Mount Everest: What happens to your body at extreme altitudes?

Eleven climbers have died on Mount Everest this season, the highest number since 2015.

The high number has been blamed on traffic jams at the summit of the mountain — leading to an extended period of time in the so-called Death Zone.

Lineups heading to and from the peak mean climbers have to wait on treacherous ledges in an inhospitable climate.

But experts say longer times in higher altitudes when you’re not used to it can slow do neurological functions in your brain and cause your body to be more tired — making it harder to climb the mountain.

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“I honestly thought I was going to die up there. I could barely move. Every step (was) a struggle for hours and is dangerous. I mean, you’re on ledges 20 centimetres wide and you make one false step and you’re going to fall,” said Chris Dare, a dentist with the Canadian Forces and mountaineer who made the ascent last week.

Dare told Global News what should have been a six-hour climb took 12 hours, and then he got caught in 60 kilometre-per-hour winds and temperatures below -50 C on the descent.

The extra time in the Death Zone means more chance of developing an extreme form of altitude sickness called cerebral edema, explained Paul Firth, an avid climber who is also a doctor at Massachusetts General Hospital.

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“A common feature of this extreme altitude is neurological dysfunction — when people get confused” Firth explained. “And once you get confused and disorientated, you can’t rescue yourself.”

That’s when fluid leaks into your brain, which can cause confusion and clumsiness — a deadly combination when on treacherous paths where every step counts.

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Here’s how higher altitudes affect your body and brain

Acute altitude sickness can start at 2,500 metres above sea level. (For comparison, Toronto is at 76 metres above sea level, while Calgary is at 1,045 metres above sea level.)

Symptoms include headaches, nausea, dizziness, fatigue and weakness.

Humans need a few days to acclimatize themselves to the high altitudes.

At 3,600 metres, more severe symptoms can occur, and the acute altitude sickness can change into cerebral or pulmonary edema — where fluid leaks into your lungs or brain, respectively.

At 5,000 metres, there’s about half as much oxygen in the air than at sea level. That means you’re breathing speeds up to compensate.

At 8,000 metres, climbers begin to use oxygen tanks to supplement their oxygen supply. Firth says this is the altitude where cerebral edema is more common — and the most dangerous.

The way to treat cerebral edema is to descend the mountain, Firth explained. But bottlenecks and traffic jams exasperate the symptoms.

“With those bottlenecks, people weren’t able to descend,” Firth said. “If you can’t get people down a mountain fast, you know, your problems will progress.

“The fact that it was so crowded, I think, contributed to some of these deaths of people not being able to be rescued.”

Firth says it’s important for people to recognize the signs of cerebral edema and altitude sickness and turn back when they know they’re being affected.

But he recognizes that’s a hard thing to do when you’re so close to a goal.

He himself attempted to summit Everest in 2004 but had to stop at around 8,000 metres because he recognized he wasn’t climbing fast enough.

“It’s very difficult to do sort of turn back when you still have reserves,” Firth said.

“It’s a lot harder not to climb Everest than it is to climb Everest.”

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Most climbers who die on Everest die on their descent, Firth said.

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“What happens is people shoot up (the mountain) quite rapidly,” Firth explained. “They’re not acclimated and then they get sickness after a period of time.”

A study published in 2008 noted that over 50 per cent of climbers died on the descent. The study noted that the cause of death for sherpas, the local people who guide mountaineers up Everest, was more likely to be due to hazards like avalanches.

*with files from Simon Little