The human body in thin air

Just the Facts

The concentration (percentage) of oxygen in the air is the same at all altitudes.

The amount of oxygen available to the human body is determined by atmospheric pressure which decreases with altitude -- the air thins leaving fewer molecules of oxygen per breath.

Atmospheric pressure determines how much air is "pushed" into the lungs with each breath.

The atmospheric pressure at the summit of Mount Everest (29,028') is about a third that at sea level.

Heart beat rate, breathing rate, and the production of red blood cells, which carry oxygen throughout the body, increases as the body acclimates to high altitude allowing more oxygen to be "grabbed" from every breath.

Acute Mountain Sickness (AMS)

Often occurs around 8,000'


  • Vomiting
  • Headeache that doesn't respond to usual medicine
  • Shortness of breath
  • Exhaustion that doesn't fade with rest

    This may be a warning sign for the onset of more serious altitude illness.

    Treatment is rest with further ascent recommended only after symptoms have ceased.

    Severe AMS

  • Altered balance or muscular coordination
  • Altered mental state
  • Extreme shortness of breath with almost any activity
  • Person is angry, combative, or incomprehensible

    Further ascent is not recommended unless there are options for easy and rapid descent should symptoms recur.

    HACE Symptoms

    Has occurred as low as 10,000'

  • Ataxia or loss of balance and muscle coordination
  • Decreased mental functioning
  • Severe headache
  • Nausea and vomiting
  • Hallucinations or stroke-like symptoms of impaired speech

    Coma and death may rapidly follow. The only treatment is immediate descent.

    HAPE Symptoms

    Rarely occurs below 8,000'

  • Extreme difficulty breathing and inability to catch one's breath
  • Very rapid pulse and breathing rate (over thirty breaths per minute)
  • Extreme exhaustion and difficulty with any exertion
  • Coughing which may progress to sounding bubbly as lungs fill with fluid
  • Fever and blue coloring, especially the lips and finger-nail beds

    Death can come on quickly. The only treatment is immediate descent.


  • Climb as high as you want during the day, but raise sleeping altitude by no more that 1,000' a day.
  • Factor in a rest day every 3,000'.
  • Spend at least one night below 10,000' before ascending higher.
  • If you don't feel good, don't raise sleeping altitude until you feel better.
  • If you don't feel better at the current sleeping altitude, descend to below where you first felt sick.

    Facts taken from Stephen Bezrucha's "Altitude Sickness: Prevention and Treatment," with permission from The Mountaineers, Seattle, WA.

  • Corfield
    Climbing at Altitude
    What happens to your body at high altitude? Why do some people get sick and even die? According to Dr. Eric Weiss, when trekkers first started going to Everest base camp, 1 in 50 died. Nowadays thanks to awareness of high altitude problems that figure is 1 in 10-50,000. The medical syndromes encountered at altitude all come down to the body's response to a fall in the pressure of oxygen.

    Atmospheric pressure is a gauge of how many oxygen molecules are available per given volume, say a breath. While there is the same percentage of oxygen in the air up high, lower atmospheric pressure at higher altitudes means fewer oxygen molecules are available with each breath.

    Pemba Sherpa resting at 25,500' on the Lhotse Face as more climbers ascend from Camp III.

    Cerebral Edema
    Many people who travel from sea level to over 8,000 feet, report symptoms ranging from headache to loss of appetite and nausea. In short, they feel hung over. Why? As the available oxygen falls, the body responds by increasing the blood flow to the brain. Indeed it can overcompensate, and fluid leaks from the blood vessels into the brain causing it to swell. The result is Acute Mountain Sickness. There is a spectrum from mild, to life threateningly severe swelling called High Altitude Cerebral Edema (HACE).

    Not surprisingly, the greater the elevation gain the more severe the swelling. In really bad cases the brain gets squeezed out down the hole for the spinal cord like toothpaste. And then you die. The way to avoid falling victim is to ascend gradually, about 1,000'/day over 8,000' which gives your body time to acclimatize properly. With experience you will find what works best for you.

    You can treat, and indeed preempt, Acute Mountain Sickness by using a diuretic called Diamox and drinking plenty of water. Signs to watch for with HACE, are persistent severe headache, loss of coordination, and acting stupid (lights on but noone at home). Treatment is immediate descent, and Decadron or oxygen if available.

    Pulmonary Edema
    Groups of trekkers are occasionally struck down by a mysterious flu like illness near the high point of their trek, and, just as mysteriously, they get better when they head down. What they had was not flu, but High Altitude Pulmonary Edema (HAPE), which is a build up of fluid in lungs, and can lead to a feeling of profound fatigue.

    A trekker who developed pulmonary edema at 12,000' is revived in a Gamow Bag, a sealed chamber that is pumped full of oxygen to simulate lower altitude.

    Pulmonary edema is a result of greatly increased blood flow through the lungs, as the body tries to get as much oxygen from the air as it can. The heart increases the flow by increasing the pressure, causing leakage from the blood vessels into the air sacs. It usually takes a few days to develop, and is exacerbated by over exertion. In bad cases, you can hear gurgling of fluid in the lungs, and the victim brings up a bloody sputum. This is a serious condition, which can kill in hours. The victim drowns in his own juices. Yuck. HAPE is best avoided by gradual ascent. Treatment is immediate descent by several thousand feet and oxygen if available.

    Too Much of a Good Thing
    The body also responds to the lower oxygen levels by putting more red blood cells into circulation. Up to a point, this is a good thing. However, if it goes too far, the blood becomes thick and prone to clotting. Clots which get dislodged float around and can cause strokes, heart attacks, and pulmonary embolisms. All bad juju. The only treatment for thick blood is to be bled. A lot. Best not to be squeamish. Since this condition takes weeks to develop, it is rarely an issue, except on the Himalayan giants.

    Charles Corfield (rt) draws a sample of blood from guide Eric Simonson to compare levels of red blood cells as his body acclimates to high altitude.

    Many climbers who have been to over 14,000' have experienced listening to their tent-mates repeatedly stop breathing, gasp, breathe heavily for a while and then stop breathing again. This is called periodic breathing, and is caused by the system which regulates breathing getting out of whack. The sleeper responds to a build up in carbon dioxide by hyperventilating. This leads to respiratory alkalosis, and the breathing center responds by shutting off respiration. CO2 levels then increase and the cycle repeats. It is a relatively benign condition, and responds well to a low dose of Diamox taken at bed time.

    The Khumbu Cough
    A common complaint of high altitude climbers is a persistent dry cough (high altitude hack), which can be so bad that some have even broken ribs. This condition results from a drying out of the bronchi caused by breathing cold dry air. The best prophylaxis is to avoid over exertion and thereby keep the breathing rate down, so that the bronchial mucosa can stay moist.

    If you would like to pursue high altitude medical topics further, I suggest "Medicine for Mountaineering" by James Wilkerson. [Click here for The Mountain Zone Bookstore], and "Wilderness Medicine," Ch.1, by Auerbach.

    -- Charles Corfield, Alpine Ascents Climber

    Photos by Todd Burleson, Lhakpa Rita, and Jane Bromet

    [Dispatches from Everest Index]