Weeks before a federal judge’s ruling led U.S. airlines to drop mask mandates, airline executives argued masks no longer should be required – in part because air filtration systems on planes create “hospital-grade cabin air.”

Now that masks are optional on every major U.S. airline, experts said the filters that remove 99% of particles, including viruses, will help keep transmission on flights low but won't eliminate the risk of spreading COVID-19 that masks helped mitigate.