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.
Common trekker questions
How long does it take to acclimatise to high altitude?
Acute acclimatisation takes about 3-5 days to meaningfully improve breathing and oxygenation. The body runs four repairs on separate clocks: harder breathing within minutes to hours, kidney compensation over 1-2 days, easier oxygen unloading over 2-5 days, and new red-cell capacity over weeks.
How much altitude can you gain per day on a trek?
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. Day-trips higher are not the problem; the altitude you sleep at is the load your body must handle overnight.
Does Diamox help you acclimatise?
Acetazolamide, or Diamox, speeds one acclimatisation step: bicarbonate excretion by the kidneys, which removes the brake on breathing. It does not add oxygen to the air. It is a prescription medicine in India, so discuss it with a doctor before you leave.
What is the Diamox dose for preventing altitude sickness?
The CDC/WMS prophylaxis dose is 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. Treatment once symptoms have started uses 250 mg every 12 hours. These are published guideline doses, not personal instructions.
Can you acclimatise before a trek by sleeping at moderate altitude?
Yes, it can be protective. Two or more nights around 2,450-2,750 m before a higher trip is the logic behind a Leh hold. CDC/WMS guidance also warns against going from low altitude to a sleeping altitude above 2,750 m in one day.
How often should you take rest days at altitude?
Plan a rest day every 3-4 days, or add an acclimatisation night for each 1,000 m of sleeping-altitude gain. The clocks are real, but they have limits: climb faster than the body can adapt and symptoms can still appear despite rest days.
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.