Climbing at high altitude can have fatal consequences for the human body, presenting a multitude of risks that need to be respected and managed to guarantee the best chances of survival. 

Around 10 to 20% of people who fly to the likes of Machu Picchu in Peru or to ski resorts in Colorado experience acute mountain sickness every year*, making it a relatively common occurrence.  

We define ‘high altitude’ as altitude that’s 2,500m or above, and acute mountain sickness can affect the body within 6 - 12 hours of exposure. Headaches which are accompanied by nausea, dizziness, vomiting and tiredness, and feeling out of breath, are typical symptoms, and while they may sound fairly innocuous in themselves, if the sickness is not managed in its early stages, life-threatening complications can ensue. 

To really put the broad spectrum of risk and potential complications of high altitude into perspective, we undertook a journey to the summit of the world’s highest peak: Mount Everest. We bumped into International Mountain Guide Sandy Allan (the fifth Scot to climb Everest), and Clinical Lead Dr Daniel Atkinson along the way, who offered us some crucial advice. 

There are various routes of ascent (and descent) of Everest, but we’ve opted for the most popular one, the South Col, along the southeast ridge, for the purposes of this article.  

The Hike to Everest Base Camp (3,000m - 5,000m)

The hike to Everest base camp from Lukla airport, high in the Himalayas, is no picnic; you should spend at least two days at 3,500m in a town called Namche Bazar first, which is a two day walk from the airport, before undertaking a further five to seven day hike to the base camp itself. 

The stop-off at Namche is very important for your acclimatisation; the effects of altitude typically start to be felt between 3,000 and 3,500m, and given that Everest base camp is situated a full 1,500m higher than Namche, it provides the ideal launchpad for acclimatising incrementally.  

Allowing the body to grow accustomed to altitude, and to changes in altitude, slowly, is key. 

‘Acclimatisation to altitude consists of many changes to the body, stretching over days to weeks, improving peoples’ functioning in these environments,’ points out Dr Atkinson.

‘If the rate of exposure to altitude exceeds the body’s natural adaptation to it, acute mountain sickness can materialise.’

You should aim to complete some physical activity to help the body adjust to the conditions. It’s best to walk at a slower pace than your average speed, and rest when you need to. A walk that takes you above Namche itself to a higher point and then descending again to sleep for example is ideal for acclimatisation; remember the mantra, ‘climb high, sleep low’. 

Remaining hydrated at altitude is also critical; alcohol consumption should be kept to the bare minimum, while smoking should be avoided altogether. Most people need to drink as much as 5 or 6 litres of water a day. This may sound like a lot, but it’s imperative that you meet this threshold. As Sandy Allan explains:

‘I was told years ago by the MEF (Mount Everest Foundation) and by Mike Ward, who was a doctor on the Everest trips, how essential it was to drink a lot of water, and it’s turned out to be some of the best advice I’ve been given on coping with high altitude.’  

Having spent a few days at Namche Bazar, the walk to Everest base camp should play by  similar rules. Foods rich in iron will help you to preserve energy levels, and given that this trek is of a considerably longer duration than the walk from Lukla to Namche, a gradual ascent to altitude is all the more paramount. There’s 50% less oxygen at Everest base camp than at sea level.

‘You should avoid rushing too much. Don’t push your acclimatisation too hard. If you or any of your party is experiencing a sore head, it’s really worth stopping at whatever altitude you’re at and resting,’ says Sandy. 

Should you start to experience symptoms of altitude sickness, a medication called Diamox is an effective form of treatment.

‘Diamox can be effective in preventing acute mountain sickness and has less side effects than drugs like dexamethasone, which can mask symptoms and subsequently put users at more risk,’ advises Daniel.

Everest Base Camp and Camps 1, 2 and 3 (between 5,000 - 7,000m)

You’ve been at base camp for about a week, you’ve fully adapted to the altitude and the weather conditions are looking ideal. It’s now time to acclimatise to the greater heights of Everest itself. 

Many mountaineers climb from base camp up to higher camps before returning back to base camp to grow accustomed to the high altitude on 8,000m + peaks like Everest. This usually takes the form of moving up and down between Camps 1, 2 and 3. Ascending too quickly between these camps can result in a dangerous complication of altitude sickness called high altitude pulmonary edema (HAPE), in which the lungs start to fill up with fluid. 

