FlyHealthy Research Team” observed the behaviors of passengers and crew on 10 transcontinental U.S. flights of about three and a half to five hours. Led by Emory University’s Vicki Stover Hertzberg and Howard Weiss, they not only looked at how people moved about the cabin, but also at how that affected the number and duration of their contacts with others. The team wanted to estimate how many close encounters might allow for transmission during transcontinental flights.

“Suppose you’re seated in an aisle seat or a middle seat and I walk by to go to the lavatory,” says Weiss, professor of biology and mathematics at Penn State University. “We’re going to be in close contact, meaning we’ll be within a meter. So if I’m infected, I could transmit to you…Ours was the first study to quantify this.”

As the study revealed in 2018, most passengers left their seat at some point—generally to use the restroom or check the overhead bins—during these medium-haul flights. Overall, 38 percent of passengers left their seats once and 24 percent more than once. Another 38 percent of people stayed in their seats throughout the entire flight.

This activity helps pinpoint the safest places to sit. The passengers who were least likely to get up were in window seats: only 43 percent moved around as opposed to 80 percent of people seated on the aisle.

Accordingly, window seat passengers had far fewer close encounters than people in other seats, averaging 12 contacts compared to the 58 and 64 respective contacts for passengers in middle and aisle seats.

Choosing a window seat and staying put clearly lowers your likelihood of coming into contact with an infectious disease. But, as you can see in the accompanying graphic, the team’s model shows that passengers in middle and aisle seats—even those that are within the WHO’s two-seat range—have a fairly low probability of getting infected.

Weiss says that’s because most contact people have on airplanes is relatively short.

“If you’re seated in an aisle seat, certainly there will be quite a few people moving past you, but they’ll be moving quickly,” Weiss says. “In aggregate, what we show is there’s quite a low probability of transmission to any particular passenger.”

The story changes if the ill person is a crew member. Because flight attendants spend much more time walking down the aisle and interacting with passengers, they are more likely to have additional—and longer—close encounters. As the study stated, a sick crew member has a probability of infecting 4.6 passengers, “thus, it is imperative that flight attendants not fly when they are ill.”


What does it mean for the new coronavirus?

As Weiss points out, we don’t know yet the preferred way that the new coronavirus transmits. It could be primarily through respiratory droplets, physical contact with saliva or diarrhea followed by oral consumption of viral material, or perhaps even aerosols.

He notes that this model doesn’t include the transmission of aerosols, though the FlyHealthy team hopes to research this topic in the future. In the study, the researchers also warn that this model cannot be directly extrapolated for long-haul flights or airplanes with more than one aisle.

Landon agrees that we don’t yet know how the coronavirus transmits, but believes the results of this study are applicable. All previous coronaviruses have transmitted through droplets, she notes, so it would be unusual if this new pathogen was different. And indeed, the new coronavirus is behaving much like SARS in many respects. Both are zoonotic, meaning they started in animals before jumping to humans, and both appear to have started in bats. The pair also transmit from human to human and have a long incubation period—up to 14 days for the Wuhan coronavirus, compared to about two for influenza—which means that people might be sick and transmitting the disease before symptoms show up.

With all that in mind, Landon suggests following the CDC guidance for infectious diseases when you’re on an airplane.

That includes washing your hands with regular soap or using an alcohol-based hand sanitizer after touching any surface—especially since there’s evidence that coronaviruses last longer on surfaces than other illnesses, around three to 12 hours.

You should also avoid touching your face and contact with coughing passengers by whatever means possible.


What’s worse, the coronavirus or influenza?

There are many ways to estimate the risk posed by a disease, but let’s focus on two numbers often used by public health researchers: the reproduction number and the case-fatality ratio.

The reproduction number—R0 or “r naught”—simply refers to the number of additional people that an infected person typically makes sick. Maia Majumder, a faculty member at Boston Children’s Hospital and Harvard Medical School, has been tracking exactly that.


Researchers from Germany are to examine the way virus particles spread in the passenger cabins of aircraft and trains using experiments and computer simulations to help fight the Covid-19 pandemic.

The research looking at virus spread in aircraft cabins is soon to start in a new laboratory at the German Aerospace Center (DLR) Institute of Aerodynamics and Flow Technology in Göttingen and is part of an EU project called Advent.

Head of the Institute Andreas Dillmann said, “Up until now, our focus has been on the comfort of passengers and the energy requirements of the air-conditioning system.

“We can now use the scientific tools that we have developed to research the spread of viruses within passenger cabins.”

The tests introduce a sick passenger into a fully occupied area. They then look at how far exhaled particles are distributed.

Computer simulations of a section of the cabin are first run where the sick passenger’s exhalation or coughing pattern is generated using a program that is usually used to simulate cabin airflow. This is supplemented by the addition of aerosol particles, which then atomize and evaporate.

For coughing, the atomization process has clear parallels with the process of fuel injection into an engine, where strong shear forces cause the droplets to disintegrate, said DLR.

The parameters used, such as the exhaled lung volume of around one to 1.5 litres and the size of the droplets, which range from smaller than one micrometre to several hundreds of micrometres, are derived from studies conducted by the FAA.

The computer program calculates the distribution and range of the particles and provides a graphical representation of their propagation.

Rolf Henke, the DLR executive board member responsible for aeronautics research said, “Aircraft cabins are self-contained systems and already have a high level of air quality control. Our research on virus spread in cabins is intended to help protect passengers from infections and find ways of making flying safe in future.”

At the same time, researchers at the generic train laboratory in Göttingen are replicating a similar situation in an experiment. Here, 24 mannequins fitted with sensors serve as the passengers. A ‘sick’ mannequin releases air with added droplets and a tracer gas from its mouth area. High-speed cameras and gas sensors track the spread of the particles within the cabin. The particles and their concentration are recorded at various points within the space.

The DLR researchers in Göttingen are investigating the spread of physical particles that are analogous to virus-laden droplets. Meanwhile, researchers elsewhere including at the DLR Institute of Aerospace Medicine in Cologne, are studying the infectiousness of the droplets and the effect of aircraft air filters.

Preliminary results from the recently initiated research studies are expected in the coming weeks. However, some of the experiments are set to continue for months. All of the findings will be published and made available to partners in industry, said DLR.

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