When it comes to health risks on long flights, it's not what you pay for your seat, but where you sit that makes the difference.

New medical recommendations dispel the myth of "economy class syndrome," the notion that cramped leg room in the cheap seats on long flights can lead to deep vein thrombosis, or blood clots in the legs.  The clots can travel through the bloodstream to block blood flow to the lungs, causing a pulmonary embolism, deadly in as many as 30 percent of sufferers, according to the U.S. Centers for Disease Control and Prevention.

Sitting in roomier first class seats won't lower the risk of developing DVT, but sitting in an aisle seat will, according to the American College of Chest Physicians.

In the new guidelines published today in the journal Chest, doctors list sitting in a window seat as a risk factor for DVT.

"Traveling in economy class does not increase your risk for developing a blood clot, even during long-distance travel; however, remaining immobile for long periods of time will," said Dr. Mark Crowther, one of the authors of the guidelines, in a statement. "Long-distance travelers sitting in a window seat tend to have limited mobility, which increases their risk for DVT."

Advanced age, pregnancy, use of oral contraception and other forms of estrogen, recent surgery and obesity, also increase the risk of developing DVT during air travel, according to the guidelines.

But there's no evidence that dehydration or alcohol intake will cause clots to form.

Crowther emphasized that passengers rarely develop symptomatic DVT on airplanes, and those who do are usually on flights of 8 to 10 hours and have at least one additional risk factor.

In 2011, tennis star Serena Williams was treated for a pulmonary embolism that doctors speculated was caused by a blood clot that formed after a combination of a recent foot surgery and a cross-country flight.

These dangerous blood clots aren't just a risk for travelers. In August, a 20-year-old British man died of a pulmonary embolism caused by a blood clot that formed after he sat playing video games for 12 hours.

Doctors say the best way to prevent DVT is to move around as much as possible, even on long, cramped flights. The ACCP guidelines say passengers on flights of 6 hours or more should get up and walk around frequently, and stretch their calves. For travelers at an increased risk of DVT, the guidelines recommend wearing below-knee graduated compression stockings.

People should also watch for the symptoms of a blood clot in the legs, such as tenderness or pain in the calf, warmth, redness or swelling. Serious signs of a pulmonary embolism can include shortness of breath, rapid heart rate, stabbing pain in the chest and an unexplained cough.