thornhappy
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Joined: 12/16/2006
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Looks like there are a lot of misperceptions regarding air flow and oxygen levels on commercial jets. Patrick Smith (a commercial pilot) addresses this in his Ask A Pilot column for Salon: http://www.salon.com/tech/col/smith/2009/09/25/askthepilot335/
"On modern aircraft, the rate and volume of airflow is pretty much automatic. On the Boeings that I fly, we have direct and accurate control over temperature, but only indirect control over flow. If you asked me to please "cut it back to 20 percent," I would politely inform you that this is impossible. The switches are set to automatic mode prior to flight, and the packs more or less take care of themselves. So long as both engines are turning and everything is operating normally, the flow is perfectly adequate. Only when there's a malfunction (an overheat, a recirculation fan failure, or some other glitch in the plumbing) are the settings changed. In over two years of flying 757s and 767s, this is something I have done exactly twice.
On the MD-80 that I previously flew, designed by McDonnell Douglas, we had two pack modes determining airflow: on and off. On is where they stayed. Even with two of them, shutting down a pack completely is a fairly big deal; it is not something done for fuel savings.
I am not as familiar with Airbus models, but let's talk to somebody who is."Airbus series aircraft, from the A320 through the much larger A380, do provide a way for pilots to vary airflow," says Dave English, an A320 captain. "But not in the way characterized by the Economist."
English explains that the Airbus controllers have three positions, labeled HI, NORM and LO. "Almost all the time you're in the center NORM position, which is automatic. The HI position is used when you need a rapid change in cabin temperature. The LO position does as the name implies. It provides some fuel savings, but they are minimal and this isn't used very often. Company guidance is to use LO whenever the passenger load is below a hundred. It's not a big change. Sitting in the cabin, it's almost impossible to notice the difference."
An Airbus pilot for United with whom I spoke says that his carrier requests switching to LO if the airplane is less than 60 percent full. He adds, "We rarely ever move the switch out of automatic.""
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Air quality in commercial aircraft - 
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