What is high altitude?

Altitude can be divided into high, very high, extremely high, and ultra high above sea level. Symptoms due to altitude can develop anytime above getting into the 'high' zone.

Altitude guide


2,500 to 4,000 metres

8,000 to 13,000 feet

Very High

4,000 to 5,500 metres

13,000 to 18,000 feet

Extremely High

over 5,500 metres

over 18,000 feet

Ultra High

over 8000 metres

over 26,000 feet

At what height does altitude illness start?

Air pressure and available oxygen get gradually lower the higher you are above sea level. The body often doesn't really notice these changes until over 2000 metres. Above 2500 metres, you may begin to develop symptoms of acute mountain sickness (AMS). This can become more severe and even life threatening, causing either fluid on the lungs - high altitude pulmonary oedema (HAPE), or fluid on the brain - high altitude cerebral oedema (HACE).

How is oxygen affected at altitude?

As air pressure drops with altitude so the effective amount of oxygen available to the body drops. By 5,500 metres there is only half the amount of oxygen in each breath compared to at sea level.

Altitude and oxygen levels
Altitude (metres) Altitude (feet) Effective oxygen % Altitude level Tourist destinations
0 0 20.9 Low
500 640 19.6 Low
1000 3281 18.4 Medium Chamonix, France (995m/3,264ft)
1,500 4921 17.3 Medium
2,000 6,562 16.3 Medium
2,500 8,202 15.3 High Aspen, USA (2,410m/7,907ft)
3,000 9,843 14.4 High
3,500 1,1483 13.5 High Cusco, Peru (3,399m/11,152ft)
4,000 1,3123 12.7 Very high Lhasa, Tibet (3,650m/11,975ft)
4500 14764 11.9 Very high
5000 16404 11.2 Very high Mont Blanc, France (4,810m/15,780ft)
5,500 18,045 10.5 Extremely high Everest Base camp, Nepal (5,150m/16,900ft)
6,000 19,685 9.9 Extremely high Kilimanjaro, Tanzania (5,895m/19,340ft)
6,500 21,325 9.3 Extremely high
7,000 22,966 8.7 Extremely high Aconcagua, Argentina (6,962m/22,841ft)
7,500 24,606 8.2 Extremely high
8,000 26,247 7.7 Ultra high
8,500 27,887 7.2 Ultra high Mount Everest, Nepal (8,848m/29,028ft)

Where in the world is high altitude a risk?

The high altitude areas of the world are shown on the map below.

Worldwide distribution of mountain lands

© Encyclopædia Britannica, Inc.

What is the best way to acclimatise at altitude?

  • People planning to ascend to over 3,000 metres (10,000 feet) should spend at least one night at 3,000 metres before they start to go higher.
  • Above 3,000 metres each new sleep height should be between 300 metres and 500 metres (1,000-1,500 feet) higher than the previous night.
  • It is possible to go higher during the day but then descend to sleep no more than 500 metres higher than the previous night - climb high, sleep low.
  • For every 1,000 metres (3,000 feet) of ascent, sleep at that height for two nights before going any higher, preferably also taking a rest day.

Does coca tea/gingko biloba/ibuprofen help acclimatise to altitude?

Despite being promoted in many mountainous areas none of these products have been shown consistently in trials to help acclimatisation to altitude. However ibuprofen will alleviate symptoms of headache in mild AMS.

What is the Lake Louise score and how does it help at altitude?

The Lake Louise score was originally a research tool. It gives a measurement of symptoms of AMS, assisting in diagnosis of AMS but not HAPE or HACE. It assesses symptoms of headache, the gut, fatigue, dizziness and sleeping. Some experts prefer not to include sleeping within the score as some sleep disturbances may not be a direct effect of altitude.

What is periodic breathing/sleep disorder of altitude?

Irregular 'periodic breathing' tends to happen when people are sleeping at altitude. Instead of regular slow shallow breaths during sleep, increasingly rapid, deep breaths are followed by prolonged pauses. This is not dangerous but can be worrying for sleep partners and sometimes also jerks the individual themselves awake, so disturbing sleep. Acetazolamide can be taken to relieve irregular breathing during sleep at altitude.

