Altitude Sickness: How to Prevent, Recognize & Treat It

Q: "At what elevation does altitude sickness start?"

"Most people can develop altitude sickness above about 8,000 feet (2,400 m), though sensitive individuals feel it lower. Risk rises the higher and faster you go. Many popular trailheads — Rocky Mountain National Park, the Colorado 14ers, the high Sierra — sit right in or above this range, which is why visitors from low elevations so often feel it."

Q: "How do you prevent altitude sickness?"

"Ascend slowly and 'climb high, sleep low' — don't gain too much sleeping elevation per day above 8,000 ft. Spend a night at a moderate altitude to acclimatize before going high, stay well hydrated, avoid alcohol, don't over-exert on day one, and eat plenty of carbs. For fast ascents or a history of altitude sickness, ask your doctor about acetazolamide (Diamox)."

Q: "What is the fastest way to cure altitude sickness?"

"Descend. Going down even 1,000–3,000 feet relieves symptoms faster than anything else, and it's the definitive treatment for severe altitude illness. For mild symptoms, stop ascending, rest, hydrate, and let your body acclimatize. If symptoms are severe or worsening (confusion, breathlessness at rest, loss of coordination), descend immediately — it's a medical emergency."

Disclosure: Some links on this site are affiliate links. If you buy through them, we may earn a small commission at no extra cost to you. We only recommend gear we'd actually carry.

Altitude sickness doesn’t care how fit you are — ultramarathoners get it and grandmothers don’t. It’s about how high you go and how fast, not strength. And because so many iconic trails start high, it catches flatlanders off guard every summer. Here’s how to prevent it, spot it, and respond — including the one rule that can save a life.

What Causes It

As you climb, the air thins and there’s less oxygen in every breath. Go up faster than your body can adjust, and that oxygen shortfall triggers altitude sickness. It typically begins above ~8,000 feet and gets more likely the higher and faster you ascend. Acclimatization — your body’s natural adjustment — takes days, which is why a rushed ascent is the main culprit.

The Three Levels (Know the Difference)

1. AMS — Acute Mountain Sickness (common, mild): Feels like a hangover. Headache plus one or more of:

Nausea or loss of appetite
Fatigue and weakness
Dizziness
Trouble sleeping

AMS is usually manageable — but it’s the warning sign that you’ve gone too high too fast.

2. HAPE — High-Altitude Pulmonary Edema (severe, fluid in the lungs):

Breathlessness at rest
Persistent cough, chest tightness, gurgling breath
Extreme fatigue, blue-tinged lips/nails

3. HACE — High-Altitude Cerebral Edema (severe, swelling in the brain):

Confusion, altered behavior
Loss of coordination (can’t walk a straight line — the key test)
Severe headache, drowsiness

HAPE and HACE are life-threatening emergencies. The treatment is immediate descent.

How to Prevent It

Ascend slowly — “climb high, sleep low.” Above 8,000 ft, limit how much you raise your sleeping elevation each day (~1,000–1,500 ft/day is a common guideline). You can hike higher during the day, just sleep lower.
Acclimatize first. Spend a night at a moderate elevation before going high. Arriving from sea level and sleeping at 10,000 ft the same night is asking for it.
Hydrate well and skip the alcohol (especially the first day or two).
Don’t over-exert on arrival — take day one easy.
Eat carbs — your body uses oxygen more efficiently burning carbohydrates.
Medication: for rapid ascents or a personal history of altitude sickness, ask your doctor about acetazolamide (Diamox), which speeds acclimatization.

How to Treat It

Mild AMS: stop ascending, rest at your current elevation, hydrate, take ibuprofen/acetaminophen for the headache, and let your body catch up. Don’t go higher until symptoms resolve.
Not improving, or getting worse: descend. Going down 1,000–3,000 ft usually brings fast relief.
HAPE or HACE (severe): descend immediately, get the person to medical care, and use a satellite communicator to call for help if you’re beyond cell service. Don’t wait.

The Golden Rule

If symptoms are getting worse, go down. If someone can’t walk a straight line or is breathless at rest, descend now — it’s an emergency.

Never leave someone with severe symptoms alone, and never keep climbing to “push through” worsening altitude illness.

Where This Matters Most

Plenty of bucket-list hikes start high. Plan acclimatization for places like Rocky Mountain National Park (trailheads at 8,000–9,500 ft, peaks over 14,000), the Colorado 14ers, the high Sierra, and the climb-out at the Grand Canyon. Arriving from sea level? Give yourself a day to adjust before the big effort. A pulse oximeter can help you monitor oxygen saturation on high trips, and electrolytes help you stay hydrated.

Bottom Line

It’s about ascent speed, not fitness — anyone can get it above ~8,000 ft.
Prevent it: go up slowly, sleep low, hydrate, skip alcohol, eat carbs.
Treat it: stop, rest, hydrate — and descend if it worsens.
Severe signs (confusion, breathlessness at rest, loss of coordination) = descend immediately.

Respect the altitude and it won’t ruin your trip. Ignore it and it can turn dangerous fast.

Climb smart. Sleep low. Come home.

Frequently Asked Questions

At what elevation does altitude sickness start?

Most people can develop altitude sickness above about 8,000 feet (2,400 m), though sensitive individuals feel it lower. Risk rises the higher and faster you go. Many popular trailheads — Rocky Mountain National Park, the Colorado 14ers, the high Sierra — sit right in or above this range, which is why visitors from low elevations so often feel it.

How do you prevent altitude sickness?

Ascend slowly and ‘climb high, sleep low’ — don’t gain too much sleeping elevation per day above 8,000 ft. Spend a night at a moderate altitude to acclimatize before going high, stay well hydrated, avoid alcohol, don’t over-exert on day one, and eat plenty of carbs. For fast ascents or a history of altitude sickness, ask your doctor about acetazolamide (Diamox).

What is the fastest way to cure altitude sickness?

Descend. Going down even 1,000–3,000 feet relieves symptoms faster than anything else, and it’s the definitive treatment for severe altitude illness. For mild symptoms, stop ascending, rest, hydrate, and let your body acclimatize. If symptoms are severe or worsening (confusion, breathlessness at rest, loss of coordination), descend immediately — it’s a medical emergency.

Free Checklist

Get the Sub-10 lb Ultralight Gear Checklist

Join the free PackLite Life newsletter — new gear guides, trip reports, and trail-tested tips — and grab the printable checklist when you sign up. No spam, unsubscribe anytime.