TORONTO, ON. – For the first time ever, an international team of experts has probed every known death on the world's tallest mountain, shedding some light on what makes Mount Everest one of the most dangerous places on earth.

The team's surprising findings shatter commonly-held beliefs about the prevalence of deaths caused by avalanches, falling ice and pulmonary oedema (lung problems) and highlight severe weather deterioration as a major factor in deaths.

"We know that climbing Mount Everest is dangerous because more than 200 people have died trying to scale it, but never before has anyone studied these deaths with such a collaborative or fine-tooth approach," says University of Toronto Mississauga Physics professor Kent Moore, one of two U of T co-authors on the report. "We now know with much more certainty what factors play a major role – and which factors do not."

The team, made up of North American and British experts in medicine, physiology and meteorology, examined 212 reported deaths on Everest between 1921 and 1996. Among their key findings, published online now in the British Medical Journal:

• Over the 86-year-period, the mortality rate above base camp on Mount Everest was 1.3 per cent. This is significantly higher than that of other tall mountains.
• For deaths above 8,000 metres, cognitive impairment and cerebral oedema (swelling of the brain) were often present where pulmonary oedema (lung problems) was not.
• Marked fatigue, late summit times and the tendency to fall behind companions were common among those who died on Mount Everest.
• Most sherpas (natives of Nepal who act as guides) were killed on the lower slopes, while most climbers died above 8,000 metres, usually while descending.
• A typical expedition to Everest usually last at least 60 days, but more than 80 per cent of climber deaths happened the day of the summit attempt or the day after.
• A severe deterioration in weather played a role in 25 per cent of all deaths.

"Because of our combined diversity and experience, we've really been able to pinpoint the most critical factors that cause death on Mount Everest," says Dr. John Semple, Professor of Surgery at U of T. "These findings will provide a foundation of improved safety for both mountaineers and sherpas."