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 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 Business Insider. "It becomes a race against the clock."

In 2019, at least 11 people died on Everest, almost all of whom spent time in the death zone. It became one of the deadliest seasons on Everest in recent memory.

Some expedition companies blamed these deaths on crowding, noting that 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.

On 22 May 2019, 250 climbers attempted to reach the summit, The Kathmandu Post reported, and many climbers had to wait in line to go up and down.

These extra, unplanned hours in the death zone might have put the 11 people who perished at higher risk, though it's hard to determine the specific causes of each death.

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

At sea level, the air contains about 21 percent oxygen. But at altitudes above 12,000 feet, there are approximately 40 percent fewer oxygen molecules per breath.

Jeremy Windsor, a doctor who climbed Everest in 2007 as part of the Caudwell Xtreme Everest Expedition, told Everest blogger 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 and filmmaker David Breashears.

Climbers need to acclimate to the lack of oxygen

A lack of oxygen results in myriad health risks. When the amount of oxygen in your blood falls below a certain level, your heart rate soars to up to 140 beats per minute, 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.

Expeditions generally make at least three trips up the mountain from Everest Base Camp (which is higher than nearly every mountain in Europe at 17,600 feet), going a few thousand feet higher with each successive trip before making a push for the top.

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

But too much hemoglobin can thicken your blood, making it harder for the heart to pump blood around the body. That can lead to a stroke or the accumulation of fluid in your lungs.

On Everest, a condition called high altitude pulmonary edema (HAPE) is common – a quick stethoscope check can reveal a clicking sound as fluid that's leaked into the lungs rattles around.

Other symptoms include fatigue, a feeling of impending suffocation at night, weakness, and a persistent cough that brings up white, watery, or frothy fluid. Sometimes the coughing is so severe it can crack or separate ribs.

Climbers with HAPE are always short of breath, even when resting.

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 isn't possible, high-altitude expert and doctor Peter Hackett told PBS.

One of the biggest risk factors at 26,000 feet is hypoxia, a lack of adequate oxygen circulation to 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). Essentially, it's HAPE for the brain.

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

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

Hypoxic climbers' judgment becomes impaired, and they've been known to do strange things like start shedding their clothes or talking to imaginary friends.

Other possible dangers include insomnia, snow blindness, and vomiting

Burke said that while climbing, she suffered from a constant, relentless cough.

"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 added. "Sleeping becomes a problem. Muscle wasting takes place. Weight loss takes place."

Nausea and vomiting from altitude-related illnesses, including HAPE and HACE, also cause a decrease in appetite. The glare from the endless snow and ice can cause snow blindness – temporary vision loss, or burst blood vessels in your eyes.

Temperatures in the death zone never rise above zero degrees Fahrenheit. "Any exposed skin freezes instantly," Burke said.

A loss of blood circulation to climbers' fingers and toes can cause frostbite, and in severe cases – if the skin and underlying tissues die – gangrene. Gangrenous tissue often needs to be amputated.

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 bag the summit try and 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 nine times (more than any other woman on Earth) previously told Business 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 article was originally published by Business Insider.


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