A Defense Department study of the risk of catching the coronavirus on a packed commercial flight concluded that a person would have to be sitting next to an infectious passenger for at least 54 hours to receive a dangerous dose of the virus through the air.
Researchers concluded that, if passengers wear surgical masks continuously, very little of the virus spreads, because of how the air is circulated and filtered on the planes.
The study, which used a mannequin expelling simulated virus particles to determine how the virus spreads as an aerosol inside an aircraft cabin, had some limitations. But it offers a new way to try to understand the risks of flying during the pandemic.
In a briefing Thursday, the scientists and Defense Department officials involved in the study were careful to note those limitations, but they said the results were encouraging.
“Within the scope of the test, the results showed an overall low exposure risk from aerosolized pathogens like covid-19 on these aircraft,” said Vice Adm. Dee Mewbourne, the deputy commander of the U.S. Transportation Command.
Significantly, the study did not examine the risk posed by the virus spreading in larger droplets that people can spread when eating or talking. Nor did it look at risks involved in getting to the airport and waiting to board a plane.
There have been few studies looking at real-world cases, with scientists hampered by limited testing and contact tracing and the difficulty of pinning down the moment of infection with a virus that incubates over several days.
The research that has been completed tends to date to March, before the wearing of masks was widespread.
United Airlines, which donated flight time for the mannequin study, was less circumspect than the Defense Department officials, hailing the new research as “landmark.”
“Your chances of COVID exposure on a United aircraft are nearly nonexistent, even if your flight is full,” Toby Enqvist, the airline’s chief customer officer, said in a statement.
The research was led and funded by Transportation Command, which operates Patriot Express, a program that uses commercial planes to transport members of the military and their families. The command wanted to determine the risks of doing that during the pandemic.
The study, run at Dulles International Airport, was carried out in late August aboard Boeing 777 and 767 jets. Researchers placed a mannequin, wearing a mask and unmasked, in different places around the planes and released fluorescent particles designed to mimic the virus. The tests were conducted on the ground and in the air. In all, the study ran 300 tests.
The researchers, which included a team from the University of Nebraska, concluded that the virus was removed by the plane’s air filtering systems 15 times as fast as in a typical home and five or six times as fast as what is recommended for hospital operating rooms and patient isolation rooms.
Using mannequins gives researchers a good deal of control and can compensate for the difficulties of studying real-world cases, but it also requires looking at fairly simple scenarios.
“You take the element of human behavior out,” said Lauren Sauer, director of research for the Johns Hopkins Hospital biocontainment unit.
The results assume continuous mask-wearing and a low number of infectious people aboard the plane. The study did not examine the risk presented by plane lavatories and did not account for people moving about on board, in airport lounges and on boarding bridges.
But passengers talking, for example, could have a significant effect on the exposure risks.
Linsey Marr, an engineering professor at Virginia Tech and an expert on air quality, said the rate at which the researchers assumed passengers would shed the virus was consistent with breathing but that someone talking might release 10 times more. Nonetheless, she called the findings “reassuring.”
“It shows the risk is quite low if people are not talking,” she said.
Researchers looking at real-world cases have identified examples of likely transmission aboard aircraft. In one case, a woman traveling from London to Hanoi in March appears to have infected as many as 15 other passengers and crew members.
In one case, South Korean researchers believe a woman might have been infected when she removed a protective mask while using an airplane bathroom.
The Centers for Disease Control and Prevention has said it has investigated about 1,600 cases of infectious people traveling, identifying some 11,000 others who were exposed to the virus as a result. But the agency has not been able to confirm a case of transmission, saying that pinpointing the exact moment someone was infected is tricky and that contact-tracing information is sometimes incomplete.
The CDC continues to say that air travel presents some risk because it involves being in close quarters with other people and encountering frequently touched surfaces.
In a recent review of studies, David Freedman, an infectious-disease specialist at the University of Alabama at Birmingham, and Annelies Wilder-Smith, a professor at the London School of Hygiene and Tropical Medicine, said the research remains limited across the globe, writing that “the opportunities for rigorous studies have been few.”
The researchers tentatively concluded that strict use of masks offers protection and said more studies to quantify the risk when they are worn should be a priority.
The team that conducted the mannequin study recommended continued use of masks and additional cleaning to guard against transmission from large droplets and surfaces. They said that it was critical for planes’ air filters to continue to run even when they are on the ground and that boarding passengers in small groups to maintain social distancing is probably beneficial.
Mewbourne, the Transportation Command official, said his team would review several policies in light of the study’s findings and might make changes to how full the Defense Department allows flights to be and tweak its contact-tracing and quarantining rules. But no decisions have been made.
The airline industry has sought to convince people that flight is safe since the virus began spreading.
Last week, the International Air Transport Association said it was aware of only 44 potential cases of the virus spreading during a flight, comparing that figure with the 1.2 billion passengers who have traveled since the beginning of the year.
“We think these figures are extremely reassuring,” David Powell, the organization’s medical adviser, said in a statement.
To back up the point, IATA highlighted Freedman and Wilder-Smith’s review. But in an interview, Freedman said he took issue with how the organization presented the data, saying the 44 cases needed to be compared with the far smaller number of passengers who were carefully tested for the virus.
“It’s just bad epidemiology,” he said. “It’s just bad math.”
IATA also pointed to new studies by Airbus, Boeing and Embraer, all of which make passenger jets, that it said underscored how planes’ cabin air filters limit the spread of viruses. The companies used a technique called computational fluid dynamics to simulate how virus particles move around the plane.
Engineers at Boeing and Airbus concluded that the risk of being exposed to the virus by someone seated next to you on an aircraft was about the same as from someone six or seven feet away in an office, a classroom or a store.