Altitude Skills · Part 2 Updated

How your body fights back.

Left alone at Leh on day one, your blood is short of oxygen. Your body does not fix that with one trick. It runs four repairs, each on its own clock.

~8 min
Overall 3-5 days for acute acclimatisation to meaningfully improve breathing and oxygenation.
Above 3,000 m 500 m maximum sleeping-altitude gain per night in the CDC traveller checklist.
Prophylaxis 125 mg acetazolamide every 12 hours, when prescribed, starting the day before ascent.
01

The instrument is time, not altitude.

Part 1 showed the plunge on arrival. This view holds altitude steady and lets days pass. The exact number varies by person; the shape is the point.

about 91% illustrative resting SpO2 line at Leh
94 91 88 0 days 16 days

The line is intentionally approximate: the research brief only supports the direction and range, not a per-person prediction.

Minutes-hours You breathe harder
Low blood oxygen drives the hypoxic ventilatory response. You blow off CO2, which causes respiratory alkalosis.
1-2 days Kidneys lift the brake
The kidneys excrete bicarbonate to correct the alkalosis, letting ventilation keep rising. Acetazolamide mimics this step.
2-5 days Blood gives up oxygen more easily
Red cells raise 2,3-DPG, shifting the oxygen-haemoglobin curve rightward so tissues can pull oxygen out more readily.
Days-weeks The red-cell signal starts, but capacity lags
EPO rises rapidly, often peaking around 48 hours, but extra red-cell mass is not the acute fix. Measurable haemoglobin-mass changes take about a week and continue longer.
02

The ascent rule is about sleeping altitude.

Above 3,000 m, CDC and WMS guidance says to raise sleeping altitude by no more than 500 m per day, with an extra acclimatisation day for every 1,000 m gained.

Avoid the first-day jump

CDC/WMS guidance warns against going from low altitude to a sleeping altitude above 2,750 m in one day.

Use rest nights deliberately

Plan a rest day every 3-4 days, or add an acclimatisation night for each 1,000 m of sleeping-altitude gain.

Pre-acclimatise if you can

Two or more nights around 2,450-2,750 m before a higher trip can be protective. That is the logic behind a Leh hold.

03

Where Diamox fits.

Acetazolamide does not add oxygen to the air. It speeds one acclimatisation step: bicarbonate excretion, which removes the brake on breathing.

CDC / WMS prophylaxis

125 mg every 12 hours, started the day before ascent and continued for the first 2 days at altitude, or longer when the ascent profile stays aggressive.

India note

Acetazolamide is a prescription medicine in India. This page is not a dosing instruction; talk to a doctor before you leave.

04

Acclimatisation can still be outrun.

The clocks are real, but they are not magic. Go higher faster than the body can adapt and symptoms can still appear. Part 3 is the recognition chapter: headache, nausea, dizziness, fatigue, and the two branches that mean the mountain has become unsafe.

Sources

  1. CDC Yellow Book. High-Altitude Travel and Altitude Illness. 2026 edition, updated April 23, 2025.
  2. Luks AM, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update. Wilderness & Environmental Medicine, 2024. PMID 37833187.
  3. Ryan BJ, et al. AltitudeOmics: Rapid Hemoglobin Mass Alterations with Early Acclimatization to and De-Acclimatization from 5,260 m in Healthy Humans. PLoS ONE, 2014. PMC4182755.

Educational only, not medical advice. If symptoms worsen at altitude, the safe default is to stop ascending and descend.