There plenty of reasons why some people don't like to fly — the stress of making it to your plane, connecting flights, crowds, germs, TSA, and the list goes on. But you actually may have a visceral reaction to the thought of taking a flight because of what flying does to your body. Hanging out between 30,000 and 40,000 feet in the sky doesn't come without a cost. Here are four things that happen to your body whenever you fly.

1. Dehydration

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If you've ever taken a trip to a destination at a higher elevation, you know you become dehydrated. Now imagine going a lot higher up.

When you drink plenty of water, you ward off headaches, deliver more oxygen to your body, regulate your body temperature, and much more. According to some experts, you can lose over 1.5 liters of water in a three-hour flight. It's essential to stay hydrated before, during, and after a flight to keep the negative side effects of dehydration away.

2. Drowsiness

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While plenty of people complain about not being able to sleep on planes, passengers doze right off. The drowsiness phenomenon is due to cabin air pressure. The cabin of an airplane is regulated to simulate the air pressure of about 6,000 to 8,000 feet above sea level. But for many people, that's still relatively low air pressure. There is plenty of oxygen for everyone on the plane to breathe, clearly, but less oxygen is absorbed by the blood, which can make you sleepy.

The drowsiness you may experience on an airplane causes no harm, but people with heart or lung diseases may have an issue flying. If that's the case for you or someone you are traveling with, make sure to contact the airline ahead of time to ensure you can bring your own oxygen supply.

3. Higher Risk of Blood Clots

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When you fly, especially on longer flights, blood starts to pool in your feet and legs. Additionally, the loss of water when you fly (see our point on dehydration) causes your blood to become thicker. For many people, this isn't a problem, other than some slight discomfort from swelling. But if you're at a higher risk for blood clots, you'll want to take some extra steps when you fly.

You can prevent blood clots by making sure you get up and walk around every few hours. Even if you don't need to use the restroom, take a stroll down the aisle anyway. Moving around becomes especially important on international flights. You can also buy a pair of those oh-so-attractive compression socks to help the blood flow in your feet, ankles, and calves.

4. Germ Exposure

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The exposure to germs when you fly is real. Think about it. People from all over, with different strains of bacteria and viruses than those found in your hometown, pass through the airport. According to an article from Real Simple, you are 100 times more likely to catch a cold from a flight. Common cold germs and nastier bacteria like E. coli can live for up to a week on hard surfaces like the bathroom counter or the armrests on your seat.

Luckily, there are simple things you can do to increase your chances of staying healthy after flying. Make sure you wash your hands often. Avoid touching your eyes, nose, and mouth. You can increase your vitamin C and probiotic intake to boost your immune system too.