What is Altitude Sickness?
Altitude sickness (Acute Mountain Sickness, or AMS) occurs when you ascend to high elevation too quickly for your body to adapt to the lower oxygen levels. It can affect anyone — regardless of age, fitness level, or prior climbing experience.
On Mount Fuji, symptoms typically begin to appear above the 8th station (around 3,000 meters). In the Japan Alps, they can occur on any route that gains more than 1,500 meters in a day, especially if you start from near sea level.
How Elevation Affects Oxygen
At sea level, the air contains about 21% oxygen. At 3,776 meters (Fuji's summit), the air still contains 21% oxygen — but the atmospheric pressure is roughly 60% of sea level. This means each breath delivers significantly less oxygen to your bloodstream. Your body compensates by breathing faster and deeper, but this takes time.
Symptoms
Mild Symptoms (Common)
- Headache (the most common symptom)
- Dizziness or lightheadedness
- Fatigue or weakness
- Shortness of breath
- Loss of appetite
- Mild nausea
- Difficulty sleeping
Action: Rest, hydrate, and monitor. Do not ascend further until symptoms improve.
Moderate Symptoms (Warning)
- Severe headache unrelieved by rest/painkillers
- Ataxia (loss of coordination, stumbling)
- Nausea and vomiting
- Shortness of breath at rest
- Confusion or impaired judgment
Action: Stop ascending immediately. Descend at least 500-1,000 meters. Seek medical attention if symptoms don't improve.
Severe Symptoms (Life-Threatening)
- HAPE: Fluid in lungs (wet cough, frothy sputum)
- HACE: Brain swelling (confusion, altered mental state)
- Inability to walk straight
- Loss of consciousness
Action: Immediate descent is the only treatment. Call for rescue if the person cannot walk. This is a life-threatening emergency.
Prevention
Before You Climb
- Acclimatize: If possible, spend a night at elevation before your climb. Matsumoto (600m) is better than Tokyo (0m) as a staging point.
- Get fit: Cardiovascular fitness helps your body utilize oxygen more efficiently.
- Stay hydrated: Begin hydrating 2-3 days before your climb.
- Avoid alcohol: Alcohol dehydrates you and worsens altitude effects.
- Sleep well: Being well-rested improves your body's adaptation response.
During Your Climb
- Pace yourself: "Pole pole" (slowly, slowly) — the mountaineer's mantra.
- Hydrate constantly: Drink 3-4 liters of water per day at altitude.
- Eat carbohydrates: Carbs are more efficient at altitude than fats or protein.
- Rest at huts: If doing a 2-day climb, the overnight rest helps acclimatization.
- Don't overexert: Keep your heart rate moderate. If you're breathing too hard, slow down.
Medication
Diamox (acetazolamide) is the standard preventive medication for altitude sickness. It helps your body acclimatize faster by increasing breathing rate.
Diamox Guidelines
- Start 24 hours before ascent
- Typical dose: 125-250mg twice daily
- Continue throughout ascent
- Side effects: increased urination, tingling fingers/toes
- Consult your doctor before use
Note: Diamox masks symptoms but doesn't prevent the underlying condition. If symptoms appear, you still need to stop or descend. It is not a substitute for proper acclimatization.
The Golden Rule
Never ascend to sleep at a higher elevation if you have symptoms of altitude sickness. Descend, rest, and try again another day.