Neither falls nor avalanches cause as many deaths on Mount Everest as brain swelling. A study conducted by  Massachusetts General Hospital, University of Toronto and three British hospitals have revealed that brain swelling occurs at high altitude especially when the climbers are descending. 212 deaths of climbers were analysed before coming to the given conclusions:

- There are 1.1 deaths for every 100 climbing attempts by Sherpas or Tibetans

- 1.6 climbers die for every 100 non-Himalayan mountaineers

- The average deaths are 1.3 per hundred or one climber out of 77.

- In the last 25 years, a period when more climbers are reaching the summit, the death rate has more than doubled, to nearly three per 100 climbers.

- A severe deterioration in weather played a role in 25 per cent of all deaths.

The international team leader Paul Firth said that the cause of many deaths appeared to have been the result of high-altitude cerebral edema.

Cerebral edema is a condition which causes low oxygen levels that lead leakage in cerebral blood vessels fluid into surrounding brain tissue, triggering swelling. This results in confusion and loss of coordination.

Many deaths occurred above 26,000 feet in an area that is called "the dead zone”. This was prevalent particularly among those climbers who already reached the summit and were climbing back down.