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Altitude sickness. You’ll no doubt have heard of it. You probably know it’s something that should be taken seriously and is something that happens high up on mountains... 

...but do you know how serious altitude sickness can be, what the symptoms to look out for are and whether you could be affected by it on your next adventure?

To ease any concerns, we’ve gathered the best information and advice on altitude sickness for you. After a few minutes reading, you’ll know what altitude sickness is, how to look out for tell-tale signs of it, and what to do should you, or someone else, start suffering from it.

The areas we will look at are as follows:

Before we go on, a little disclaimer: While the advice below follows general guidance and has been thoroughly researched, it is the responsibility of every mountaineer, hiker and adventurer to seek his or her own professional advice from medical and mountaineering experts before heading to high altitudes. Always go to unfamiliar heights with a professional guide, and we’d advise no-one to ever head up a mountain, familiar or unfamiliar, alone. Now, all that said, here is some worldly wisdom on altitude sickness to help you to stay safe on your adventures.

What are the Symptoms of altitude sickness?

Altitude sickness is the body’s response to the changing conditions in the atmosphere as you head up a mountain. 

As you climb above 2,500 m (about 8,000 ft), both air pressure and the volume of oxygen in that air decreases to levels that can start to affect your body’s ability to function. Other factors such as high exposure to the sun and weather extremes can also contribute to it.

Altitude sickness strikes when the body is given insufficient time to adjust to the changing conditions. It is most simply prevented with decent periods of acclimatisation, though this may not always prove to be sufficient.

Altitude sickness can manifest in a number of ways, and it is vital that the first symptoms, even very mild ones, are taken extremely seriously. While the mild form of altitude sickness just feels a bit like a bad hangover, it can be a signal that you should be taking immediate action. 

Briefly, the three forms of altitude sickness are:

  • AMS (Acute Mountain Sickness) – the mild form of altitude sickness, manifesting as nausea, headaches, fatigue, decreased appetite and loss of coordination. 
  • HAPE/HAPO (High Altitude Pulmonary Edema/Oedema) – a potentially fatal excess of fluid on the lungs, manifesting as a constant breathlessness, fever and coughing up liquid. 
  • HACE/HACO (High Altitude Cerebral Edema/Oedema) – a potentially fatal increase in pressure on the brain, manifesting as drowsiness, loss of balance and extreme emotions and can lead to a comatose state. 

While the latter two forms are clearly very serious, so is AMS. If these mild symptoms are left untreated, they can easily develop into a fatal condition. So before you climb high on a mountain, learn how to recognise altitude sickness, and how to react should it arrive.

Who does it affect?

Fitness, age, build and experience – none are safeguards against altitude sickness. Even athletes and highly experienced mountain climbers, like Sir Edmund Hillary, get affected by it. 

It is estimated that at least 25% of people will be affected by altitude sickness. But rather than wait to find out if you’re one of them, it is much wiser to take steps to prevent it.

It may come as a shock to many that the man who climbed Everest declared 30 years later that  his “big mountain days were over” because of altitude sickness. This was after his second bout of the condition when advising younger climbers on their attempt on Everest’s East Face. That 1981 expedition ended for Hillary when he was forced to stumble down from the mountain in a state of confusion. 

How bad is altitude sickness?

Altitude sickness can be fatal. So, how bad can it get? That bad!

To return to Sir Hillary, when he first suffered altitude sickness, during an ascent of Akash Parbat (or Sky Peak) in India in 1977, he must have known little about it. Aged 58, Hillary couldn’t be woken up and had to be dragged unconscious off the mountain by his fellow climbers from 5,500 m (around 18,000 ft), wrapped in his tent. 

Sir Hillary had suffered from HACE, a form of altitude sickness that can be fatal. While asleep he effectively passed out as his brain swelled within his skull (the body’s oxygen uptake is lowest during sleep). Left any longer at those altitudes and he may well have never seen the final 31 years of his life.

At what height could altitude sickness affect me?

There is a general consensus that changes in atmospheric pressure and oxygen concentration don’t have negative effects on a body used to sea-level conditions below 2,500 m or 8,000 ft. Below this level, oxygen levels in the air are still above 15% and air pressures are still high enough to allow most bodies to function more or less normally.

Above 2,500 m (8,202 ft), it is crucial to begin paying attention to the tell-tale signs of altitude sickness. Air pressure at these heights can easily be 30% lower than at sea-level, vastly decreasing the body’s ability to take on vital oxygen. 

These two very different stories of altitude sickness demonstrate how unexpectedly and quickly altitude sickness can arise, even in a fit young 19-year-old in holidaying Peru or a “rugged young doctor and experienced climber”.

