On Earth, all life exists in the troposphere -- the lowest and densest layer of the atmosphere, composed of 78% nitrogen, 21% oxygen, and trace amounts of other gases. Nearly all organisms require oxygen to survive; humans rely on the oxygen in every breath of air that enters their lungs. The chemical composition of the troposphere is fairly uniform, so it might seem that humans should be able survive at any elevation. However, as altitude increases and atmospheric pressure decreases, there is less oxygen available in each breath.

Earth's gravity pulls on the gas molecules in air, creating atmospheric pressure. Air that is closer to the ground is compressed by the weight of all the air above it. At sea level, the weight of air on one square-inch of area is about 14.7 pounds. At higher altitudes, there is less pressure because less air is pushing down from above. Less atmospheric pressure means that the density of the air is lower. With fewer air molecules in a given volume of space, there are fewer oxygen molecules available, even though the air is still 21% oxygen. For mountain climbers at extreme altitudes, such as at the top of Mount Everest where the air is only about one-third as dense as the air at sea level, it is a challenge to be able to get enough oxygen in each breath. The decreased atmospheric pressure and low levels of oxygen can have very dangerous effects on a climber's body.

Even though there is less oxygen available to the climbers, their bodies can learn to compensate. The human body survives by breathing air into lungs. The oxygen from the air is transferred from the lungs to the blood, which then transports it throughout the body. Mountain climbers' bodies can adjust to high-altitude air in several ways, such as increasing the frequency of breathing and increasing blood flow. However, if climbers do not allow enough time for their bodies to acclimatize, or if the air is simply too thin, they may suffer from high-altitude sickness. At a towering height of about 8,800 meters (29,000 feet), Mount Everest seems to be at the limit at which the human body can survive. Most climbers cannot reach the summit of Everest without the help of supplementary oxygen, though there have been a few extraordinary individuals whose bodies have been able to successfully adapt.

Determine how effectively common clothing fabrics insulate against cold in this NOVA classroom activity.

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