Travel, whether by plane, train or automobile, involves doing the same thing: sitting for an extended period. This can be risky for some, as it can increase the risk of a dangerous group of blood clots known as venous thromboembolism (VTE) — including deep vein thrombosis (DVT) and pulmonary embolism (PE).

Compression socks, also called graduated compression socks or compression stockings, are often touted as a way to lower the risk of VTE on flights and other modes of transportation. Let's explore who should use compression socks when they travel and why.

What is venous thromboembolism (VTE)?

Venous thromboembolism, or VTE, is a blood clot that blocks blood flow in the veins. When VTE occurs in the deep veins, most commonly in the legs or arms, it's called deep vein thrombosis (DVT). When a DVT moves through the body to the lungs, it's called a pulmonary embolism (PE). A pulmonary embolism is considered a medical emergency.

"VTE is quite common," says Dr. Thomas Loh, a vascular surgeon at Houston Methodist. "We have about a million cases per year in the United States, and it is something that affects both young and old."

Who is at risk for blood clots while traveling?

Dr. Loh says that there are three main categories of risk factors for VTE:

• Long periods of immobility or not moving around, called stasis
• Trauma to the veins related to an injury or surgery
• When the blood is thicker than it should be, called hypercoagulability

"The main thing to recognize with venous thromboembolism is that the risk factors are all additive," Dr. Loh says. "Travel, especially long-distance travel, is a risk factor for everyone. But because the risk factors are additive, the people who are at highest risk, regardless of age, are those who have other risk factors for venous thromboembolism."

Since our risk increases with the number of risk factors we have, it's important to know and consider all of them before traveling. Dr. Loh says other common risk factors include:

• Age: VTE can occur in someone of any age, but the risk nearly doubles every 10 years after age 40, according to the National Heart, Lung and Blood Institute
• Being overweight: Obesity can affect the blood vessels due to chronic inflammation
• Cancer: People who have experienced cancer or who are in cancer treatment can be at elevated risk for blood clots
• Personal or family history of blood clots or blood clotting disorders: People who previously experienced a blood clot or blood clotting disorder or have a family history of blood clots are at higher risk
• Recent pregnancy: Those who are currently pregnant or in the postpartum period are at elevated risk for VTE

What can you do to lower your risk for VTE while traveling?

"In order to understand what to do, you really have to go back to the basics of how the legs circulate blood," Dr. Loh says. "Your heart circulates blood down to your toes by pumping. The problem is that that blood then has to get all the way from the toes back up to the heart. And the primary pump of the legs is actually the calf muscles. So, when the calf muscles squeeze, it actually propels the blood out of the legs."

Dr. Loh emphasizes that when we're on a long-haul flight, driving a car long distance or sitting on a train, the calf muscles aren't moving or squeezing as they should — and you're not propelling the blood up to the heart as efficiently as you should, causing blood to pool and the legs to swell.

"All of the different interventions or things that we do to improve the blood flow out of your legs are geared towards accentuating blood flow," Dr. Loh says. "For example, compression stockings work by closing the veins that are close to the skin and forcing the blood to go only into the veins that are inside of the muscle. And that way, every time you move at all, most of the blood in your legs then circulates out of them in the same way."

Another issue for our circulation while traveling is dehydration. When we avoid drinking fluids —whether because the seatbelt sign is on, we're in the middle seat or only want to stop driving to get gas — we create an environment for blood clots to thrive.

"When we dehydrate ourselves, our veins actually collapse and are not as efficient at getting the blood out of our legs because we actually have less blood volume," Dr. Loh says. "And you're not moving around, so your calf muscles aren't squeezing, and so doing calf raises, or moving your legs around even while seated, can make a big difference in terms of circulating the blood out of legs."

Dr. Loh advises moving around as much as you can before boarding an airplane. And once you're on the plane, stay hydrated within reason (this means saying no to alcoholic beverages and yes to water). And then if your flight is longer than three to four hours, you really want to try and move your legs, whether it's getting up and walking up and down the aisle or doing calf raises (lifting your heel and keeping your toes planted) or toe lifts (lifting your toes while keeping your heel planted).

Who should wear compression socks for travel?

"The medical societies only recommend routine use for people at higher risk for blood clots, so anybody with many risk factors should consider using compression stockings," Dr. Loh says.

Dr. Loh emphasizes an additional at-risk group: Even if it's their only risk factor, pregnant people and those in the postpartum period should definitely wear compression socks.

