Spending long hours sitting down could put Penn State students at risk for getting a type of fatal blood clot.

The condition, known as deep vein thrombosis (DVT), affects the deeper veins of the leg, arm and pelvis, said Dr. Teresa Dolan, a physician at University Health Services. She said DVT is caused by a continuum of three factors known as Virchow's triad.

The first cause is a change in blood flow, which can be a result of decreased activity. Being bedridden, wearing casts, or sitting for extended periods of time can cause blood to pool in the legs, where it becomes thicker and can clot more easily, Dolan said.

Another factor in the development of DVT is damage to the endothelium, a layer of flattened cells that lines the inside of some body cavities.

A bruise or fracture in the lower extremities can damage the blood vessels themselves, which can lead to clotting.

The last element of the triad of causes is a change in the consistency of the blood coursing through the veins. A number of hereditary conditions, such as factor V Leiden or protein deficiencies, can create a predisposition to blood clots.

Dolan said a combination of two or more of the risks is particularly dangerous.

Women are more predisposed to DVT than men because estrogen can actually cause blood clots to form. This information is printed on the packaging for many oral contraceptive pills. Pregnancy is another increased risk factor.

Smoking also increases the risk of developing DVT, Dolan said.

Symptoms of the disease include pain and swelling in one leg or arm without acute evidence of trauma. Dolan said those with DVT will report suddenly having pain while walking or even at rest.

The extremity with the clot can be red and warm, and the veins may show prominently. Many sufferers experience pain in calf muscles.

Dolan said there are several ways to prevent DVT from developing. Riders on long car rides should try to stop every two hours or so and take a walk to keep blood circulating.

Those working on computers should not cross their legs, since this can cause blood to pool. Dolan said taking aspirin every day may lower the risk of getting a clot, but this option should first be discussed with a doctor.

Amy Kudwa, a spokeswoman for US Airways, said the airline realizes the dangers of sitting for extended periods of time.

It recommends passengers "make a proactive effort to have a healthy travel experience," by avoiding sitting with their legs crossed. She said there have been no reported cases of getting DVT while on one of their flights.

Dolan said she has not seen any cases of DVT in the three months she has been at Penn State, but that the disease is not rare in students. Level of activity, not age, is the primary factor.

The danger of DVT lies in the ability of parts of the clot to break off and go to the lungs, preventing breathing and possibly causing death. This is known as a pulmonary embolism.

The Associated Press reports that up to two million Americans develop DVT each year. Nearly 600,000 of those cases can lead to pulmonary embolisms, and 60,000 to 100,000 of those cases result in fatalities.