What is AMS?

AMS is acute mountain sickness. It can feel like a bad hangover with:

  • A headache that is not caused by dehydration (dehydration headache should settle by taking paracetamol and drinking a litre of water).
  • Fatigue and weakness.
  • Dizziness and light-headedness.
  • Difficulty sleeping and irregular breathing during sleep.
  • Loss of appetite, nausea, and vomiting.

What is HAPE?

HAPE is High Altitude Pulmonary oEdema (fluid on the lungs) with symptoms of:

  • Trouble breathing, breathlessness at rest, breathing rate taking a long time to recover after physical exertion.
  • Coughing.
  • Frothy spit, becoming bloody.
  • Lips, tongue, nail beds become blue.
  • Fast pulse and fast breathing at rest.

What is HACE?

HACE is High Altitude Cerebral oEdema (fluid on the brain) with symptoms of:

  • Severe headache.
  • Becoming clumsy - unable to walk heel to toe in a straight line or to touch finger to nose with eyes closed.
  • Odd behaviour - unhelpful, violent, lazy, unable to think straight, or to do simple sums.
  • Non-stop vomiting.
  • Blurred or double vision.
  • Confusion.
  • Seeing haloes around objects, hearing or smelling odd things, hallucinations.
  • Reduced consciousness, coma.

Are HAPE and HACE really that serious?

Yes. HAPE or HACE are both potentially fatal conditions, with most deaths at altitude being caused by HAPE. HAPE is twice as common as HACE. Immediate descent by at least 1,000 metres (3,000 feet) and medication (including acetazolamide for HACE) can be life saving.

How is AMS, HAPE, HACE treated?

The most important part of treatment is descent by a minimum of 1,000 metres (3,000 feet).

Acetazolamide (250mg twice daily) is used to treat AMS and HACE but other medication is also usually prescribed.

Treatment for HACE may also include dexamethasone (8mg immediately, followed by 4mg 6 hourly for at least 3 days).

Treatment for HAPE may include: nifedipine (10mg immediately, followed by 20mg three times a day for 3 days); sildenafil/Viagra (50mg 6 hourly); and salbutamol/salmeterol inhaler (2 puffs 2 hourly).

If oxygen is available, it should be used for severe AMS, HAPE and HACE.

What dose of acetazolamide should I use to aid acclimatisation to altitude?

Acetazolamide doses for effects of altitude
Dose of acetazolamide
Acclimatisation 125mg (half a tablet) twice daily
Irregular breathing during sleep at altitude 125mg (half a tablet) 2 hours before sleep increasing to 125mg twice daily if necessary
Mild Acute Mountain Sickness (AMS) 250mg (one tablet) twice daily

If there are side effects of tingling extremities should acetazolamide be stopped?

No. The tingling is a mild side effect which in most cases can be tolerated. It indicates that acetazolamide is having a positive effect on the blood chemistry, so helping the body to acclimatise to altitude. The tingling is caused by an effect on the local chemical balance around skin nerve endings.

If I take other medicines, can I take acetazolamide?

Acetazolamide may interfere with the action of some other medication.

Acetazolamide should not be taken at the same time as aspirin, lithium, sodium valproate, or zonisamide.

Extra caution is needed, and you should discuss with your regular doctor if considering taking acetazolamide with:

  • Epilepsy medication.
  • Heart or blood pressure medication or water tablets (diuretics).
  • Blood thinning medication.
  • Diabetic medication.
  • Any medication known to affect folic acid, eg methotrexate, trimethoprim.
  • Steroids.
  • Glaucoma medication.
  • Amphetamines.
  • Ciclosporin.

I take anticoagulant (blood thinning) medication - can I take acetazolamide for altitude problems?

There is no direct interaction between acetazolamide and anticoagulant tablets.

When travelling to high altitude, there is an increased risk of developing blood clots because of thickening of the blood. Most people taking anticoagulant tablets have a higher risk of blood clots due to some underlying health issue, so adding to this risk by high altitude travel would not be recommended. Discuss with your regular doctor, before travel.