Both these stories show that the debilitating effects of AMS occurred only a little way over 3,000 m (about 10,000 ft), far below the height of many of the world’s great mountains in the Himalayas, the Alps or the Andes. David’s story shows how altitude sickness needn’t be a barrier to success, but that you must learn how to “deal with it”.

How high is 2,500 m (8,000 ft)? 

It’s as high as Breithorn (2,504 m/ 8,215 ft) in Austria, Mount St Helens (2,549 m/ 8,365 ft) in Washington State and Mount Taranaki (2,518 m/ 8,261 ft) in New Zealand. It’s also 271 m (892 ft) higher than the whole of Australia! 

Put in a broader context however, and altitude sickness occurs far below the vast majority of the summits of the mountains listed on our Mountains Info list.

What causes altitude sickness?

To help understand what happens to the body at altitude here is a concise step-by-step run down of what happens as you climb higher:

  • As you climb, the volume of oxygen in the air decreases, along with the air pressure. This means there is less of the vital oxygen that helps your body turn stored energy into energy your body needs. Decreased air pressure means breathing becomes harder, as the air simply doesn’t flow between the air and your blood through the membranes in your lung tissue as easily as it does at lower altitudes.
  • The higher you go the drier the air becomes. This is because there are lower levels of water vapour in the air and means you lose more water from your body than normal. In fact, you can lose up to a litre a day more through your breath alone when at higher altitudes. All this leads to a higher risk of dehydration.
  • A dehydrated body is a more acidic body. And in an acidic environment, the body simply cannot take in oxygen as efficiently. So the amount of oxygen is lower in the atmosphere and the body is less able to absorb it, all of which leads to hypoxia or oxygen deprivation.
  • By now, your body will be telling you that something’s wrong. First of all, heart rate and blood pressure increases, as the body tries to get the oxygen to where it needs to go. Then the body begins to shut down non-essential elements, redirects nutrients and blood to essential functions and triggers the warning signals associated with AMS such as headaches and muscle cramps. Heed these warnings carefully.
  • If HAPE develops, it is because the lungs, now starved of oxygen, constrict the blood supply to prevent the heart pumping oxygen deficient blood around the body. While this is normally a useful survival mechanism, at altitude it means fluid in the blood passes out into the lungs. The build up of this fluid decreases the spaces available for exchange of air and, essentially, the person begins to drown or suffocate in their own lungs.
  • In the case of HACE, more blood supply is directed to the brain, to keep that most crucial organ alive. However, it is believed that a chemical reaction also occurs that weakens the walls of blood vessels in the brain. This means the increased fluid volume leaks out into the brain tissue, increasing the pressure on the brain and potentially leading to a fatal outcome.

How to prevent altitude sickness

While you can take steps to lessen the effects or possibility of suffering from altitude sickness, there is no absolute prevention. The best cure is time, so don’t rush to get higher as the rewards will never be worth risking your life for. 

Here are some tips for helping to prevent altitude sickness:

Go slow.

Acclimatise, acclimatise, acclimatise. The first point on almost any advice list for preventing altitude sickness is go slow. General guidance varies between safe ascent speeds being between 300-500 m (1,000-1,604 ft) a day above the 2,500 m (8202 ft) mark.

How long? Count on a couple of days at first, and plan for other rest days on your way up. Read the accounts of any famous mountain climb and you’ll notice they normally have rest days after establishing each camp. These days will have been as crucial to a successful ascent as the hard slog of climbing days, giving the body time to make the physiological adjustments that will allow you to go higher.

Sleep low.

While asleep, your blood oxygen levels are at their lowest. As Sir Hillary discovered, you also can’t monitor your condition while you’re asleep either. Where possible, always descend to a safer level when sleeping, especially if you have noticed any of the symptoms of AMS.

For more serious adventures at higher altitude, it is advisable to set up camps at lower altitudes and sleep at the lowest camp that will allow the ascent to continue. 

Drink enough water with electrolytes.

Dehydration is a contributing factor to altitude sickness, so drinking enough water is key (remember how you lose a litre a day more at altitude through breathing alone?) 

Ensure that you also take on electrolytes as part of your fluid intake. Electrolytes help maintain the balance of fluid in your cells and ensure fluid goes where it should rather than where it shouldn’t. This also decreases the onset of fatigue and muscle cramps, making you more able to deal with the changing conditions effectively.

Learn the symptoms and don’t be a hero.