A blue hue to the skin, breathing problems (even while at rest) and coughing up white or pink frothy liquid are usually indicators of the condition, and without immediate treatment, it can be deadly. 

In the event that you experience these symptoms, you should descend to a lower altitude or  lower camp at once and give yourself sufficient time to recuperate. Nifedipine is a medication that will help to alleviate tightness in the chest, and bottled oxygen, if available, may also provide relief. 

‘The importance of taking your time at altitude can’t be stressed enough - and that applies to strong climbers as well as beginners,’ says Sandy.

‘The best thing you can do if you’re experiencing pulmonary edema is to get to your nearest campsite, ideally back to base camp, and take a rest day.’

Above 5,500m, your risk of developing frostbite also starts to increase more and more as you continue your ascent. Camps 1, 2 and 3 all sit above the 5,500m mark, and your extremities (the hands, feet, nose, ears and lips) are particularly vulnerable to the cold. 

As the body starts to prioritise blood flow to your vital organs over your extremities, a throbbing and tingling pain in the affected areas (frostnip) and a ‘pins and needles’ sensation, progressing to numbness, may start to be felt.

Further exposure to the cold can lead to the skin whitening, or turning blue or blotchy, and eventually, tendons, muscles, nerves and bones may become damaged. This constitutes deep frostbite and requires immediate medical attention to prevent tissues from dying altogether (tissue necrosis). 

When tissue dies, there’s no longer a blood supply to it, and so the body’s white blood cells aren’t able to combat infection. This may lead to the development of bacterial infections such as tetanus, or in more severe cases, sepsis or gangrene (which can lead to amputation of the affected area). 

If you or any of your teammates are not feeling well, don’t climb any higher,’ advises Sandy.

‘If you carry on climbing regardless, you’re really risking putting yourself in harm’s way.’

Withdrawal from the cold environment into a warmer one and descending to as low a camp as  as possible can restrict the damage caused by frostbite. Hypothermia also tends to go hand in hand with frostbite, which makes retreating to warmer conditions doubly important. 

Some base camps on high altitude peaks are staffed with physicians; Everest for example has an ER unit who work on a seasonal basis to provide a range of medical relief. Should you run into any difficulties, try to descend to such facilities for treatment. 

Camp 4 - ‘The Death Zone’ (7,800m)

You’ve made it to your highest camp on Everest, from which point (health and weather permitting), you can launch your bid for the summit. It’s at this altitude however where conditions on the mountain are at their most dangerous, hence the nickname,‘The Death Zone’. 

‘The Death Zone’ is a term applied to areas of mountains that stand at 8,000m and above. There’s so little oxygen at these heights that the brain starts to swell; it becomes a race against the clock, and spending as little time as possible at this altitude is critical to survival (guidelines recommend as little as 16 - 20 hours max). The use of supplemental oxygen, should you have chosen to use it, could absolutely be the difference between life and death at this stage. 

Temperatures in ‘The Death Zone’ never exceed freezing point on Everest, and winds can reach up to 50mph (and at least 100mph on average on the summit itself). The wind chill at 50mph is extremely cold, and you should climb in wind speeds no greater than 20mph to minimise the risks of frostbite. 

Another significant threat to the body at 8,000m and beyond is hypoxia, which can develop when there isn’t a sufficient oxygen supply to organs such as the brain. Common symptoms include dizziness, shortness of breath and impaired judgement or cognitive functioning, which presents in a similar way to drunkenness and can make climbers very disorientated.

Suffice to say, hypoxia has the potential to be extremely serious, and even fatal, if you stay above 8,000m for any real length of time. It may also lead to further complications such as high altitude cerebral edema (HACE), where fluid accumulates in the brain and causes swelling. The build-up of fluid increases pressure inside the skull, which restricts blood flow to the brain and lessens the amount of oxygen it receives. 

