A Third of
the Air

The air thins along a cliff you cannot see

Climb, and the fraction of oxygen never changes — the pressure holding it in place does. Halfway up Everest by height, you are already down to half the oxygen per breath.

How to read the strip: shading is the pressure of the atmosphere as a share of sea level, derived from the International Standard Atmosphere; the 8,849 m endpoint is the measured value (253 mmHg). Real summit pressure runs higher than the standard model predicts, because a bulge in the stratosphere over the equator piles extra air above the Himalaya — a discrepancy West called the difference between an impossible climb and a merely brutal one. For scale: airliners are certified to keep the cabin no thinner than 2,438 m (8,000 ft).

The lowest arterial oxygen ever recorded in a living person

In 2007, four climbers descending from the summit stopped at 8,400 m, exposed a femoral artery, and drew blood while breathing ambient air. In a hospital, those numbers describe a patient in extremis. These four then walked down.

Mean arterial PO₂ was 24.6 mmHg (range 19.1–29.5). Their bodies had spent weeks compensating — thickening the blood with red cells until, at 7,100 m, the oxygen content of arterial blood still matched sea-level values. That defence finally broke above it: at 8,400 m arterial oxygen content had fallen 26%, from 197.1 to 145.8 ml per litre. Grocott et al., New England Journal of Medicine, 2009 (Caudwell Xtreme Everest).

At the top, a fit body's ceiling is a slow walk

Aerobic capacity scales with the oxygen you can deliver. On the 1981 American Medical Research Expedition, climbers breathing a gas mix that matched the summit's inspired oxygen pressure — 43 Torr — were pushed to exhaustion.

A maximum of 15.3 ml/kg/min is roughly four times resting metabolism — the effort of strolling on flat ground at sea level. That is the entire budget available for climbing the last 500 vertical metres of the planet, in −30 °C air, in a down suit. It is why summit day is measured in steps between breaths, and why only 232 of 13,737 ascents of Everest — 1.7% — have been made without bottled oxygen since Messner and Habeler proved it possible in 1978.

Fourteen doorways, one 2,000-kilometre arc

Every point of land on Earth above 8,000 m sits in a single crumple zone where India is still driving into Asia. Fourteen summits, five countries' borders, one continuous ridge system.

Annapurna I is the deadliest 8,000er on record, and the reason is not its thin air but a single avalanche-swept slope at 5,900 m between Camps II and III. Everest kills far more people in total; per person who reaches the top, it is five times safer. Both ratios are falling: Everest's death rate dropped from 1.4% of climbers (1921–2006) to 0.7% (2007–2024).

The thinnest air on Earth now has a queue

Everest ascents per year. The two zeroes are not weather: 2014 ended after a serac collapse killed 16 workers in the Khumbu Icefall, 2015 after the Gorkha earthquake, and 2020 was closed by the pandemic.

What the thin air costs, and who pays it

On 1 September 2025 Nepal raised the spring Everest royalty by 36% — its first increase in a decade — earmarking the money for waste removal and for insurance for the high-altitude workers who take most of the risk.

The 2023 season remains the deadliest on record. Above 8,000 m a rescue is often physically impossible: a helicopter's rotors have a third of the air to bite into, and carrying an incapacitated adult down the Hillary Step is beyond what an oxygen-starved team can do. The mountain's arithmetic is unforgiving in both directions — the same thin air that makes the summit a prize makes it a place you cannot be saved from.