Before you go, learn the tell-tale signs of AMS, HACE and HAPE. This little bit of study could save your life. Keep an eye on fellow adventurers and make sure that you all agree before setting out that you will keep each other updated at the first signs of headaches, dehydration, and so on.

How to cure altitude sickness

The first thing you do on noticing the symptoms of AMS, HAPE or HACE is tell your fellow adventurers. It is crucial that you don’t ignore or try to hide the symptoms, however mild they may be. 

Once you’ve told them, it’s time to turn around and head back down the mountain. 

Do it slowly and with the support of at least one other adventurer. As a loss of coordination and dizziness are two of the common symptoms, not having somewhere to help guide you down safely is as foolish as ignoring the symptoms. 

The advice is to descend to at least the elevation that you last woke up feeling fine at. Given that symptoms can arrive slowly, that is likely to be the height you slept at two days previously. If in doubt go lower, and if you still feel the symptoms continue all the way off the mountain.

It may only be necessary to spend a day or two recovering at lower elevations. Should you feel well enough to continue back up, ascend very slowly, certainly slowest than the first ascent. Don’t try to ‘catch up’ with your party, however tempting, or you could put them all in danger (imagine how hard carrying an unconscious person down from a mountain would be and you’ll understand why!) 

If these steps are not possible (because the symptoms have progressed to HACE or HAPE), there are several other measures. 

The first is to receive pressurised oxygen, as all high-altitude adventurers should be carrying with them. The second is that you may need to be given/take pharmaceutical drugs to help limit the development of the sickness. 

One common drug used is a corticosteroid called Dexamethasone (commonly known as Dex, in the climbing community), which is designed to prevent the release of substances that cause inflammation. There is much controversy surrounding its use. Others include: Acetazolamide, which reduces fluid build up and fluid pressure in the body and nifedipine, which lowers blood pressure - again extreme caution is advised in its use.

Where possible, a portable hyperbaric chamber should be carried by teams going to extreme elevations. This chamber reverses the pressure decreases, and can offer the sufferer air pressures up to three times higher than at sea level. This enables the lungs to absorb much more oxygen and help the sufferer recover to a degree where they can safely descend. 

Of course, before heading to high altitudes, you should visit your doctor to get advice on what drugs to carry with you. Consulting professional mountain guides, and going with the best of them is obviously another wise preventative step to take.

Finally, if someone is suffering from HACE, do all you can to keep them conscious. Keep talking to them and stop them from giving in to the drowsiness. Once unconscious, their condition can easily progress into a coma and then become fatal. 

altitude sickness Symptoms in more detail.

AMS - Acute Mountain Sickness

  • The mildest form of altitude sickness. Should be treated as a precursor or warning signal of the more dangerous forms. 
  • Symptoms include: headaches, nausea, fatigue, aching, loss of coordination and decreased appetite. Imagine a bad hangover and it will likely feel somewhere close to that.
  • Symptoms are caused by the sudden changes in air conditions and the body not being given enough time to acclimatise
  • If left unchecked, this condition can develop into either HACE (more likely) or HAPE.
  • Sufferers should head to lower elevations to recover and reassess after a couple of day’s rest.

HACE/HACO - High Altitude Cerebral Edema/Oedema

Usually develops from AMS. Most common above 4,500 m (14,765 ft) though statistics suggest it only affects 1-2% of people.

Symptoms include: confusion, loss of balance, clumsiness, excessive emotional states, violence, inability to pass urine, drowsiness and loss of consciousness (coma). Odd behaviours and the loss of survival instincts can also accompany HACE, in a similar way that hypothermia can affect sufferers. If allowed to continue, develops into a pulmonary embolism, which involves a blood clot travelling to the lungs, which eventually prevents your body from circulating oxygen.

  • Symptoms are caused by an increase of fluid volume in the brain causing it to swell and push against the skull.
  • If left unchecked, this condition is almost certainly fatal.
  • Sufferers should receive oxygen and descend to lower levels immediately.

HAPE/HAPO - High Altitude Pulmonary Edema/Oedema

Most common in climbers going above 4,000 m (14,000 ft) and can affect up to 10% of climbers at this level, especially on too speedy an ascent. At this point, oxygen levels in the air area down to below 12.5%, less than two-thirds of what’s found sea-level. Anyone who has had a recent upper respiratory tract infection is particularly susceptible.

  • Symptoms include: breathlessness, fever, high temperature, weakness and coughing up fluids.
  • Symptoms are caused by an increase of fluid in the lungs.
  • If left unchecked, this condition is almost certainly fatal.
  • Sufferers should receive oxygen immediately and slowly descend to lower elevations.


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