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500 W Pacific Ave
Telluride, CO, 81320
United States



Altitude physiology

General Information

So you're coming to Colorado to visit? Ski? Climb? Hike? Jeep? Chill? Colorado offers the best in mountain recreation, with clean air and beautiful vistas. It also offers thin air, with less oxygen. This section of the web site addresses possible concerns about high altitude and offers tips on maximizing your enjoyment of our beautiful State.

What defines high altitude?

High altitude to a physiologist starts around 5000ft, the altitude where the body senses changes in the oxygen level and starts to respond by increasing breathing. Ski resorts in Colorado range from base areas of 6-9,000 ft, to elevations of close to 13,000 ft, while 54 peaks go to over 14,000 ft.

Why is there less oxygen in the air?

The pressure in the atmosphere decreases as you gain elevation. The percent of oxygen is actually the same at all altitudes, 21%; however, it is 21% of a smaller number as one goes higher. The barometric pressure at sea level is 760 mmHg, and at 10,000 ft, it is 534 mmHg. Breathing the air of Telluride is the equivalent to breathing air with only 15% oxygen at sea level, instead of 21%. The net result is that there is 29% less oxygen in the air at Telluride compared with sea level. At 14,000 ft, the air has 43% less oxygen than at sea level. Because of the reduced air pressure at high altitude, the volume of air you breathe into you lungs contains less oxygen molecules in each breath.

Physiological changes

Your body must adjust to the lower oxygen levels, and this process of adjustment is called acclimatization. Many changes occur in your body during acclimatization. One of the first things you will notice is that you breathe faster and deeper, to take in more oxygen. Therefore, you might feel short of breath for the first 2-3 days, especially with physical activity.. Some shortness of breath with exercise is normal. Heart rate also increases, to supply more oxygenated blood to the tissues, and this might be noticeable the first few days; after that, it goes down towards a more normal rate. Increased urination is a response to changes in your body's acid/base balance and helps you in the acclimatization process; this is usually noticeable on the second day. Some people experience mild swelling in their hands, feet, and face, which is not serious.

Sleep Disturbances

Trouble sleeping is quite common at high altitude. The low oxygen directly affects the sleep center of the brain. Frequent awakenings, a light sleep and less total time of sleep are the main problems, and these usually improve with acclimatization after a few nights. Some persons, however, will have trouble sleeping despite acclimatization.

What is Periodic Breathing during Sleep?

This condition, which can cause trouble sleeping, happens quite frequently but is not associated with altitude illness. It results from a battle within the body over control of breathing during sleep. Oxygen sensors in the body command the brain to increase breathing, which causes the lungs to blow off CO2. But CO2 sensors in the body then tell the brain to stop breathing, because CO2 is getting too low. So breathing then stops for about 12 seconds, until the oxygen sensors take over again. The result is an irregular pattern of breathing, with 4 or so large breaths followed by no breaths. The first large breath will sometime wake up a person, with a sensation of feeling breathless or suffocated. This pattern may continue throughout the day as well, but typically is most disturbing at night as it frequently wakes a person multiple times. Although uncomfortable it is not dangerous. It is easily treated with a small dose of Diamox® (62.5 or 125 mg) taken before bedtime; this smooths out the breathing and improves sleep and raises blood oxygen.

What can I take to help me sleep?

  • If you are not sleeping well because of periodic breathing, Diamox® should be the first choice of medication to improve sleep.
  • Benzodiazepines such as temazepam (Restoril®), lorazepam (Ativan®), diazepam (Valium®), and alprazalam (Xanax®) should generally be avoided as they decrease the breathing drive, especially when combined with alcohol. Sleep medication such as zolpidem (Ambien®) and eszopiclone (Lunesta®) are safe at altitude and seem to work well without affecting the breathing drive.
  • Some folks use over-the-counter sleep aids such as Tylenol PM® which contains Benadryl®, an antihistamine. Antihistamines have not been shown to affect breathing and may be taken safely.
  • If you are taking trazadone for sleep, there is no evidence that it decreases the breathing drive and it is likely safe at altitude.
  • If you have obstructive sleep apnea (OSA), this can worsen your sleeping symptoms at altitude. If you sleep with CPAP (continuous positive airway pressure) you should make sure to use it at altitude.


Because of less oxygen available to the working muscles, exercise performance is decreased at high altitude. For example, it is impossible to run a mile at high altitude as fast as at low altitude. And this applies to any aerobic event, which is any muscle activity lasting more than two minutes. In addition one cannot expect to perform with the same intensity as at low altitude, and one's pace has to be adjusted accordingly. This means running, cycling or walking a bit slower at high altitude, and taking more breaks and rest stops, to avoid exhaustion. For those who keep track of VO2 max, an overall measure of physical performance: VO2 max drops 3% per thousand feet of altitude gain, starting at around 5000 ft.