Field Notes · Summit
The oxygen arithmetic of 5,000 metres
Altitude is a number before it is a feeling. Air pressure falls with height, oxygen falls with it, and the body adapts at a rate that has been measured for decades. The Altitude Simulator applies the arithmetic to your nights; this is the arithmetic.
At sea level the air presses down at about 1,013 hectopascals and a fifth of it is oxygen. At Leh, 3,500 metres up, the pressure is around 660 and the oxygen is still a fifth of it, which means the lungs get about 65 percent of what they are used to. At the Khardung La, 5,359 metres, it is about 52 percent. At Jangothang in Bhutan, where the tents are at 4,080 metres, the number is 60 percent. These are not feelings. They come out of the barometric formula, and the Altitude Simulator computes them for every night of an itinerary.
What the body does about it
Within hours of arriving high the breathing deepens and speeds up, which is why the first night is restless. Within days the kidneys shed bicarbonate to let the breathing stay fast, and within a couple of weeks the blood carries more red cells. The adaptation is real and it has a speed limit, and the mountaineering guidelines have measured it: above about 3,000 metres, the sleeping altitude should rise by no more than 300 to 500 metres a night, with a rest day every thousand metres of gain.
Break the rule and the odds of acute mountain sickness rise sharply: headache, nausea, sleeplessness, loss of appetite. Most cases resolve with a day of not going higher. A few progress to the fluid in the lungs or the brain that kills people on mountains, and the treatment for both is descent, immediately.
The itineraries that break it
The flight from Delhi to Leh breaks the rule in an hour and a quarter: 200 metres to 3,500. That is why the Ladakh page insists on two rest days before the passes. The drive from San Pedro at 2,400 metres to the geysers at 4,320 before breakfast breaks it too, which is survivable because you come back down to sleep. The Bolivian salt crossing from San Pedro climbs to a refuge at 4,300 and a pass at 4,900 in two days; the direction matters, because starting in San Pedro means two nights at 2,400 first.
The simulator shows all of these as a profile of nights and marks the ones that gain too much. It also shows the sleep-low, climb-high rule in action: an afternoon walk to a viewpoint five hundred metres above camp, back down to sleep, counts as acclimatisation. The rest day at Jangothang is a walk to the lakes, not a day in the tent.
What does not help
Fitness. Fit people arrive at the dangerous night faster and feel invincible when they get there. Alcohol, which suppresses breathing at night. Sleeping pills, for the same reason. Pushing on because the group is pushing on. What helps is water, food, a slow pace and the willingness to say that the headache is worse this morning, which is the sentence that saves lives.
Medication
Acetazolamide speeds the kidney part of the adaptation and is used on most of the itineraries on this site. It is a prescription decision between you and a doctor; the simulator does not model it because the itinerary should work without it. If the profile only works with the drug, change the profile.
Questions people ask
Does fitness protect against altitude sickness?
No. Fit people climb faster and arrive at the dangerous night sooner. Prior history at altitude and the rate of ascent are what matter.
What about medication?
Acetazolamide speeds acclimatisation for many people and is widely used on the itineraries here. It is a decision for you and a doctor, and the rest days work without it.