Often travel to high altitude is in remote places, distant from medical help. This could be a problem if you have a fall and are injured whilst taking anticoagulants. If there is no quick access to medical facilities, bleeding will be more difficult to manage, and potentially fatal, especially if there is brain or internal bleeding (haemorrhage).

Can I take acetazolamide with my contraceptive pill?

There is no direct interaction between acetazolamide and combined hormonal contraceptives - combined pill, vaginal ring, or contraceptive skin patch, or with progestogen only forms of contraception - progestogen-only pills (POP or minipill), injections, or implants.

There is an increased risk of blood clotting when taking any combined hormonal contraceptive with the most risk in the first year after starting. There is also an increased risk of developing blood clots from being at high altitude. You may prefer not to add to this risk, by continuing your combined hormonal contraception, so you may choose to swap to a different form of contraception. However, there is a risk of irregular bleeding when first starting progestogen-only contraception, which may be disruptive at altitude. It is best to discuss your options with your contraception provider, or GP. On balance many women on short term trips (less than 1 month) to altitude will choose to continue their previously stable form of hormonal contraception/contraceptive pill.

Should I drink more water at altitude?

Risk of dehydration is increased at altitude for several reasons. There is an increased breathing rate and dryer air at altitude, also an increase in urine production by the kidneys, and often there is more sweating from increased exertion. It is recommended to drink 1 to 1½ litres more than usual per day when at altitude. Drinking too much fluid though can cause problems in itself. Aim for 3-4 litres of water each day and check that your urine (pee) is clear and not too dark.

Is it OK to drink caffeine at altitude?

Some people worry about caffeine at altitude, but being a mild stimulant to the brain, kidneys, and breathing means that in small amounts, if anything, it is more likely to have mild benefits.

Is it OK to drink alcohol at altitude?

It is not recommended to drink alcohol at altitude. Alcohol slows breathing during sleep which will reduce acclimatisation. The effects of alcohol and especially the symptoms of a hangover, can be confused with those of AMS and HACE.

Does taking acetazolamide mask symptoms of altitude sickness?

Acetazolamide works alongside the body, speeding up the natural acclimatisation process. It will not mask symptoms of altitude sickness.

I'm superfit - do I need to worry about altitude sickness?

Unfortunately yes! You still need to take care to acclimatise. Being superfit is no protection against altitude sickness and in fact many of those who have to turn back and don't make it to the summit of Kilimanjaro, are fit men in their 20s and 30s, who probably push themselves too much and ignore the early warning signs of altitude sickness.

What is a Gamow bag?

This is a portable hyperbaric chamber, 'bag' which increases the pressure of available oxygen in the air. It can help with symptoms of HAPE and HACE if it is difficult to make an immediate descent. The patient is zipped inside the bag for several hours and the pressure inside is increased, using a foot pump. It is a short term help to buy time but descent as soon as possible is still the best treatment.

Photo of Gamow Bag

What about oxygen from cylinders for altitude sickness?

Using oxygen from cylinders will not help acclimatisation, as the body needs to adjust to lower levels of available oxygen.

Oxygen, if available, should be used in cases of severe AMS, HAPE and HACE. But the patient should still be taken to lower altitude and treated with acetazolamide and other medication. Oxygen from a cylinder can also be fed into the air inside a Gamow bag.

Should I take a pulse oximeter with me to altitude?

Pulse oximeters measure the heart rate and oxygen percentage in your blood through a small finger-tip device. They are now easily available to purchase on the internet.

As yet, there is no clear expert consensus on the normal oxygen values and heart rates whilst acclimatising, but a pulse oximeter may be a helpful additional tool in assessing acclimatisation. An indication of blood oxygen levels and heart rate is very useful in managing severe AMS, HAPE, or HACE.

Order acetazolamide 250mg
Dr Amanda Wood

Authored 12 September 2022 by Dr A. Wood
MB ChB Manchester University 1984. Former NHS GP in Bristol. GMC no. 2855422

Reviewed by Dr C. Pugh, Dr B. Babor
Last reviewed 04 October 2022
Last updated 19 July 2024