In the jet's wheel well where he apparently hid, the air became so thin that passing out was inevitable.
At the cruising altitude of 38,000 feet, the outside temperature would have been about 85 degrees below zero.
In response to those conditions heart rate and brain activity slow. But they can continue, and return to normal as the plane descends to conditions more hospitable for human survival.
Despite the hazards, people survive stowaway attempts with surprising frequency.
According to data kept by the Federal Aviation Administration, about 1 in 4 of the 105 known airplane stowaways have survived.