Most commercial aircraft in service recirculate 50 percent of the air delivered to the passenger cabin for improved control of cabin circulation, humidity and fuel efficiency.

CNN: Commentary: Flu and your health on a plane

"There's only so much swelling brain can tolerate until there's pressure on the nerves that control breathing and circulation, " said Dr. Roger Bonomo, director of stroke care at Lenox Hill Hospital in New York.

CNN: STORY HIGHLIGHTS

The US Federal Reserve has a system to control the amount of currency in circulation.

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The speculation is that something went wrong with the circulation of this drilling mud such that it could no longer control the reservoir pressures.

FORBES: Blowout Suspected in Transocean Rig Explosion