Do you feel inexplicably crappy — tired, dehydrated, and headachy — every time you fly?
It's not your imagination. People talk a lot about the many awful aspects of flying nowadays, but one that gets less attention is the way that sitting in a small, pressurized metal tube at 35,000 feet for several hours wreaks havoc on your body.
"Anytime you fly, you're exposing yourself to a different environment than your body is used to," says Jeffrey Sventek, director of the Aerospace Medical Association and a longtime aerospace physiologist for the Air Force.
For some people, this environment — with lower oxygen levels than the ground, extremely little humidity, and sudden changes in air pressure — can cause a bunch of negative symptoms. This is how flying can make you feel terrible.
Planes have lower oxygen levels
As a plane flies, air that flows through the engine gets sucked in, compressed, cooled, filtered, and pumped into the cabin. If this didn't happen, everyone inside the plane would die, as the low air pressure at the elevations planes fly (typically 35,000 feet or so) means there isn't enough oxygen present for your body to function.
Still, the amount of air pumped inside doesn't result in quite as much oxygen as you'd normally breathe at sea level. "The cabin is only pressurized to simulate an elevation of 6,000 to 8,000 feet on modern jets," says Brent Blue, a doctor and longtime pilot. In other words, to your body, flying is like sitting on a 6,000 to 8,000 foot mountain for several hours. As Blue says, "that's a significant difference for people who live at sea level, and aren't used to it."
For people with conditions — like heart or lung disease — that cause them to have special oxygen requirements, this is a big deal, and means they might need to fly with an oxygen concentrator, or not fly at all. But even for healthy people who are used to the abundant levels of oxygen present at sea level, it can have an effect.
"If you're flying for six hours and dropping your blood's oxygen saturation by five or ten percent, the fatigue factor is significant," Blue says. Even if they don't cause fatigue, reduced oxygen levels can also make your thinking a bit less sharp.
What's more, other aspects of the flying environment exacerbate this effect. Sitting for extended periods of time causes your blood to disproportionately pool in your thighs and feet, which means your body is less efficient at circulating and oxygenating the blood, and your brain gets even less oxygen.
Theoretically, planes could be somewhat more heavily pressurized to eliminate this effect. Federal regulations only require them to be pressurized to 8,000 feet, though some experts have criticized this as a substandard level.
Moreover, because planes are designed to be lightweight, they aren't nearly strong enough hold in enough air to simulate pressures that you'd find closer to sea level. (The new Boeing 787 Dreamliner, it's worth noting, will have slightly higher levels of air pressure and humidity.) Finally, intaking and pumping all this air uses fuel — so airlines are reluctant to pressurize any more than they need to.
What you can do to stay sharp
Though you can't solve this problem entirely, there are a few things you can do.
1) Don't drink alcohol. This makes the oxygen problem even worse, by interfering with your cells' metabolism so they too are less efficient at taking in oxygen. (This is also why it's easier to get drunk on a plane, or for that matter, at high elevations on Earth.) Alcohol also exacerbates the problem of dehydration (more on that below).
2) Get up and walk around during flights, or do in-seat leg exercises if you don't want to deal with the dirty looks from flight attendants. This will improve circulation, and also reduce the chance of a blood clot in your legs.
3) Blue recommends taking an aspirin the day before and the day of flying, to further improve blood flow, and to wear support compression stockings, to reduce the amount of blood pooled in your legs.
How head colds make flying even worse
When a plane descends below 6,000 feet, this pressurization system is shut down. This causes the air pressure inside the plane to fluctuate as it matches the pressure outside the plane.
"As this happens, you'll notice your ears popping," Blue says. "That's a good thing — it's the opening and closing of the eustachian tubes, which connect the oral cavities to the middle ear." The air flowing through these tubes allows the pressure inside your middle ear to match the pressure outside your head.
If you have a head cold or sinus infection, however, these tubes won't open and close as easily — so the pressure won't be equalized as easily. This causes your eardrums to bulge inward, which hurts like hell. In extreme cases, this can even cause the eardrum to rupture.
If you're already very congested and scheduled to fly, Sventek recommends getting a prescription decongestant. Additionally, as your plane is descending, try to chew, yawn, or swallow repeatedly to get your eustachian tubes to open. If that doesn't work, you can try what scientists call the Valsalva maneuver: pinch your nose and forcefully exhale, which forces air into your middle ear.
Why planes make you dehydrated
Because the air drawn into the plane to pressurize it comes from extremely dry, high-altitude regions of the atmosphere, the plane environment itself is drier than a desert. "About 30 minutes after takeoff, the relative humidity in the cabin drops down to nearly zero," Blue says. "This makes it relatively easy to get dehydrated, especially on long overseas trips.
The most obvious effect of this is dry mouth, as much of the moisture inside it is quickly evaporated into the surrounding air. But on longer flights, this effect — coupled with little water consumption — can cause your body as a whole to get dehydrated, leading to headaches and dizziness.
The key to avoiding this, Sventek says, is to hydrate before you fly, and keep drinking water when you're onboard. Bring a water bottle so you don't need to rely on flight attendants' refills. And don't drink alcohol or caffeinated beverages, because they act as diuretics and dehydrate you further.
Further reading: The Aerospace Medical Association has a number of detailed publications on the health effects of flying.