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.
Wanderingly Yours,
Science
Send your science questions to
dearscience@thestranger.com
I have not noticed the farting. I will take note.
I believe I fart more at sea level. The rich, moist air comforts me and the relaxed feeling is felt by my butt, as I release my sweet air upon my surrounding world. Aah, sweet ass air.
In addition to the air being thinner (with pressurization sometimes closer to 10,000 feet rather than a thick 8,000), there is an addition reason for feeling tired after flying: dehydration.
The combination of near-0% humidity caused by pressurization and the reduced pressure means that you are certain to lose a significant amount of water even just sitting in your seat without exercise.
Pressurizing air for the cabin causes the air to heat which must in turn be air conditioned back to normal temperatures. As a result of the process of cooling, the pressurized air is dehumidified down to nearly 0% relative humidity.
The reduced pressure increases the rate of vaporization of water from the lungs and nose with each breath.
It’s this dehydration that was the original reason for serving the complementary non-alcoholic drinks on flight. That’s pretty much stopped due to economics but the dehydration effects of any flight never ceased.
Then how come I feel so energetic in Breckenridge? With a 9,000 ft. base and 13,000 ft. peaks, wouldn’t that be more exhausting than a plane ride? When I’m there, I just want to snowboard all day, then explore that wonderful town. I think planes are tiring, because we are crammed in there like cattle. It’s very stressful.