Preventing altitude illness in adults: preparing travelers for >2500 m

What is altitude illness and who might be affected?

Altitude illness is the collective term for syndromes that can develop in unacclimatised people who travel to elevations above 2500 m. The spectrum includes acute mountain sickness (AMS), high altitude cerebral oedema (HACE), and high altitude pulmonary oedema (HAPE). AMS is the commonest presentation and can progress to the life threatening conditions HACE and HAPE if not recognised and managed appropriately.

Symptoms usually appear during or after the first night at high altitude rather than immediately on arrival. On initial exposure, mild lightheadedness or tachypnoea (increased respiratory rate) can be normal physiological responses to hypobaric hypoxia. However, when symptoms suggestive of altitude illness arise at altitudes above 2500 m, clinicians and travellers should assume altitude illness until an alternative cause is established.

Epidemiology and risk estimates reported in the source text include that AMS may occur in up to about a quarter of people with rapid ascent above 2500 m. HACE is less common but has been reported in up to approximately 1% of travellers at 4000–5000 m, and HAPE in up to about 6% of travellers at around 4500 m. Peripheral oedema can occur at high altitude but is not specific to altitude illness.

Pathophysiology across the spectrum is driven by hypobaric hypoxia — reduced availability of oxygen at altitude owing to decreased barometric pressure rather than a change in oxygen concentration. AMS and HACE represent cerebral responses to hypoxia, including increased cerebral blood volume and intracellular oedema. HAPE is a non‑cardiogenic pulmonary oedema caused by exaggerated hypoxic pulmonary vasoconstriction and raised pulmonary artery pressures.

Commonly visited destinations and altitude categories referenced in the source include:

• High altitude (2500–3500 m): some ski resorts and cities with high‑altitude airports (examples cited include Bogota, Quito, Cusco, La Paz, and Lhasa).
• Very high altitude (3500–5500 m): trekking routes such as Everest Base Camp and the Annapurna circuit or travel across the Altiplano.
• Extremely high altitude (>5500 m): high peak ascents such as Mount Kilimanjaro or Aconcagua.

Those most at risk include people with a prior history of altitude illness and individuals who ascend rapidly (defined in guideline terms as more than 500 m sleeping altitude gain between consecutive nights). Recreational trekkers, professional mountaineers, support teams (for example, porters), people travelling to high‑altitude work sites (miners, military personnel), and others undertaking high‑altitude exposures without adequate acclimatisation are typical at‑risk groups.

Key mountain medicine terms highlighted in the source and important for clinical discussions with travellers are:

• Sleeping altitude: the elevation at which a person sleeps; considered more relevant for risk than daytime maximum altitude.
• Sleeping altitude gain: increase in sleeping altitude between two consecutive nights.
• Rate of ascent: sleeping altitude gains per day above 3000 m; guideline safe ascent rates are generally cited as 300–500 m per day.
• Rapid ascent: sleeping altitude gain >500 m between nights, increasing altitude illness risk.
• Acclimatisation: physiological adaptation to hypobaric hypoxia; can be aided by adding extra days at the same sleeping elevation (commonly one day after each ~1000 m gain, or operators may schedule extra rest days every 2–3 days assuming a 300–500 m/day ascent).

If altitude illness is suspected during travel, the practical advice given in the source excerpt is to stop further ascent for mild–moderate AMS, and to descend and seek medical attention for severe AMS, HACE, or HAPE.

How are people assessed before high altitude travel?

Optimal practice is for travellers to seek advice before booking travel. Pre‑travel assessment aims to stratify health and altitude‑illness risk and to plan mitigation. Clinicians should emphasise that higher baseline physical fitness and younger age do not confer protection against altitude illness; intense exertion increases oxygen demand and can raise risk irrespective of fitness level. Including extra acclimatisation days in itineraries reduces exertional demands and helps decrease risk.

Initial assessment recommended by the 2024 Wilderness Medical Society (WMS) and UIAA Medical Commission guidance includes asking about:

• Current comorbidities and whether these are stable.
• Personal history of known or suspected altitude illness.
• Detailed travel plans: destinations, planned maximum sleeping altitudes, and whether descent of at least 500 m per day is feasible if needed.
• The planned timetable and mode(s) of travel, including any rapid travel segments that might lead to fast elevation gain.
• Whether extra acclimatisation days are incorporated into the itinerary and the expected level of physical exertion during the trip.
• Availability of local medical support during travel, including whether a medical professional will accompany the group and the capabilities of local facilities.
• Travel medical insurance that covers in‑country treatment and medical evacuation from remote high‑altitude locations to lower altitude urban centres with adequate care.

Clinicians are advised to use validated stratification tools referenced in the 2024 WMS guidance to estimate individual risk. The source notes that tables in the guidance summarise risk stratification schemes, with the rate of ascent identified as the most important modifiable risk factor. The travel itinerary usually provides the necessary information to determine the rate of ascent and sleeping altitude gains and thereby to guide preventive advice.

Where appropriate, risk can be managed by modifying travel plans (slower ascent, extra acclimatisation days) and, in some cases, using pharmacological prevention. The provided source excerpt references the 2024 WMS and UIAA guideline updates and the authors’ clinical experience as the basis for recommendations. Specific pharmacological prevention options, detailed thresholds, and the remainder of the guideline recommendations were not included in the provided excerpt and therefore are not reported here.

Note on source text: the supplied source content ends partway through a discussion of risk stratification and the importance of rate of ascent. The remainder of the original article, including full tables, detailed prevention measures, pharmacological guidance, and any algorithmic or checklist material, was not present in the excerpt and therefore is not summarised or restated here.