Why are we tired after a plane flight? We haven't really done any physical activity, so why are we exhausted?
Zonked at 30,000 Feet
Science is sitting on an airplane bound for his family in Detroit while writing this. The air is dry and thin, as it always is on a plane. Let's start our speculation here.
Nearly every cell in our body requires oxygen—whether we're on the ground or flying in a plane. Our lungs allow our blood to grab the oxygen out of the air, by making the air we breathe in as close as possible to the blood it meets through a thin membrane. That means warming and wetting the air until it is saturated with water. The dry air in a plane means our lungs have to work overtime to moisten each breath. That's a start in answering your question.
Let's think about air—on the ground in Seattle and on a plane. Whether we're on the ground or in a plane, air is made of water vapor, nitrogen, and oxygen. Dry out the air until there is no water vapor in it, and the remainder roughly consists of about 80 percent nitrogen and 20 percent oxygen molecules. The number of molecules per volume of air determines how thick the air is, its pressure. Pressure is a force caused by gas molecules bashing into things—the more molecules, the more bashing and the higher the pressure, measured with a unit like torr. Each gas in the air has its own partial pressure, the contribution to the total pressure made by its molecules; our lungs only care about the partial pressure of oxygen.
As we go up in altitude, the air becomes thinner—with fewer molecules per volume. At sea level, the air pressure measures at about 760 torr—570 of which is nitrogen, 150 is oxygen. Most airplanes are pressurized to the equivalent of about 8,000 feet, a total pressure of 560 torr and an oxygen pressure of only 100 torr—only two-thirds of the oxygen pressure we're used to in Seattle. In other words, the number of oxygen molecules available per breath decreases when we're flying.
If we need the same amount of oxygen to keep our cells running, and we get less oxygen per breath, we have to take more breaths to keep everything functioning. Test it out the next time you travel. Count how many breaths per minute your companion makes while sitting quietly on the ground. Later, in flight, compare how many she takes.
All that extra breathing wears us out. The drop in air pressure on a plane is also probably why we fart so damn much when flying. The pressurized gas in our intestines has even more reason to escape.
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