If it doesn't turn out to be a hoax, a 15-year-old boy probably defied long odds against death from hypoxia (oxygen shortage) and cold by a fluke of the conditions he faced in the wheel well of the jet where he stowed away from California to Hawaii, according to an expert on altitude and health.

The Santa Clara runaway "got cold enough to protect his brain, but not cold enough to stop his heart," said Peter Hackett, director of the Institute for Altitude Medicine in Telluride, Colorado, who lectures world-wide on the effects of altitude on the body. He almost certainly spent the trip unconscious, with perhaps just enough heat in the wheel well to endure temperatures of 80 degrees below zero or worse, Hackett said.

"The body temperature drops and that cools the brain, and neuronal activity is suppressed. So the brain shuts down--it goes into what is almost like hibernation, without causing irreparable damage. And then, when it warms up, you can be normal." It's fairly common nowadays for doctors to cool the brain to protect it from damage after heart attacks, he said.

Indeed media reports say the young man emerged from the wheel well about an hour after the plane landed in Maui, conscious but unharmed, to the shock of the ground crew.

Hackett said the stowaway would have been breathing 5 percent oxygen at 30,000 feet, rather than the 21 percent his body was accustomed to at sea level. By 26,000 or 27,000 feet, he would have lost consciousness, if he hadn't earlier. Exposure to the cold for much longer than the 5 1/2 hours of his flight over the Pacific probably would have killed him, Hackett said.

Update 5:25 p.m.: Hackett said that if the plane reached 38,000 feet, as reports suggest, the teen-ager would have just 15 percent of the oxygen he was accustomed to at sea level.

"Chances are you're going to die. In fact it's happened before," Hackett said.

According to a 2003 Slate post, few people survive as wheel well stowaways. In 2000, there were 13, according to the FAA, three of whom lived. In 2001, all six people who tried to enter the United States that way died. In 2002, five perished and one survived.  The overall wheel well survival rate at that time was 20.3 percent, and that doesn't include anyone who might have tumbled out over water or remote areas and died without being counted.

However, Fidel Maruhi, a native of Tahiti, survived a 7 1/2 flight from Papeete to Los Angeles, according to Slate.  When he was discovered, Maruhi's body temperature was just 79 degrees, about 6 degrees lower than what is usually considered fatal. (Normal body temperature is 98.6 degrees Fahrenheit).

In 1996, brothers Pardeep and Vijay Saini stowed away on a jet from Delhi to London. Pardeep survived the 11 1/2 hour flight, but his brother died.

A positive sign for the teen's prognosis, Hackett said, is the fact that he apparently regained consciousness. "I think he'd be fine," he said. "...Prolonged unconsciousness would indicate he wasn't going to do well. But if he wakes up after that insult, he's going to be fine."