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At 8,400 metres above sea level, near the summit of Mount Everest, four climbers submitted to an extraordinary (and uncomfortable) experiment.
One after another, they pulled down enough of their insulated clothing and underwear to expose the femoral artery in the groin. A colleague then pushed a needle into the vessel and drew arterial blood.
“For an arterial sample, you need to get into an artery, and we had quite a lot of debate about how we were going to do that,” Hugh Montgomery, a professor of intensive-care medicine at University College London, told ZME Science.
The researchers had initially considered the radial artery in the wrist, a common site for arterial blood sampling. But in the extreme conditions high on Everest, Montgomery said, that just wouldn’t work.
“So, in the end, we went for the one in the groin,” he said. “We all had to pull our underwear down and expose our groin because there’s a great big artery in there, the femoral artery, which is relatively big, pretty easy to stick something into, easy to press on and isn’t really going to go into spasm.”
This experiment was just one bit of a remarkable research project.
The Everest as a laboratory
The Caudwell Xtreme Everest expedition wasn’t about breaking some mountaineering records. It meant to probe an important medical mystery: why do some people tolerate severe oxygen deprivation while others deteriorate rapidly?
Doctors like Montgomery see this all the time in intensive care.
“There are situations in illness where low oxygen delivery is very common,” Montgomery told me during an interview at the Cheltenham Science Festival. “It might be, let’s say, that your lungs are knackered and they’re not taking up much oxygen, or that your heart’s not pumping the blood round well, or you don’t have enough red cells or haemoglobin to carry the oxygen, or the cells themselves don’t work very well.”
“And yet some people cope quite well with it, and some people die,” Montgomery said. “We wanted to try to work out how humans adapted to cope well with lack of oxygen.”
Obviously, you can’t experiment on critically ill patients. A patient may arrive with infection, inflammation, organ damage, chronic disease and the effects of several medications. Many biological systems fail at once, making it hard to isolate the effects of oxygen deprivation itself. Even basic measurements are hard to rely on in this context.
“If you look at a patient on intensive care, everything you look at is up the creek,” Montgomery said. “Everything you measure will be weird and wrong because the patient’s sick.”
That’s when the Everest came up as an idea.
The researchers could start with healthy people at low altitude. They could then test those same bodies repeatedly as the participants climbed into progressively thinner air. Instead of trying to separate oxygen deprivation from a web of serious diseases, they could watch it develop gradually in otherwise healthy volunteers.
The expedition recruited 198 trekkers and 24 investigators. Of the 198 volunteers who left Kathmandu, 190 reached Everest Base Camp at 5,300 metres. Fourteen members of the research team continued higher on the mountain, as high as 8,400 metres. The ascent created a series of increasingly severe exposures to low atmospheric pressure and scarce oxygen.
Basically, Everest became a giant physiological stress test.
Bike exercises at 8,000 meters
The whole experiment was carefully designed. At the South Col, a high-altitude mountain saddle 7,950 metres above sea level, they set up a full laboratory. Out of the 12 participants who reached that point, five completed ramped cardiopulmonary exercise tests, pedaling against increasing resistance while colleagues monitored how their bodies responded. The expedition also measured cerebral blood-flow velocity and microvascular blood flow at that altitude.
“We had exercise bikes on the South Col with full AC power in tents with machines,” Montgomery recalled. “We were able to run all the sort of experiments you’d run at sea level up there.”
Montgomery himself was involved directly; not just as a doctor or researcher, but as a participant.
“We had a rule that if you designed an experiment, certainly at altitude, you had to be the first person to do it because, of course, there’s no precedent for these sorts of experiments at all,” he said. It turned out to be a pretty important role, as Montgomery himself found out the hard way. “I certainly had designed a few experiments that turned out not to be very safe at all, which I discovered when I did them.”
The climbing researchers also needed experience above 8,000 metres and medical expertise that allowed them to understand the risks.
“You had to be the first person to take that risk,” Montgomery said.
The team had tested its equipment in cold rooms, hypobaric chambers and earlier field expeditions. But no laboratory simulation could fully reproduce the hazards of conducting invasive medical experiments on Everest. Of course, for the tests to be complete, climbers also had to give blood.
The researchers had hoped to collect blood on the summit itself, but the conditions proved too severe. Instead, they erected a small sampling station at the Balcony during the climbers’ descent. Four subjects provided the record-setting samples there. The balcony is a small area at 27,700 ft (8,440 m).
The blood readings were stunningly abnormal. Under normal conditions, the observed levels would be considered a medical emergency and doctors would expect confusion, collapse or even death.
“These were oxygen levels you’d have thought would be lethal, but they were taken from someone who was walking around on the summit.”
The limits of the human body
You might expect the body to respond to a crisis by pushing everything harder and faster: breathing more, pumping more blood and forcing more oxygen through the system. That’s also what doctors thought.
“When I was a medical student, I was told that the way you’d adapt was to deliver more oxygen,” Montgomery said. “You breathe harder, your heart would pump harder, pump more per beat. Your blood would get thicker. It would carry more oxygen. It was all about improving delivery.”
