As the travel industry continues to flounder during the COVID-19 pandemic, a new modeling report funded by an airline suggests that the risk of contracting coronavirus aboard an airplane is minimal, even during long-duration flights at full capacity.
But the report shouldn’t give flying a clean bill of health, since it oversimplifies human behavior and leaves out variables that could affect transmission risk.
The report was funded by United Airlines and the U.S. Department of Defense and is one of the largest to date testing aerosol transmission aboard planes. On October 15, the United Airlines Twitter account posted a link to the results of the simulation and wrote, “Your risk of exposure to COVID-19 is almost non-existent on our flights (yes, even on a full flight).”
The airline industry has cratered due to travel restrictions and lack of demand. In an effort to start making money again, United Airlines in particular has been at the forefront of trying out measures such as rapid tests and a digital "health pass."
Experts agree that the filtration systems of airplanes combined with mask mandates lower the risk of contracting the virus that causes COVID-19. But just how low are your chances of getting sick on a flight, really? Probably not as low as modeled by this study, which represents a "best-case scenario" that isn't likely to match reality, according to experts.
“I think we should have a healthy degree of skepticism, since it was funded by the industry which is trying to get us to fly,” said Linsey Marr, a civil and environmental engineering professor at Virginia Tech who studies aerosols. “But at the same time, I think the methods they used are sound, and the people who did it are careful researchers. The results seem reasonable to me—they are a best-case scenario.”
Researchers fed a nebulizer through the head of a mannequin to simulate an infected person breathing and released tiny fluorescent and DNA-tagged particles to measure their spread throughout the cabins of a Boeing 767 and 777 when the mannequin did and didn’t wear a surgical mask. They tested surface deposition of aerosols by placing small sheets of stainless steel at various high-contact surfaces throughout the cabin and simulated passengers with sensors mounted on tripods and heated blankets to mimic the thermal plumes humans emit. They ran experiments on the ground and at flying altitude and found extremely low levels of particles captured either on surfaces or “passengers.”
By their estimates, it would take a minimum of 54 hours of sitting next to a person with a symptomatic COVID-19 infection to be exposed to an infectious dose of the virus.
But two recent case studies suggest that the risk of coronavirus exposure on board flights was real, at least during the early stages of the pandemic. Both studies report clusters of cases that stemmed from long flights, though neither flight required masks be worn and the transmission dynamics of these clusters remain unclear.
The DoD-funded report assumes a best-case scenario that is likely at odds with human behavior, Marr said. For instance, the simulation did not account for talking and turning heads toward an index case, which could increase both the frequency at which viral particles are emitted and the likelihood of coming into contact with particles. The report also did not take into account airplane bathrooms and what the researchers referred to as the “uncertainty of human behavior.”
“As soon as you put in people who are talking, not just breathing, and they're moving around, those conditions could easily increase the risk by a factor of 10 to 100,” Marr said.
There are two factors working together inside the cabin of a commercial airplane to lower the risk of aerosol transmission: heavy-duty air filters and a high ventilation rate. Marr said that 737s and larger planes are equipped with HEPA filters that remove up to 99% of particles in the air, and airplanes exchange all the air in their cabins at least every two minutes, if not faster.
Marr said that there are still unknowns about the risks of boarding, deplaning, and spending time in terminals and on skybridges. But, she added, the report helps address the limited question of exposure risk during a flight.
“I think the risk of actually sitting on the plane itself is low," she said.
While we're still learning more about the virus, it's clear that flying isn’t a zero-risk behavior—very few things are—regardless of how convincing United’s Twitter account tries to be.