This is general safety information, not medical advice. Consult a doctor before travelling to altitude or taking any medication. In an emergency, call the local emergency number — 112 anywhere in the EU, 911 in the US and Canada.
Altitude illness is unusual among mountain hazards in one happy respect: the current medical guidelines about it are public, specific, and free to read. The reference of record is the Wilderness Medical Society's 2024 practice guideline update; the UIAA Medical Commission and the UK's TravelHealthPro publish the field-facing versions. Everything below is drawn from those documents. Nothing here is a treatment protocol — the job of a trekker is to recognize the illness, stop, descend, and get help.
Three conditions, one picture
| AMS (acute mountain sickness) | HACE (high-altitude cerebral edema) | HAPE (high-altitude pulmonary edema) | |
|---|---|---|---|
| Typical altitude | Above 2,500 m (susceptible people lower) | Above ~4,000-5,000 m (below that possible but rare) | Above ~3,000 m |
| Typical onset | 6-10 hours after ascent | Commonly develops out of AMS | More than 24 hours after ascent |
| What it looks like | Headache, loss of appetite, nausea or vomiting, fatigue, dizziness, poor sleep | Confusion, altered consciousness, incoordination (ataxia) | Breathlessness on exertion and a dry cough, progressing to breathlessness at rest or lying flat, a wet cough, blood-stained sputum |
| What it is | The common, usually self-limiting form | Swelling of the brain — an emergency | Fluid in the lungs — an emergency |
The details, in the guidelines' own framing. TravelHealthPro (the UK national travel-health network) states that altitude illness "usually occurs at altitudes over 2,500m, however… susceptible individuals can experience illness below this altitude," with AMS symptoms typically appearing "6–10 hours after ascent"; the UIAA Medical Commission gives the onset window as more than 4 and less than 24 hours after arriving at a new altitude. The NHS lists the same AMS symptom set.
HACE is "caused by swelling of the brain… characterized by the onset of confusion, altered consciousness and or incoordination (ataxia). HACE is commonly preceded by AMS" — and in the UIAA's words, AMS "can quickly lead to HACE which is a life threatening emergency." HAPE is "the respiratory form of altitude illness," and the UIAA flags reduced exercise tolerance as an early warning.
One sourcing note: the often-quoted "6-12 hours" for AMS and "2-4 days" for HAPE are figures we could not verify against any live official page, so we print the verified ones — 6-10 hours, and onset beyond 24 hours — instead.
Sources: TravelHealthPro — altitude illness; NHS — altitude sickness; UIAA MedCom Consensus No. 2 (PDF).
How common it actually is
The verified figures, from TravelHealthPro: "Approximately 9–25 percent of unacclimatised individuals ascending to 2,000–3,000m develop AMS compared to 35–50 percent of those ascending to 3,500–4,500m. HACE and HAPE are much less common than AMS. Both are extremely rare below 2,800m and seem to occur at an incidence of around 1–2 percent at altitudes between 4–5,000m."
Read that as a planning fact, not a curiosity: on a trek that sleeps at 3,500-4,500 m, something between a third and a half of unacclimatized walkers will develop AMS. It is the normal cost of going up too fast — which is why the prevention rules below are itinerary rules, not medicine.
Source: TravelHealthPro — altitude illness.
The self-check: the 2018 Lake Louise score
The standard field self-assessment is the Lake Louise AMS Score, revised in 2018 (Roach et al., High Altitude Medicine & Biology). The revision removed sleep disturbance, leaving four scored symptoms:
- Headache
- Nausea or vomiting
- Fatigue
- Dizziness or light-headedness
Each is rated 0-3, and the definition is strict: "For a positive AMS definition, it is mandatory to have a headache score of at least one point, and a total score of at least three points." No headache, no AMS by this definition — but the golden rules below still apply to whatever you are feeling.
Sources: PMC — The 2018 Lake Louise Acute Mountain Sickness Score; PubMed record.
The golden rules, verbatim
The UIAA Medical Commission compresses field judgement into four sentences. They are worth memorizing exactly as written:
"All symptoms at altitude must be considered as altitude related until proven otherwise."
"No further ascent with symptoms!"
"Rest immediately, never continue ascent!"
"Descend if symptoms do not improve or worsen within 24 hours."
The first rule is the one that saves lives, because the alternative explanations — dehydration, a bad night, "just tired" — are exactly what people tell themselves while ascending into HACE.
Source: UIAA MedCom Consensus No. 2 (PDF).
Prevention is an itinerary, not a pill
The guideline numbers for acclimatization are concrete enough to plan a trek around. The UIAA: "Acclimatize properly! Above 2500-3000m the next night should not be planned more than 300-500m higher than the previous one. Have two nights at the same altitude after every 2-4 days of ascent. On this day you may climb higher but return to sleep" — the classic "climb high, sleep low."
