A brain swelling condition related to low oxygen levels in the air may have caused many of the deaths of people climbing Mount Everest, researchers said on Tuesday.

An international team led by Paul Firth of Massachusetts General Hospital studied the 212 reported deaths from 1921 to 2006 on Mount Everest, the highest on Earth.

Hazards awaiting those who dare to climb the 29,000-foot (8,850 meter) Himalayan mountain include extreme cold, whipping winds, changing weather, treacherous climbs and avalanches. Oxygen content in the air is only a third of that at sea level.

"Nobody was attacked by any Yeti or anything else," Firth said, referring to the "abominable snowman" of legend.

Firth said that while the cause of some deaths could not be determined with certainty, many appeared to have been the result of high-altitude cerebral edema.

In this condition, low oxygen levels cause cerebral blood vessels to leak fluid into surrounding brain tissue, triggering swelling. Confusion and loss of coordination follow.

Many deaths occurred above 26,000 feet in an area dubbed "the dead zone," particularly among people who already reached the summit and were climbing back down.

"Of the guys who died up at 8,000 meters (26,000 feet), a large number of them were developing neurological symptoms. In other words, they were getting confused, comatose or they were having a loss of coordination," said Firth, whose findings appear in the British Medical Journal.

This seemed consistent with high-altitude cerebral edema, he said in a telephone interview. "If you go too fast and you haven't adapted to the low oxygen levels, then you can get various types of high altitude illness."

He had expected to find more lung problems but in fact they were rare.

The researchers speculated that many of the deaths attributed to falls or the person vanishing during the climb may have been due to high-altitude cerebral edema.

The study showed that 1.3 percent of mountaineers who climbed above their Everest base camp died.