About altitude sickness
If you ascend to an area of high altitude, you are likely to experience some form of altitude sickness. Usually, the symptoms are mild and will improve if you descend.
Altitude sickness is most likely to affect you if you ascend quickly - especially at a rate of more than 500m per day - or if you don't allow yourself time to get used to the height (acclimatise).
Acute mountain sickness (AMS) is the most common and mildest form of altitude sickness. If you go on a climbing expedition or stay in an area of high altitude, you may develop AMS. High-altitude pulmonary oedema (HAPE) and high-altitude cerebral oedema (HACE) are much more severe forms of altitude sickness. They usually affects you only if you are at very high or extreme altitude.
What counts as high altitude?
High altitude refers to heights that are between 1,500 and 3,500m above sea level.
Very high altitude is from 3,500 to 5,500m above sea level.
Extreme altitude is from 5,500 to 7,500m above sea level.
Examples of commonly visited areas of high altitude include Cusco in Peru (about 3,300m) and La Paz in Bolivia (about 3,600m). Mountain climbers may reach higher altitudes on Mount Kilimanjaro (5,895 metres) or the Inca Trail (4,400 metres).
Causes of altitude sickness
At high altitudes, the air is at a lower pressure than it is nearer sea level. This makes it harder to get oxygen out of the air you breathe in, and into your bloodstream. Your body responds by increasing your breathing and heart rate.
There are certain factors that may make you more likely to develop altitude sickness; for example, if you:
- have had altitude sickness before
- do strenuous activity or exercise at high altitude
- rapidly ascend to high altitude
- have a lung infection
If you have diabetes, or a heart or lung condition, you should check with a doctor before travelling to places at altitude. Many people with such conditions can cope very well but certain severe conditions may mean it's unwise for you to travel to, or climb in, areas of high altitude.
Symptoms of altitude sickness
You may notice symptoms of altitude sickness about six to 24 hours after you've arrived at an area of high altitude. However, this varies from person to person and will depend on the speed of your ascent. If you've been climbing or ascending slowly, the onset of your symptoms will be more gradual.
If you have altitude sickness in its mildest form (AMS), you may:
- have a headache
- feel tired
- feel sick or vomit
- lose your appetite
- feel dizzy
- have difficulty sleeping
Symptoms of AMS usually start to ease within about two days as your body acclimatises to the high altitude, particularly if you don't ascend any further. If your symptoms get worse, the best thing you can do is descend as quickly as possible. There are some medications that can help ease your symptoms and treat complications, but only descending will deal with the cause.
Complications of altitude sickness
If you ascend to an area of high altitude too quickly and don't allow your body time to acclimatise, you may develop a serious form of altitude sickness such as HAPE or HACE. These conditions are rare, but potentially fatal if you don't descend immediately and receive treatment.
HAPE occurs when fluid builds up in your lungs. Symptoms usually develop between two and four days after a rapid ascent over 2,500m. You may:
- have a dry cough
- feel breathless at rest
- have a fever
- feel confused
- have pink or bloody spit
- have a bluish tinge to your skin, lips and nails (cyanosis)
If you have severe symptoms of HAPE, you may gasp for breath and make gurgling sounds when you breathe.
HAPE can occur by itself or in conjunction with HACE. HACE occurs when excessive fluid collects in your brain, causing it to swell. Fewer than two percent of people with altitude sickness develop HACE and it rarely occurs below 4,000m. If you have HACE you may:
- have a severe headache
- feel very confused
- lose co-ordination
- have blurred or double vision
- notice changes in your behaviour - for example, you may feel irritable or be unhelpful
- lose consciousness
- hallucinate (see, hear, feel and smell odd things that aren't really there)
- fall into a coma (this is rare)
The symptoms of HACE may progress rapidly from mild to life-threatening within a few hours. If you have these symptoms, you should descend to a lower altitude as soon as possible and seek urgent medical advice.
You may develop other symptoms at high altitude. For example, your face, arms or legs may swell up (peripheral oedema). This usually lasts a few days and then disappears. At altitudes of 5,000m and over, tiny blood blisters can form at the back of your eye (retinal haemorrhages), but you may not notice any symptoms and they only occasionally interfere with your vision.