(CNN) — A new study released Thursday suggests that people don't need to worry about circulating air spreading coronavirus on airplanes.
The US Department of Defense study supports earlier research showing the ventilation systems on aircraft filter the air efficiently and take out particles that could transmit viruses.
The study, which was released without peer review, did not take into account other ways that people could catch the virus on aircraft -- including from others coughing or breathing directly on them, from surfaces or from confined spaces such as restrooms.
The US Transportation Command, The Defense Advanced Research Project Agency (DARPA) and the Air Mobility Command used Boeing 777-200 and 767-300 aircraft loaded up with sensors meant to duplicate the effect of a fully loaded passenger flight.
A dummy wearing a surgical mask simulated a coughing passenger infected with a respiratory virus.
The team used fluorescent aerosol tracers to see where particles emitted from the coughing "passenger" went. They were sucked quickly into the ventilation system, the team concluded, and were unlikely to contaminate nearby surfaces or blow into the breathing zones of people seated nearby.
"Testing assumes that mask wearing is continuous, and that the number of infected personnel is low," the research team wrote. "Contamination of surfaces via non-aerosol routes (large droplets or fecal contamination) is more likely in lavatories and other common areas and is not tested here," they added.
"These alternative routes of exposure are more challenging to predict because of uncertainty in human behavior."
Other reports have found people became infected with coronavirus on flights, perhaps when they took off masks to use restrooms.
"Testing did not include substantial movement throughout the plane or in the airport, lounge or jetway, where air change rates and human interactions will vary," the researchers added.
"Similarly, the mannequin remained facing forward, uncertainty in human behavior with conversations and behavior may change the risk and directionality in the closest seats to an index patient, especially for large droplets."
A lot left to learn
Much is still unknown about Covid-19 transmission aboard planes. Two previous studies documented real-life cases of suspected transmission aboard flights.
Both studies involved cases connected to long flights early in the pandemic, before airlines began requiring face masks.
Another study documenting a case of suspected coronavirus transmission aboard a flight involved a woman who wore an N95 mask throughout her flight except when she used the lavatory.
A passenger seated three rows away who developed coronavirus but who never had symptoms also used the lavatory.
The US Centers for Disease Control and Prevention notes that "most viruses and other germs do not spread easily on flights because of how air circulates and is filtered on airplanes," adding that the lack of social distancing on crowded flights that may require passengers to sit within six feet of others for long periods "may increase your risk of getting Covid-19."