HACE can, in some cases, even cause extremely vivid hallucinations, if there’s continued exposure to altitude above 8,000m. Numerous climbers have reported sightings and sounds of people climbing with them who are completely illusory (a phenomena known as ‘third man factor’) and there are even accounts of climbers mistakenly thinking that they can jump from the summit of mountains right down to the bottom, so as to shorten their descent*. 

‘You don’t necessarily realise that you’re hallucinating,’ explains Sandy.

‘I saw a rabbit running back and forth in the snow behind my climbing partner. I thought, that rabbit must be absolutely freezing as it didn’t have its boots on.

‘I thought I wasn’t hallucinating at all, but then I realised that the rabbit wasn’t leaving tracks in the snow, and at that point I was able to rationalise it properly.’

Visions of phantom helpers, functioning as companions and spurring climbers on to the summit, or accompanying them during the first stages of the descent, are also not all that unusual.

‘I saw Snoopy for days and days, sitting on a rock,’ notes Sandy.

‘I felt really safe. It was as if he was looking after me.

‘On Everest, we were convinced that there was someone else on the mountain, but there was no-one really there at all. It happens a lot.’

The best course of action, should you have illusory experiences, is to descend, advises Sandy.

‘I was fortunate in that we were descending further and further all the time, and once we got to about five and a half thousand metres, the hallucinations died off.’

Setting out for the summit of Everest at 11pm from Camp 4 should mean that you reach the top at around 9am the following morning, which gives you sufficient time to descend before the weather deteriorates in ‘The Death Zone’ (usually mid-afternoon onwards). Climbers usually try to make it to the summit and back to Camp 4 within 24 hours. 

The Summit (8,848m)

You’ve made it to the summit, and are likely feeling overwhelmed; you are standing at the highest point on the planet, after all. Given that this is the case, the temptation might be to linger, but you really must resist the urge to. You should spend no longer than 20 minutes at the top; having already spent a considerable amount of time in ‘The Death Zone’ by this point, it’s absolutely vital that you descend to lower altitude. The weather, particularly at such an elevation, can deteriorate suddenly and unexpectedly, with extremely high winds, which can be strong enough to blow people off the summit (and off the mountain) altogether.    

It’s easy to overlook the fact that reaching the summit is only half the journey, and that most incidences of climbers running into difficulties happen on the descent. Provided that you’ve taken the time to acclimatise properly, that you haven’t exhausted yourself on the ascent, that you stay hydrated, and that you use supplementary oxygen if you’re struggling to breathe at high altitude, there’s a very strong chance that you’ll make it back to base camp fatigued, and sore, but without any long-lasting damage to the body. 

Some basic guidance on climbing Everest

Climbing at high altitude without bottled, supplementary oxygen is extremely dangerous, and the vast majority of people who attempt Everest choose to climb with it. Only a select number of professional, seasoned mountaineers who have climbed above 8,000m previously (with at least one experience above 8,500m) or high-altitude Sherpas should try to summit Everest without oxygen (if they wish to). 

It is worth noting that it’s generally considered irresponsible in climbing circles for guides and Sherpa guides to choose to climb without oxygen. This is due to guides potentially being less able to look after their clients should they start to experience difficulties themselves at altitude. 

People usually start breathing bottled oxygen at the 7,000m point on Everest. 

Typically, only highly experienced professional climbers who fully appreciate the various rigours and dangers of the mountain itself first-hand (as well as the routes to Everest base camp) should embark on the expedition without professional guides. So unless you fall within this criteria, it’s best to attempt the peak under the supervision of experts. 

There are numerous companies to choose from, and when you contact them, find out who would be leading your climb. Try to establish how many occasions they’ve guided on Everest, and if the organisation has more than one guide who can lead in the event of sickness or injury to the designated person. 

Ascertain what the climber to guide ratios are, as well as how many other climbers there will be in your team. You should also ask for confirmation of a company’s safety record as well as their success record.

Generally speaking, you should spend at least 12 months getting yourself into sufficient physical condition for climbing Everest. Focus on a combination of uphill walking whilst carrying a loaded bag, strengthening the lower body and core muscles, and both cardiovascular and flexibility training. 


J Travel Med. Hypoxia-related altitude illnesses. 2013.

** Time. Climbing Mountains Can Cause Psychosis, Researchers Say. 2017.