China has some of the world’s richest high-altitude terrain: Tibet (average elevation 4,000m), Qinghai (most of the Qinghai-Tibet Plateau), Yunnan (parts above 4,000m), and western Sichuan (Gongga, Daocheng) are the most popular high-altitude outdoor destinations. Altitude sickness and acclimatization are essential medical knowledge for all high-altitude travelers.
## Physiological Mechanisms of Altitude Hypoxia
As elevation increases, atmospheric pressure decreases and oxygen partial pressure (PO₂) falls: sea level PO₂ is approximately 159mmHg; at 3,500m it’s ~125mmHg (a 21% reduction); at 5,000m ~100mmHg (a 37% reduction). Low PO₂ triggers a series of physiological compensatory responses: increased respiratory rate (expelling CO₂ to raise blood pH), increased heart rate, increased EPO secretion (stimulating bone marrow to produce more red blood cells), and renal bicarbonate excretion adjustment (compensating for respiratory alkalosis). This adaptation process requires days to weeks — the biological basis of acclimatization.
## Distinguishing and Managing Three Types of Altitude Illness
**Acute Mountain Sickness (AMS)**: most common, occurring 12-24 hours after rapid ascent above 2,500m. Symptoms: headache (required) + at least one of nausea, decreased appetite, fatigue, dizziness. Assessment tool: Lake Louise Score (≥3 diagnoses AMS). Management: stop ascending, descend at least 500m if necessary; acetaminophen/ibuprofen for headache; acetazolamide (Diamox, 250mg twice daily) for prevention and treatment.
**High Altitude Cerebral Edema (HACE)**: severe progression of AMS with ataxia (inability to walk a straight line) and altered consciousness (drowsiness, confusion). Management: immediate descent (first priority) + dexamethasone 8mg IV or IM (4mg every 6 hours maintenance) + portable hyperbaric oxygen chamber (Gamow Bag). HACE is a high-altitude emergency — do not wait.
**High Altitude Pulmonary Edema (HAPE)**: the most lethal altitude illness (untreated mortality rate up to 50%). Symptoms: excessive dyspnea on exertion → dyspnea at rest → coughing pink frothy sputum. Management: immediate descent + supplemental oxygen + nifedipine (to lower pulmonary artery pressure).
## Prevention Strategies
“Ascend slowly” is the gold standard: above 3,000m, ascend no more than 300-500m per day, with one acclimatization rest day for every 1,000m gained. Acetazolamide (Diamox) 125-250mg twice daily can prevent AMS; begin 1-2 days before ascent; contraindicated in sulfa allergy.
See [Hiking Science Guide](https://sunqi.org/hiking-science-guide-en/) and [Wilderness Medical Society altitude illness guidelines](https://wms.org/).
