FAA did a study in 2011 that only covered flights up to 5 hours and wanted to do a more expansive study for longer flights, but was not funded. The study found that those with cardio-pulmonary issues as well as the elderly were at increased risk from flying. Not surprising as POCs are regularly required by pulmonary docs, what was surprising was that these people were asymptomatic during flight, but suffered post flight. HAST test rather than just 6 min walk may be more appropriate pre-flight test. My wife, who at the time had PLCH and mild PH, and used her POC occasionally on the ground and through her entire 9-10 hr flight at 2L, experienced severe breathing issues upon landing, which have been chronic. No other cause but the flight. Did not feel need to test her Sats during the flight as was asymptomatic. Likely airlines knew of FAA study as Boeing built an aircraft for longer flights that reduced cabin pressure from 8000 ft to 6000 ft. Never been further FAA studies, but should be.