How to remain conscious and alert in high altitude flying is taught airplane pilots with the aid of a sealed low-pressure tank that duplicates atmospheric conditions at 35,000 feet. Before entering the tank for a practice “flight” the crews must “wash” all the nitrogen out of their blood, which requires a half hour of mild exercise while breathing pure oxygen. Otherwise, when they reach thin air, this gas would expand into tiny bubbles in the blood vessels, causing a painful disease called aeroembolism - the aerial equivalent of a diver’s “bends.” Once denitrogenized, the fiyer continues wearing his oxygen mask even at sea level to avoid breathing new nitrogen. In the strato-chamber he undergoes “stratosphere conditioning” at greatly reduced air pressure, where he develops confidence for journeys into the stratosphere. Without an oxygen mask, the average man can retain consciousness up to about 18,000 feet. If his oxygen supply is cut off at 20,000 feet, the flyer will pass out in 10 minutes; at 25,000 feet, in three minutes, and at 29,000 feet, consciousness is lost in less than one minute, with complete coma soon after. Above 35,000 feet, a mask may furnish enough oxygen, yet breathing becomes difficult because the rarefied atmosphere doesn’t supply enough lung pressure. With present-day oxygen masks, 40,000 feet seems to be the ceiling of human endurance. To fly at higher levels, sealed cabins or individual pressure suits might be used. The test chamber is used at Boeing Aircraft Company.