https://www.zerohedge.com/technology/number-fume-poison-events-airplanes-soaring-what-you-should-know
"
most planes, about half the air in the aircraft is recirculated. That refers to air that's already on the plane that's breathed in and exhaled by passengers with the remaining oxygen filtered and then pumped back inside the cabin. The other half is pulled from outside via the aircraft's engines using a system known as "bleed air." Engines are used for air supply on every modern aircraft, with the exception of Boeing's 787..."
