Flying can be physically uncomfortable for many reasons—but did you know that one the most serious causes of discomfort is dehydration? 

According to LifeHacker, if you step off of a plane and feel like crap, it’s not because you’re sick, but probably because you haven’t been drinking or retaining enough water. Dehydration can make you feel fatigue, headaches, nausea, and more. And airplanes are the worst environment for draining you of all moisture.

Why Does Flying Cause Dehydration?

Basically, you’re flying in a sky desert, where the humidity hovers around 10-20% (drier than the Sahara) thanks to the plane’s air circulation system. Compare that to the typical 30-60% humidity you’re used to on the ground and it’s no wonder flying dehydrates you. Physiotherapist Yasmin Badiani tells Marie Claire that a lot of your water loss is a combination of the cabin’s lack of humidity and its oxygen-thin air, which increases your breathing rate. Plus, you’re probably doing some sweating if you have any nerves when you fly. Badiani says that men can lose around two liters of water during a 10-hour flight (4% of your bodily water), and women can lose around 1.6 liters.

You Should Drink 8 Ounces per Hour in the Air

How much water you need to drink depends on how long your flight is. According to the Aerospace Medical Association, you should drink about eight ounces of water for every hour you’re in the air. Peter Hackett, M.D., the director of the Institute for Altitude Medicine tells Condé Nast Traveler this is especially the case if you’re on flights longer than three or four hours. Flights shorter than that won’t be too dehydrating otherwise, says Hackett.

But you shouldn’t drink all of that water at once. If you drink too much water too fast, you dilute your blood, which causes your kidneys to excrete water faster, and that means you’re peeing out all that water before your body can use it to hydrate itself. No chugging!

Make Up for Any Booze You Drink

Okay, so the drink cart is coming around and you’re dying for a Bloody Mary (make mine a double). That’s fine, but ask for a cup of water in addition to your cocktail. Just one eight-ounce cup of water for every drink will be sufficient. And if you can help it, try to drink as little alcohol as possible. I know it’s tempting.

Pack a Snack Like Fruit, Yogurt or Tea

Water isn’t the only way to keep your body from shriveling up into a dried husk. You can also get plenty of hydration from coffee—don’t believe the myths about it dehydrating you more—or tea. Just be aware that airplane water isn’t always that clean, and that’s what’s used to make them. Some flight attendants won’t even dare to drink the coffee and tea.

If you find that worrying, you can always pack a hydrating snack instead. Fruits, vegetables, and even yogurt can help a lot. Don’t eat foods with too much salt, though. If you get dry sinuses, a saline nasal spray can help keep your nose from drying out too, which may help you avoid a headache. Lastly, the cabin’s dry air takes a toll on your skin as well. A small, less-than-3-ounce bottle of lotion can do wonders for your skin. 

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