Altitude Sickness: Prevention Basics and Red Flags

Altitude illness is ordinary physics meeting human biology: go up faster than your body can adapt and you get sick - predictably enough that the consensus guidelines read like a recipe. This guide covers the three illnesses, the ascent rules that prevent most of it, and the symptoms that end all discussion. It is not medical advice.

The three illnesses

AMS - acute mountain sickness. The common form: headache plus nausea, fatigue or poor sleep, arriving hours after arriving at altitude. Unpleasant, usually self-limiting, and the only treatment that always works is time or descent. It is also your early-warning system - AMS is the body's notice that ascent is outpacing adaptation.

HACE - high-altitude cerebral edema. AMS's rare, lethal progression: fluid in the brain. Confusion, drowsiness, and the classic field test - an inability to walk heel-to-toe in a straight line. Descend immediately; HACE kills in hours.

HAPE - high-altitude pulmonary edema. Fluid in the lungs, often without AMS first. The signs: breathlessness that persists at rest, a wet cough, cyanosis. Also an immediate-descent emergency. HAPE famously recurs in susceptible people, and a history of it changes trip planning permanently.

The prevention rules

The Wilderness Medical Society consensus boils down to a schedule for sleeping altitude - because acclimatization happens where you sleep, not where you day-hike:

• Above 3,000 m: no more than +500 m of sleeping altitude per day. Climb higher during the day if you like ("climb high, sleep low"), but come down to sleep.
• A rest day every 1,000 m gained. A same-altitude day - day hikes welcome.
• Below 3,000 m, few rules. Most itineraries can get to 2,500-3,000 m in a day; it is the next 2,000 m that punish haste.
• Day one matters. Easy exertion and generous fluids on the first day at a new sleeping altitude measurably reduce AMS.

The altitude sickness risk calculator scores an itinerary against exactly these rules - per-day gain, acclimatization nights, starting altitude and personal history - and returns a risk band with the matching advice.

What actually helps on the mountain

• Time is the only natural acclimatizer. Every drug and gadget modifies the schedule; none replaces it.
• Hydration keeps you honest. It does not prevent AMS, but dehydration feels like altitude and blurs your read on yourself.
• Descend for symptoms that persist or worsen overnight. The rule is conservative on purpose: at altitude, descending is nearly always safe; waiting to see is not.
• Know your history. Prior AMS roughly doubles the odds of a repeat; prior HAPE or HACE is a different league - preventive medication and a conservative profile belong in the plan from the start.

Where the environment helps

Altitude pairs its thin air with cold nights and slow stoves - the same trip needs the altitude temperature calculator for the evening, the boiling point calculator for the kitchen (see the altitude cooking guide), and the wind chill calculator for exposed ridgelines.

Frequently asked questions