• Climbers who ascend higher than 26,000 feet on Mount Everest enter the "death zone." 
• In this area, oxygen is so limited that the body's cells start to die, and judgment becomes impaired.
• Climbers may also experience heart attack, stroke, or severe altitude sickness.

Human bodies work best at sea level. Down here, oxygen levels are adequate for our brains and lungs. At much higher altitudes, our bodies cannot function properly.

But if climbers want to summit Mount Everest, the tallest peak in the world at 29,029 feet (8,848 meters or 5.5 miles) above sea level, they have to brave what's known as the "death zone." 

This is the area with an altitude above about 26,000 feet (8,000 meters), where there is so little oxygen that the body starts to die, minute by minute and cell by cell.

In the death zone, climbers' brains and lungs are starved for oxygen, their risk of heart attack and stroke is increased, and their judgment quickly becomes impaired.

"Your body is breaking down and essentially dying," Shaunna Burke, a climber who summited Everest in 2005, told Insider in 2019. "It becomes a race against the clock."

In May 2023, a 50-year-old Chinese climber was found unconscious at 27,000 feet with severe frostbite. Her equipment had gotten stuck on a rope and her oxygen had run out. She survived after a fellow Chinese climber and his Sherpa found and rescued her.

She was lucky. Already this year, at least 12 people have died, and another 5 have gone missing and are presumed dead on Everest, making 2023 one of the deadliest climbing seasons ever.

According to sherpas and climbers some past deaths are due to crowding, the New York Times reported in 2019. The peak became so choked with climbers during a rare period of good weather that people were stuck in the death zone for too long. 

One day in 2019, 250 climbers attempted to reach the summit, The Kathmandu Post reported, and three died. Many climbers had to wait hours in line to go up and down.

By the end of the season, these extra, unplanned hours in the death zone were one of the main causes of 11 fatalities, Insider reported.

One mountaineer said climbing Everest feels like 'running on a treadmill and breathing through a straw'

At sea level, the air contains about 21% oxygen. But at altitudes above 12,000 feet, oxygen levels are 40% lower.

Jeremy Windsor, a doctor who climbed Everest in 2007, told mountaineering writer Mark Horrell that blood samples taken from four mountaineers in the death zone revealed that the climbers were surviving on just one-quarter of the oxygen they needed at sea level.

"These were comparable to figures found in patients on the verge of death," Windsor said.

Five miles above sea level, the air has so little oxygen that even with supplementary air tanks, it can feel like "running on a treadmill and breathing through a straw," according to mountaineer David Breashears.

Climbers need to acclimate to the lack of oxygen

A lack of oxygen can cause myriad health risks. When the amount of oxygen in your blood falls below a certain level, your heart rate can spike, increasing your risk of a heart attack.

Climbers have to give their bodies time to acclimate to the lung-crushing conditions in the Himalayas before attempting to summit Everest. 

After reaching Everest Base Camp (which is higher than nearly every mountain in Europe at 17,600 feet), they'll typically trek several overnight rotations, where they climb to slightly higher camps with each rotation in order to acclimatize before making a push for the top, according to National Geographic.

Over the course of those weeks at high altitudes, the body starts to make more hemoglobin, which is the protein in red blood cells that helps carry oxygen from the lungs to the rest of the body. 

But too much hemoglobin can thicken your blood, making it harder for the heart to move blood around the body. That can increase your risk of stroke according to the American Heart Association. 

On Everest, a condition called high-altitude pulmonary edema (HAPE) occurs in up to 15% of climbers — a quick stethoscope check can reveal a crackling sound as fluid that's leaked into the lungs rattles around. 

Other symptoms include fatigue, shortness of breath even at rest, weakness, and a persistent cough that brings up white, watery, or frothy fluid. Sometimes the coughing is so severe it can fracture ribs. 

In the death zone, your brain can start to swell, which can lead to nausea and a form of psychosis

Acclimatization to death-zone altitudes simply is impossible.

One of the biggest risk factors at 26,000 feet is hypoxia, which occurs when there isn't enough oxygen circulating to vital organs like your brain. 

If the brain doesn't get enough oxygen, it can start to swell, causing a condition called high altitude cerebral edema (HACE). It's often considered to be a severe, end-stage version of acute mountain sickness.

This swelling can trigger nausea, headache, and difficulty thinking and reasoning.

An oxygen-starved brain can cause climbers to enter a delirium that some experts consider a form of high-altitude psychosis. 

In this state, hypoxic climbers' judgment becomes impaired, and they've been known to do strange things like shed their clothes or talk to imaginary friends.

Other possible dangers include insomnia, snow blindness, and vomiting

Burke said that while climbing, she suffered from a constant, relentless cough. This persisted even while she slept. 

"Every second or third breath your body gasps for air, and you wake yourself up," she said.

The air was so thin that she was unable to sleep properly. 

"Humans will start to deteriorate," Hackett said about altitude sickness to PBS. "Sleeping becomes a problem. Muscle wasting takes place. Weight loss takes place." Nausea and vomiting from altitude-related illnesses, including HAPE and HACE, can cause a decrease in appetite. 

The glare from the endless snow and ice can cause snow blindness, which may result in temporary vision loss, according to the American Academy of Ophthalmology.

There's also exposed skin to worry about. Average wind chill temperatures on Everest are -49 degrees Fahrenheit. "Any exposed skin freezes instantly," Burke said.

Frostbite can occur when the water in your skin freezes to the point it crystalizes, which damages the skin cells and tissue thereby preventing blood to flow to those parts of the body. Frostbite is most common in fingers, toes, the nose, and ears.

In severe cases — if the skin and underlying tissues die — this may lead to gangrene. Gangrenous tissue may require amputation.

All this physical weakening and impaired vision can lead to accidental falls. Fatigue is ever-present, according to Burke.

"It takes everything to put one foot in front of the other," she said.

Poor decision-making can also lead climbers to forget to clip back into a safety rope, to stray from the route, or fail to properly prepare life-saving equipment like oxygen tanks.

Mountaineers climb through the death zone in a day, but they can wind up waiting in line for hours

Climbing in the death zone is "a living hell," as Everest climber and 1998 NOVA expedition member David Carter told PBS.

Typically, climbers attempting to reach the summit try to make it up and down in a single day, spending as little time as possible in the death zone before returning to safer altitudes. But this frenzied push to the finish line comes at the end of weeks of climbing.

Lhakpa Sherpa, who's reached Everest's summit ten times as of January — more than any other woman on Earth — previously told Insider that the day a group attempts to summit Everest is by far the most difficult period of the trek.

In order to summit successfully, everything must go right. Around 10 p.m., climbers leave Camp Four at 26,000 feet. The first chunk of their climb is done in the dark, lit by starlight and headlamps.

About seven hours later climbers typically reach the summit. After a brief rest filled with celebrations and photographs, the expeditions turn around, making the 12-hour trek back to safety and arriving (ideally) before nightfall.

This post has been updated. It was originally posted in May of 2019.

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