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What is a blood clot? When a blood vessel is injured, platelets and proteins in the blood stick together to form a plug (clot) over the site of the wound to prevent excessive bleeding. When blood flow is restricted, clots can sometimes form within the blood vessels, even if no injury exists. Prolonged immobility during long trips can lead to the formation of unnecessary blood clots in a major vein (commonly in the leg). This condition is called "deep-vein thrombosis" (DVT).

Most of the time, blood clots will naturally dissolve and go away on their own. But in some cases, a blood clot may travel from the leg into the lung, where it becomes lodged and prevents blood flow. This life-threatening condition is called a "pulmonary embolism" (PE) and should be treated immediately.

"It is estimated that more Americans die each year from pulmonary embolism than breast cancer, HIV disease and motor vehicle crashes combined," said Nancy Berliner, M.D., president of the American Society of Hematology. "The good news is that blood clots are highly preventable if you recognize the risk factors and take simple steps to promote good circulation."

Blood clot warning signs include sudden pain, redness or discoloration, skin that feels unusually warm and swelling of the leg, ankle or calf. Signs of PE include chest pain and shortness of breath. Anyone experiencing these symptoms should immediately contact a doctor.

There are simple steps people can take to help prevent blood clots from forming while flying. The American Society of Hematology (ASH) offers the following tips:

- Walk around the plane every few hours.

- Drink lots of fluids.

- Wear loose clothing.

- Avoid drinking alcohol before and during travel.

- Store carry-on luggage overhead to give yourself leg room.

Women taking oral contraceptives have a higher risk of developing clots, as do pregnant women, smokers, seniors and obese individuals. For more information, visit ASH's Web site www.bloodthevitalconnection.org.