Trekking holidays in places like Nepal, Peru and climbing Kilimanjaro are becoming ever more popular amongst travellers. 30,000 people attempt to climb Kilimanjaro every year, with 6 to 8 people dying on the mountain each year. (1)
This is a really good question! Ready for the science bit? Altitude generally refers to the height above sea level. High altitude is generally considered anywhere above 2500m.
At high altitude, atmospheric (air) pressure is lower than that at sea level. Decreased air pressure means there is less oxygen. So for example, there is approximately 66% less oxygen in each breath on the summit of Everest than at sea level.
Very High Altitude
|Between 2400-3658m||Between 3658-5500m||Between 5500-8848m|
La Paz, Bolivia -3658m
|Mt Everest summit -8850m|
|Bogota, Colombia- 2645m
|Lhaza, Tibet -3685m
Quito, Ecuador -2879m
|Everest Base Camp -5500m|
|Cuzco, Peru -3225m|
So what’s so important about oxygen? Simply put, it is needed to keep your body alive. It gives you energy, it’s important for brain function and for all those other things that it does for the body to function normally without you even realising it!
Our bodies are accustomed to working where we live, and when we go higher our bodies need to adapt. Altitude starts to have an effect on our bodies around 1500 – 2000m. So it starts to behave slightly differently as it tries to compensate for the change in oxygen levels. The decrease in pressure means you take in less oxygen and this can make it more difficult to breathe. Our bodies adapt by breathing faster and deeper, increasing the heart rate, and adjusting its blood chemistry such as making more red blood cells to carry oxygen around.
This response or adaption is called acclimatisation. It takes approximately 3-5 days to occur but can vary between individuals and conditions.
Because of these changes in your body, there are some ‘normal’ symptoms that you will notice at higher altitudes while your body is acclimatising and adjusting to the reduced availability of oxygen. They include:
The lack of oxygen above 2400 metres (8000 ft) can cause serious illnesses.
There are three forms of altitude sickness, the first two are neurological forms of altitude illness:
Symptoms of AMS appear 6 to 12 hours after you climb to or arrive at high altitude, although it can take up to 24 hours to develop. Symptoms include headaches, sleep disturbance, tiredness, shortness of breath, loss of appetite, nausea and vomiting. If you get symptoms of AMS you should take this as a warning sign that you may develop more serious altitude sickness called high altitude cerebral oedema (HACE). 10% of people with AMS will develop HACE. 
This is as a result of swelling of the brain and is typically characterized by the onset of confusion, altered consciousness and or loss of coordination. AMS commonly precedes HACE.
This affects the respiratory system where fluid builds up in the lungs and is also very dangerous. HAPE symptoms normally develop 2- 4 days after arrival at high altitudes above 2,500 metres and include: breathlessness, cough, bubbling sounds in the chest and pink spit. HAPE can develop even if you have not had symptoms of AMS.
Both HACE and HAPE are uncommon, but can develop very quickly and can be fatal within hours. If you develop symptoms of HACE or HAPE you need urgent medical attention and should go down to a lower altitude immediately.
It is difficult to predict the susceptibility of a traveller to AMS, and physically fit travellers are not necessarily at lower risk. If you have previously had AMS you may get it again.
Rapid ascent without a period of acclimatisation puts a traveller at higher risk. In a study, 84% of trekkers who flew directly to 3,860m were affected by AMS. (5)
Follow the Golden Rules
If you feel unwell at altitude it is altitude illness until proven otherwise.
Never ascend with symptoms of AMS.
If you are getting worse (or have HACE or HAPE), go down at once. (7)
Reduce your risk by:
If you rest and do not climb any higher symptoms of AMS usually improve in a few hours or days. DO NOT climb any higher.
Taking over the counter medication such as paracetamol may help with headaches and anti-sickness drugs like promethazine will help nausea and vomiting. Keep hydrated by drinking plenty of clear fluids and avoid alcohol. If you do develop AMS make sure you do not venture off on your own as you could develop more serious symptoms at any time.
If symptoms get worse or do not improve you MUST rapidly go (or be taken) to a lower altitude (at least 500 to 1,000 metres lower).
You should NEVER leave anyone with AMS on their own – they can become very ill very quickly.
Acetazolamide (Diamox®) can be prescribed to try and prevent AMS. Diamox reduces the headache of AMS and increases blood oxygenation at high altitude by altering the body’s acid-base balance. Diamox should be taken 24 hours before arrival at high altitude and continued for 2 days once the highest altitude is reached. A trial course of 250mgs per day for 2 days is recommended before going to a remote location where a severe allergic reaction could prove difficult to treat if it occurred.
Diamox is not suitable for everyone. Your nurse will make a risk assessment to see if it is suitable for use.
Acetazolamide is not a ‘magic pill’, and cure of AMS is not immediate. It speeds up a process that may normally take 24-48 hours to about 12-24 hours.
Put simply acetazolamide (Diamox) works by forcing the kidneys to excrete bicarbonate, the base form of carbon dioxide. It speeds up the acclimatisation process, stimulates breathing during the night and lessens the symptoms of AMS.
Common side effects include:
Less common side effects include:
Acetazolamide hides symptoms of AMS – FALSE
Acetazolamide speeds up acclimatization. As acclimatization occurs your symptoms resolve, reflecting improving health. Acetazolamide does not cover up anything, if you are still unwell with AMS, you will still have symptoms. If you feel well, you are well.
Acetazolamide will prevent AMS from worsening during ascent – FALSE
Acetazolamide does not protect against worsening AMS with continued ascent.
It does not change GOLDEN RULE 2 – NEVER ASCEND WITH SYMPTOMS OF AMS. Lots of people who have developed HACE AND HAPE believed this myth.
If Acetazolamide is stopped, my symptoms will worsen – FALSE
There is no rebound effect. If acetazolamide is stopped, acclimatization slows down to your own intrinsic rate. If AMS is still present it will take longer to resolve. If AMS is not present, then you don’t need to accelerate acclimatization if you ARE already acclimatized. You won’t become ill by stopping your acetazolamide. (6)
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