TravelHealthPro, restating the Wilderness Medical Society's risk categories, draws the same line from the other side: high risk includes "All individuals ascending >500 m/day (in sleeping elevation) above 3,000m without extra days for acclimatisation"; the same ascent rate with an extra acclimatization day every 1,000 m drops to moderate risk. What matters is sleeping altitude — how high you walk during the day matters far less than where the night is spent.
Sources: UIAA MedCom Consensus No. 2 (PDF); TravelHealthPro — altitude illness.
Acetazolamide: what the guidelines say, and what a doctor is for
We quote here and prescribe nothing — medication for altitude is a decision for you and a doctor, before the trip. TravelHealthPro, in line with the WMS guidance, states: "Acetezolamide (Diamox®) is the preferred drug (unlicensed). The recommended dose is 125mg twice daily to be commenced one day prior to ascent to high altitude and then continued for at least two days after reaching the highest altitude." It flags a caution for people with sulfonamide allergy — a history of anaphylaxis or Stevens-Johnson syndrome rules it out, and lesser sulfa allergies are a conversation with a clinician. The UIAA adds that drug prophylaxis "should be restricted to some special situations" rather than used as a default.
And the sentence that outranks all of the above, from the same guideline: "Preventative medications are not a substitute for gradual ascent." The itinerary is the prevention; the pill is an adjunct some doctors recommend for some travellers.
Sources: TravelHealthPro — altitude illness; UIAA MedCom Consensus No. 2 (PDF); PubMed — WMS 2024 guideline.
When it stops being discomfort and becomes an emergency
For AMS, the guideline response is modest: stop ascending, rest, treat the headache with "basic analgesia (painkillers) such as ibuprofen or paracetamol" in TravelHealthPro's words, and descend if there is no improvement within 1-2 days.
HACE and HAPE are different in kind. They are emergencies, and the response has two parts, in this order:
- Descend. The UIAA's instruction for both, in capitals in the original: "DESCENT… TO BELOW ALTITUDE WHERE SYMPTOMS BEGAN." For HAPE it adds: keep the person sitting upright, and keep them warm.
- Call rescue. 112 anywhere in the EU; 911 in the US and Canada. A person with ataxia, confusion, or breathlessness at rest needs evacuation, not encouragement.
You may read about oxygen, portable hyperbaric chambers and drugs such as nifedipine or dexamethasone. TravelHealthPro's framing is the correct one for a trekker: these "are sometimes carried by experienced climbers, medics and guides. When available, these can be used to aid descent" — aid descent, not replace it. None of it is trekker self-treatment, and none of it changes the two-step response above.
Sources: UIAA MedCom Consensus No. 2 (PDF); TravelHealthPro — altitude illness; European Commission — 112; NPS — Climbing: staying safe.
Who needs a doctor before the trip
Two groups get named in the guidelines. People with pre-existing medical conditions "should consult with their healthcare provider prior to travel" — remote areas mean remote care, and specific conditions carry specific altitude risks that only a clinician can weigh. For children, "travel to altitudes up to 2,500m is considered to be low risk in healthy children," with the evidence base limited; the UIAA's Consensus No. 9 (Children at Altitude) is the dedicated guidance.
For readers who want the primary documents: the WMS 2024 guideline update (Luks et al., Wilderness & Environmental Medicine 2024) is the reference of record, and the UIAA Medical Commission maintains a free public library of advice sheets — field management of AMS/HAPE/HACE, drugs at altitude, women at altitude, children at altitude.
Sources: TravelHealthPro — altitude illness; UIAA MedCom No. 9 — Children at Altitude (PDF); UIAA — medical advice library; PubMed — WMS 2024.
Sources
- TravelHealthPro — altitude illness factsheet — thresholds, onset, incidence, WMS risk categories, acetazolamide statement, treatment framing
- NHS — altitude sickness — AMS symptom set
- UIAA MedCom Consensus No. 2 — AMS/HAPE/HACE (PDF, V3.3) — golden rules, acclimatization rules, onset windows, emergency response
- PMC — The 2018 Lake Louise Acute Mountain Sickness Score — scoring criteria
- PubMed — Lake Louise 2018 record — citation of record
- PubMed — WMS Clinical Practice Guidelines, 2024 Update — the anchor guideline
- PubMed — WMS 2019 Update — prior version, for comparison
- UIAA — medical advice library — free advice-sheet shelf
- UIAA MedCom No. 9 — Children at Altitude (PDF) — children-specific guidance
- European Commission — 112 — the EU emergency number
- NPS — Climbing: staying safe — 911 guidance