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Imagine that you are an experienced, physically fit mountain climber. You've just reached 18,000 feet. All day you’ve felt dizzy and have feared passing out. You are continuously out of breath and have to rest between putting on your boot and tying your shoelace. You are in your tent and can't for the life of you think clearly enough to remember which sleeping bag was yours. Why are you feeling this way?

Very high altitudes can have serious, and deadly, effects on the human body. Altitude itself is perhaps the greatest threat to the life of any high-altitude climber. As you may know, the higher you climb, the thinner the air becomes. At the top of Mount Everest, the percentage of oxygen in the air is the same as that at sea level (about 20% oxygen). However, atmospheric pressure decreases as altitude increases, and the atmospheric pressure at the summit of Everest is 33% that of sea level. Therefore, there is 66% less oxygen than there is at sea level. Kerosene will not burn. Helicopters cannot fly. Yet somehow, your metabolism is able to continue its chemical conversions inside of you, turning food into heat and energy.

At high altitudes breathing becomes a task. Because there is so little oxygen, your body must work harder to pass more air through your lungs. Yet your increased breathing rate taxes your body of more energy. So you must rest. Have you ever felt light-headed after blowing up a balloon? Imagine having that feeling all day long, while trying to climb a hundred flights of stairs, wearing 35 pounds of gear, breathing only through a straw. This may give you an idea of the tremendous challenge that high-altitude climbers face with every single step they take.

Altitude sickness occurs most often when there is rapid ascent, and the body does not have sufficient rest time at intermediate elevations to acclimatize. If a person were flown to the top of Everest and released at the summit without oxygen, he or she would die within an hour. Some ways that your body copes with lower oxygen levels are increased production of red blood cells (which carry oxygen throughout the body) and increased production of the enzymes that aid in the release of oxygen to the tissues. In the 1968 Olympics held in Mexico City (elev. 7300 ft/3335m), four of the five endurance track events were won by men who lived at high altitudes.

A slow ascent with ample time for acclimatization do not safeguard against illness. Individual susceptibility to altitude sickness varies greatly, and is independent of a person's state of physical fitness. In most cases of altitude sickness, the most effective and urgent treatment is descent. Many symptoms will be abated when the affected is transported to lower altitudes. Following are some of the more common illnesses:

Acute Mountain Sickness (AMS)
This is a commmon set of symptoms, easily hidden behind common daily ailments, related to rapid ascent to high altitudes. Usually AMS is characterized by: headache, dizziness, fatigue, dry cough, loss of appetite, nausea, vomiting, disturbed sleep, and malaise.

High Altitude Cerebral Edema
This occurs in a small portion of people who are afflicted with AMS, usually above 12,000 feet. Cerebral Edema is due to excessive accumulation of fluid in the brain, and can cause death or permanent brain damage. Symptoms include: unsteady gait, inability to perform minor daily tasks, confusion, loss of memory, hallucinations, psychotic behavior, and coma.

High Altitude Pulmonary Edema
This, like Cerebral Edema, occurs along with AMS, but is the most dangerous of the common types of altitude sickness. It can also lead to the development of Cerebral Edema. If the afflicted becomes unconscious, descent or oxygen must be administered within 6 to 12 hours or death is likely. Pulmonary Edema occurs when the lungs fill with fluid, blocking the transfer of oxygen from inhaled air to the red blood cells. It is characterized by: undue shortness of breath with moderate exertion, tightness in the chest, sensation of impending suffocation, weakness, debilitating fatigue, persistent cough accompanied by a white, watery, or frothy material, rapid pulse, cyanotic (blue) lips and nail beds, and bubbling or crackling sounds in the chest. Confusion and delerium are indicative of advanced cases.

Altitude sickness can occur beginning at around 8,000 feet. In general, there are three ranges of elevation with different risk of illness:

8,000 to 14,000 feet (2400 to 4300 meters): These altitudes are generally encountered by climbers and tourists in the continental United States. (Aspen, CO 7850ft/2390m, Rocky Mountain National Park 100 peaks over 11,000ft/3353m including Longs Peak 14,255ft/4345m, Mt. Rainier 14,410ft/4392m). At 5,000 feet (Denver, CO 5,280ft/1,609 m) the average person will notice a decrease in athletic performance. The point at which altitude sickness will typically onset (however not acutely) is at 8,000 feet (2400m).

14,000 to 18,000 feet (4300 to 5500m): Only experienced climbers will usually attempt climbing to these altitudes. Acclimatization prior to reaching this range is essential if illness is to be prevented.

18,000 to 29,000 feet (5500 to 8800m): At these altitudes, only the most acclimatized climbers can function, and those who are more susceptible to altitude sickness will have turned back.

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