Altitude sickness on a hike, and the descent that cures it

The only cure for altitude sickness is to travel down to a lower elevation.

1. 01

   Stop going up

   It's the cheapest thing you can do and the one that changes the most. One of the CDC's three rules is never to ascend to sleep at a higher altitude while you have symptoms of altitude illness, however minor they seem. Standing around on a cold ridge is a miserable way to spend an afternoon, and it still beats a summit push.

2. 02

   Give it a couple of hours where you are

   Sit down, eat something, drink to thirst, take a non-opiate painkiller for the headache. Mountain sickness generally settles within 12 to 48 hours provided nobody climbs any higher, and plenty of what feels catastrophic at 12,000 feet turns out to want lunch and an hour off your feet.

3. 03

   Run the heel-to-toe test

   Scratch a line in the dirt and have them walk it heel to toe with their arms out. Someone with ordinary mountain sickness can do it. Someone who staggers, steps off, or has to be caught has ataxia, and that's cerebral edema. Test whoever looks worst, then test them again in an hour.

   Test the person, not the argument. People with HACE are often confident they're fine.

4. 04

   Watch the breathing when nobody's moving

   Sit them down for ten minutes. If they're still short of breath, or coughing, or their chest feels tight and full, treat it as pulmonary edema. It usually builds over a day or two rather than hitting during the climb, and blood oxygen sits at its lowest while you're asleep, so a strong day doesn't rule it out.

5. 05

   Go down, and keep going until it helps

   A descent of 1,000 feet or more usually improves mountain sickness. Carry their pack so they aren't working on the way down. For the two serious versions there's no number to aim at: you go down until they're better, and then you keep going.

6. 06

   Don't sleep on the decision

   Whatever you choose, choose it before dark. A descent that's straightforward at six in the evening is a rescue at two in the morning.

1. A week out

   Pick the trip by its elevation profile, not its mileage

   A twelve mile day at 6,000 feet and a six mile day at 13,000 feet are different problems, and only one of them is about your legs. Look up the trailhead elevation and the high point before you look at the distance.

2. Two or three nights before

   Sleep somewhere in between first

   If you only do one thing off this list, do this one. The CDC describes two to three nights at roughly 8,000 to 9,000 feet, before going higher, as markedly protective against mountain sickness. Check the elevation of the town you were going to sleep in, because plenty of gateway towns sit below that band. A night at the Whitney Portal campground isn't downtime before the trip. It's the trip's first day.

3. The night before

   Expect to sleep badly, and don't chemically fix it

   Above about 9,000 feet, periodic breathing is close to universal and it will wake you. That's normal and it passes. The nightcap is the part to skip: the CDC says to avoid alcohol for the first 48 hours, and it names alcohol and opiates as respiratory depressants that shouldn't be used to sleep at altitude. If you want a sleep aid, that's a question for your doctor before you leave.

4. On the hike

   Go slower than feels necessary, and keep eating

   Nobody's quick on their first day high, and the people who blow up are usually the ones who felt strong at the trailhead. Hold a pace you could talk through. Keep eating, because appetite is one of the first things altitude takes away from you.

5. If it runs overnight

   The sleeping elevation is the one to cap

   Once you're sleeping above about 9,800 feet, the CDC's cap is no more than 1,650 feet of gain in sleeping elevation per night, plus an extra night to acclimatize for every 3,300 feet gained. That rule is the Wilderness Medical Society's, and its 2024 update now writes the second half as a rest day every three or four days. Walking high during the day and dropping back down to sleep is a separate practice, and an old one: climb high, sleep low.

Hypoxia

Not enough oxygen reaching your tissues. At altitude it happens because falling air pressure puts fewer oxygen molecules in each breath, not because the composition of the air changes.

Acclimatization

The set of changes your body makes over days at altitude: harder breathing first, then a correction to your blood chemistry by the kidneys. Extra red cells come later, over weeks, and do nothing for you in the first few days. Measured in nights, and it fades over the couple of weeks after you come home.

Acute mountain sickness (AMS)

The common form of altitude illness. A headache plus at least one of nausea, loss of appetite, dizziness or heavy fatigue, usually 2 to 12 hours after arriving somewhere higher.

High altitude cerebral edema (HACE)

Fluid swelling in the brain, and effectively end-stage mountain sickness. Altered mental state, confusion, drowsiness, and above all ataxia. A descend-now diagnosis.

High altitude pulmonary edema (HAPE)

Fluid in the lungs, which is why it shows up as breathlessness at rest, a dry cough and a tight chest rather than as a headache. Also a descend-now diagnosis.

Ataxia

Loss of coordinated movement. The field test at altitude is walking heel to toe along a straight line, and failing it is what separates a bad headache from an emergency.

Sleeping altitude

The elevation you spend the night at, as distinct from the high point you walked to. Nearly all the ascent-rate guidance is written about this number, because it's the one that drives the risk.

Periodic breathing

The waxing and waning breathing pattern almost everyone develops asleep above about 9,000 feet, with pauses that wake you up. Unpleasant, normal, and not by itself altitude sickness.

Acetazolamide

A prescription drug that speeds acclimatization by nudging your blood chemistry in the direction it was already heading. A conversation with a doctor before a trip, not something to start on a ridge.