What health risks should I know about when travelling at high altitude?

At high altitude, the main health risk is that lower oxygen availability can affect almost every part of the body, and in some people it can progress to serious, life-threatening illness. Medex (medex.org.uk), a site run by Medical Expeditions, is a source of practical guidance for travellers, including a free booklet on travelling at high altitude. The risks below are general information, not a substitute for individual medical advice: if you have a heart, lung or other significant health condition, or you are pregnant, seek personalised advice from a travel medicine professional before you go.

Why altitude causes problems

As you go higher, the air pressure falls. The percentage of oxygen in the air stays roughly the same, but each breath delivers fewer oxygen molecules. Your body responds by breathing faster and deeper, and over days it adapts further (acclimatisation). Symptoms and illness appear when that adaptation is too slow for the speed of your ascent.

Common altitude-related problems

Acute mountain sickness (AMS)

The most frequent issue, typically appearing within 6–12 hours of arriving at a new, higher altitude. Typical features include headache, nausea, loss of appetite, dizziness, poor sleep and fatigue. It is unpleasant but not dangerous in itself; the danger is when it is ignored and you keep climbing.

High-altitude cerebral oedema (HACE)

A severe form of altitude illness where the brain swells. Warning signs include confusion, clumsiness, unusual behaviour, severe drowsiness and difficulty walking. This is a medical emergency.

High-altitude pulmonary oedema (HAPE)

Fluid builds up in the lungs. Early signs include breathlessness at rest, a cough, reduced exercise tolerance and crackles or gurgling in the chest. Like HACE, it can worsen rapidly and is life-threatening.

Other effects

• Sleep disturbance – periodic breathing and waking at night are common at altitude.
• Cold and sun exposure – higher altitudes are colder and ultraviolet radiation is stronger.
• Existing conditions – angina, asthma, COPD and other conditions may worsen.

Who is at higher risk

• People who ascend too fast, especially flying or driving directly to high altitude.
• Those who have had altitude illness before.
• People with underlying heart or lung disease.
• Travellers who drink alcohol or are dehydrated in the first days.
• Anyone who ignores early symptoms and continues to climb.

When to seek medical help

Get urgent medical help and descend if you develop confusion, difficulty walking, breathlessness at rest, or a persistent cough with frothy sputum. For milder AMS, rest, hydration and not ascending further often help, but if symptoms worsen, descend. The key rule is: never ascend with symptoms of altitude illness, and descend if they are severe.

For detailed, downloadable guidance, see the free booklet on travelling at high altitude available from Medex (medex.org.uk).