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How plane air circulation works.
https://diqn32j8nouaz.cloudfront.net...n_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...517#post173897
https://www.homemadetools.net/forum/...797#post183189
https://www.homemadetools.net/forum/...516#post189416
https://www.homemadetools.net/forum/...616#post216049
Does outside air really circulate in the cabin? With greatly reduced partial pressure of oxygen at 35,000 feet, shouldn’t everyone get hypoxic?
Cabin Pressure is typically around 8.000 feet for most legacy aircraft and with the Boeing 787 it was "decreased" to a much more comfortable 6,000 feet along with increased humidity. Oxygen content is replenished with pre-heated, compressed outside air during flight.
Regarding the "Antique" aircraft, like the Boeing 727, I once worked with a man who as an airline mechanic had to service those systems back in the day when smoking was legal on flights. He said crawling back where they were to replace the standard for the time, paper elements was a disgusting task. After a few thousand flight hours, that's only one year of airline service, those filters would be literally dripping with tobacco tar and nicotine. It disgusted him... and he was a chain smoker!