Those things do happen. The Everest study found that rising haemoglobin concentrations helped maintain the oxygen content of arterial blood at or above sea-level values until the climbers reached 7,100 metres. But above that point, that strategy started to lose the battle. At 8,400 meters, arterial oxygen content was 26% lower than at 7,100 metres. The winning strategy then becomes more about cutting costs and making things more efficient
“The adaptive process is all about efficiency and the way in which you use the oxygen,” Montgomery says. He compares the body to a car travelling a long distance.
If you’ve got a full tank and no concern about fuel consumption, you can just push the pedal and let the engine roar. But the calculation changes when you don’t have that much gas in the tank.
“You would strip it out, make it light, switch off everything that didn’t need any oxygen, tune the engine to its maximum efficiency,” Montgomery said. “And that’s what humans do.”
Under extreme oxygen scarcity, the body can alter which fuels it uses and reduce energy expenditure elsewhere. Burning carbohydrate generally yields more usable energy for each unit of oxygen than burning fat. The body may also limit expensive processes, including the continual construction and maintenance of proteins.
Simply put, it’s more about austerity than pumping things faster. But this carries a price.
“You switch off things you’re not using, particularly making protein,” Montgomery said. “So you waste away. You lose muscle mass enormously at altitude because you’re just saving the energy it takes to make muscle and protein.”
Sherpas revealed a different way to use oxygen
The researchers later returned to Everest to study Sherpa physiology more directly.
Sherpa populations have lived at high altitude for many generations. Their adaptation does not simply involve producing ever-thicker blood. Their metabolism also appears to extract more useful energy from limited oxygen.
“If you look, for instance, at Tibetans or Sherpas who’ve been living there multiple generations, they’re genetically selected for lower amounts of haemoglobin in red cells, not more,” Montgomery said.
In the Xtreme Everest 2 project, researchers compared lowlanders with Sherpa participants as they travelled toward Everest Base Camp. A subsequent study found that the Sherpa group had a lower capacity for fatty-acid oxidation, more efficient oxygen use, better maintenance of muscle energy and greater protection against oxidative stress.
Essentially, this strengthened the expedition’s central idea. The body doesn’t adapt to oxygen scarcity only by carrying more oxygen, it can also change its cellular machinery so that each available unit accomplishes more.
The insight could eventually matter in intensive care, where doctors routinely treat patients whose organs receive or use too little oxygen. But the researchers have never claimed that an acclimatised climber perfectly represents a critically ill patient. Their methods paper explicitly acknowledges that the model’s relevance to complex diseases remains uncertain.
The body’s limits don’t end on Everest
Montgomery’s interest in the limits of human survival isn’t restricted to tall mountains. In fact, he’s very much concerned about the long-term survival of human civilization.
Montgomery co-chairs the Lancet Countdown on Health and Climate Change, an international collaboration involving scientists and health experts from around the world. Its latest global assessment01919-1/abstract) concluded that human-caused climate change is increasingly claiming lives and damaging health through heat, drought, food insecurity and other interconnected threats.
But Montgomery no longer thinks the language of “health impacts” captures the scale of the danger.
“I tend not to talk any more about health because I talk about survival,” he told ZME Science. “Climate change threatens my son’s survival, and he’s 20.”
Extreme heat and disasters can kill directly. We’re already seeing that every year. Yet Montgomery worries just as much about the systemic failures that can follow: damaged harvests, disrupted supply chains, rising prices, migration and political instability. He describes climate change as a systemic threat to the country’s food system, with extreme weather already disrupting production and increasing volatility.
The danger, he argues, is difficult to communicate because humans respond most powerfully to threats that feel immediate and personal. A crisis unfolding gradually, across borders and over decades, rarely triggers the same instinctive alarm as danger to someone standing directly in front of us.
“It turns out that humans aren’t wired to be able to project themselves into the future. You can’t really see your future self beyond seven years. After seven years, you become an ‘other’ you. So it’s a problem for human psychology, because we don’t really care about other people dying, and if we are going to die after about seven years, we can’t really view that as particularly a threat. But we do tend to care about our children, for those people who have them.”
“We don’t really care about other people dying,” Montgomery said bluntly. “But if it’s your partner or your child, you definitely do.”
Global figures clearly show that people are overwhelmingly aware that climate heating is happening and want stronger action. According to 2025 figures, 89% of the public want their governments to do more to tackle the climate crisis, even though they don’t know they’re the majority. The problem is reaching global leaders, but Montgomery isn’t pleased at all by what he’s seeing.
“Most politicians aren’t trained in science at all. They don’t have the understanding of biology, chemistry, physics or mathematics to be able to understand meteorology or the biological impacts of this. So they’re not equipped intellectually. And through a political lens, some of them won’t even hear about it. They say, ‘It doesn’t exist, therefore I will not hear about it.’ That’s a very dangerous situation of ignorance. We’re seeing the rise of people who are ignorant and making very, very dangerous decisions based on ignorance, and that’s the bit that’s unforgivable for me.”
The Everest showed that the human body can adapt remarkably well, but there are limits to this adaptation. These physical limits don’t care whether people understand them, believe in them or find them politically convenient. In the death zone, denying the lack of oxygen does not make the air any thicker.
Climate change is pushing a similar reckoning on human societies, but unlike the human body, our civilization has no automatic survival response. It must choose to act—and leaders can still deny the danger, misunderstand it or postpone the costs. The lesson from the mountain is therefore not that humans can adapt to anything. It’s that adaptation only buys time, always carries a price and cannot begin until the threat is acknowledged.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
