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, assuming that 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 DOD officials involved in the study were careful to note those limitations but 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 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 infection with a virus that incubates over several days. The research that has been completed tends to date back to March, before the wearing of masks was widespread.

United Airlines, which donated flight time for the mannequin study, was less circumspect than the DOD officials, hailing the new research as “landmark.”

“Your chances of COVID exposure on a United aircraft are nearly non-existent, 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 on board Boeing 777 and 767 jets. Researchers placed a mannequin both wearing a mask and unmasked in different places around the planes and released fluorescent particles designed to mimic the virus. The tests were conducted both on the ground and in the air. In all, the study ran 300 different 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 faster than in a typical home and 5 or 6 times faster than is recommended for hospital operating rooms and patient isolation rooms.

The results assume continuous mask wearing and a low number of infectious people onboard the plane. The study did not examine the risk presented by plane lavatories and did not account for people moving about onboard, in airport lounges and jetways.

Researchers looking at real-world cases have identified examples of likely transmission on board 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.

Almost all the published studies date back to the earliest days of the virus’ spread before mask-wearing was common, although 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 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, 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 human and financial resources to trace, interview and test hundreds of passengers from a flight have been lacking,” they wrote. “Governments and the very large travel industry may face economic and political considerations in supporting overly detailed investigations.”

The researchers tentatively concluded that strict use of masks offers protection and said more studies to quantify the risk when they’re 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 it was critical for planes’ air filters to continue to run even when they’re on the ground and that boarding passengers in small groups to maintain social distancing is likely 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, which all 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, classroom or a store.

The Defense Department has a long-standing interest in how to safely move infectious people around the globe. After the West Africa Ebola outbreak that began in 2014, the military developed a system it called the Transport Isolation System, which uses large containment chambers in the back of a C-17 cargo plane.

The system was first used in April to transport three patients who had tested positive for the coronavirus from Afghanistan to an air base in Germany.

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