"There is a very potent risk factor for blood clots during pregnancy and the postpartum period, and so it's a special population that should consider the use of compression stockings when traveling in both cars and airplanes," Dr. Loh says. "Wearing them has been shown to significantly reduce the risk of DVTs and PEs. While it's a population that we think of as being generally younger and healthier, there's still a pretty substantial increase in risk of blood clots during that time."

Those who travel extensively for work, including pilots, flight attendants and truck drivers, should also consider using compression socks as part of their regimen.

"We definitely see an increased risk of getting blood clots in not only pilots but also truck drivers or anybody in their vehicle for long periods," Dr. Loh says. "Compression stockings can help mitigate that, especially in those that have other risk factors."

But what if I'm not high risk? Can I still use compression socks?

"There's no data to support wearing compression stockings in lower-risk individuals," Dr. Loh says. "That said, it's not dangerous. Compression stockings will help return the blood to the heart more efficiently, and so you'll get less leg swelling."

Dr. Loh notes other benefits of compression stockings during travel. Venous insufficiency, a condition where the leg veins have difficulty circulating blood back to the heart, causing swollen legs or varicose veins, affects about 20-30% of the adult population in the United States. For those who experience venous insufficiency, wearing compression socks during travel can help minimize the swelling that can occur during a trip.

"Personally, I wear compression stockings anytime I get on an airplane, regardless of how long the flight is," Dr. Loh says. "The main reason why I wear compression stockings is because of venous insufficiency rather than preventing blood clots. I have varicose veins that run in the family. So that is a separate but very important reason to wear compression stockings."

From a comfort standpoint, Dr. Loh says wearing compression socks can also help the legs feel better during and after a flight for those with venous insufficiency.

"When I get where I'm going, there's a good chance that my legs will be swollen if I'm not using my compression stockings," Dr. Loh says. "And so that's why, to me, there's the additional benefit to lowering my risk of a blood clot."

Dr. Loh emphasizes that no matter your personal risk profile for blood clots, there are more benefits to wearing compression socks the longer the flight. A research review from the Cochrane Database of Systemic Reviews showed that the risk of VTE is 1.5-4 times more likely on a flight four hours or longer.

Who shouldn't wear compression socks for travel?

Dr. Loh says that while most people can safely wear compression socks, a small subset of the population should only wear them with the direction of their physician.

"Anyone with a history of circulation problems in their legs should really talk to their physician before starting to use medical-grade compression," Dr. Loh says. "You also don't want to use compression stockings if there are open wounds on the legs unless that is being actively managed by a physician, primarily from an infection prevention standpoint and to help prevent disrupting the healing process. Seeking out medical advice is always a good idea. Still, outside of those groups, almost everybody can tolerate and should consider adding compression socks to their travel regimen."

What kind of compression socks should you use for travel?

There are two things to keep in mind when choosing compression socks: compression level and fit. Compression socks have a pressure measurement, millimeters of mercury or mmHg, that tells you how tight the sock will be.

"Compression stockings generally come in three levels, a low, medium and high," Dr. Loh says. "The numbers associated with the medium level are 20 to 30 mmHg. That is what we consider the lowest medical-grade compression, and that's usually the level where I tell people to start."

However, finding the right fit is as important as the level of compression. Dr. Loh says that you can't benefit from compression socks if they're not on your legs — and if they're uncomfortable, you're not going to wear them.

"Compression stockings are challenging because all of our feet, ankles and calves are all slightly different sizes," Dr. Loh says. "You want them to be tight on your leg, but you should be able to tolerate that for the whole day. It doesn't do you any good to go really tight or really high up on the leg if you can't keep them on."

Dr. Loh says there can be some trial and error when finding compression socks that are a good fit — and you may have to try a few different brands at the same compression grade to find your Goldilocks fit.

"We don't recommend any specific brands. There are about 40 major brands out there," Dr. Loh says. "If something doesn't fit quite right at the compression level you want to use, changing brands to get the fit a little bit better is important because they don't do you any good sitting in the sock drawer."

You can work with a physician to understand what compression grade would be suitable for you, as well as to get a prescription for compression socks. Many online retailers and medical supply stores also offer compression socks and stockings without a prescription.

Afraid of flying? Compression socks can aid long drives, bus or train trips

Whether on a plane or in a car, bus or train, we face many of the same issues for our circulation. Following some of the same advice can keep you healthy as you make it from point A to point B.

"Every time you stop for gas, you should get out of the car and walk around," Dr. Loh says. "There's no replacement for using your legs the way they were intended, which is to walk around and squeeze those calf muscles. We really dehydrate ourselves when we travel, which is the opposite of what you want for your veins. So you want to try to stay reasonably hydrated when you're traveling, but compression stockings can also really help."