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.
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.
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.
Where Diamox fits.
Acetazolamide does not add oxygen to the air. It speeds one acclimatisation step: bicarbonate excretion, which removes the brake on breathing.
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.
Acetazolamide is a prescription medicine in India. This page is not a dosing instruction; talk to a doctor before you leave.
125 mg is the prevention dose. Treatment once symptoms have started uses 250 mg every 12 hours; Part 4 covers where medicines fit in the turn-around decision.
Acclimatisation can still be outrun.
The clocks are real, but they have limits. Climb 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
- CDC Yellow Book. High-Altitude Travel and Altitude Illness. 2026 edition, updated April 23, 2025.
- 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.
- 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.