The study, which was conducted for the Department of Defense, found that high air exchange rates, the filtration system on planes, and downward ventilation reduced the risk of transmitting the virus through the air by 99.7%, if passengers are all wearing masks.
The research team included members from United Airlines, Boeing, and the University of Nebraska’s Medical Center and National Strategic Research Institute. The study was prepared for two military agencies that use planes to move both people and cargo, the U.S. Transportation Command and the Air Force’s Air Mobility Command.
Researchers used fluorescent tracer aerosols to trace and study the droplets released when breathing, coughing, or sneezing. They studied over 11,500 “breathing zones” throughout the plane and found the risk of contamination on an airplane is less than the risk in private residences. This is largely because of airplanes’ filtration systems and the high air exchange rate. On average, the air exchange rate on a plane is every three minutes. Only 25% of the air is recirculated, and the other 75% comes from outside the plane.
Researchers did account for mask mandates in this study, as the risk of spreading COVID-19 increases when passengers do not correctly comply with them.
Researchers looked at infection rates of passengers seated in the same row and nearby cabins on Boeing 767 and 777 planes. These are larger planes used for longer flights, where it was anticipated the virus would spread more easily through the recirculated airplane air.
These new findings contradict earlier studies. Two studies published in the scientific journal Emerging Infectious Diseases found there was a considerable risk of spreading COVID-19 on airplanes. The studies looked at two long-distance flights and found that seating proximity was strongly associated with the risk of infection. Both of these studies looked at flights that occurred in March, however, and it’s unclear how many passengers were wearing masks at the time.
Though the Department of Defense study found the risk of contracting COVID-19 on a plane was low, it did not look at the risk of contracting the virus while waiting in the airport. Airports have significantly worse ventilation. The CDC advises travelers to be cautious in the airport, where they are likely to come in close contact with other travelers and frequently touched surfaces.
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