Causes

Are you young & very fit?  Then you are at high risk of DVT.

Understanding how DVT occurs is vital for our protection. DVT is often "silent" i.e. without symptoms until the situation is serious. Many people have had DVT without knowing it.

We need to understand normal blood circulation. The heart pumps blood around our body, but to return the blood uphill from the legs our main leg muscles are used. Every time our leg muscles contract they pump blood up our deep veins past one way valves. 

We are designed to rest & sleep lying down. When our pulse rate drops and we are sitting up, (fit persons tend to have a low resting pulse) blood tends to pool in our lower legs. When blood pools it tends to clot. If we are inactive or asleep sitting upright at a desk or in a car, bus, train or cinema then the blood tends to pool in the lower legs and that encourages clots to form.

• Any damage, e.g. bruising to our veins encourages blood clots.
• Reduced hydration tends to make the blood thicker and more prone to clot.
• Diseases that cause inflammation, cancers, surgery to hips, legs & pelvis increase the risk.

Remember 94% of blood clots are symptom free and eventually dissolve.

The major risk is that a clot that has formed may break off, sometimes days later, and travel to the heart & lungs. It seems to be simply a question of chance. Death rates from P.E. pulmonary embolism, heart attack, and stroke are rising. We do not yet have studies that show the hard statistics that directly relates that to modern transport & lifestyle, but the evidence is mounting.

By ignoring the evidence and failing to address the DVT issue, we are playing "Russian Roulette". Simple awareness and precautions can do much to protect us.

Altitude effect
Bjorne Bendz' study in Norway put twenty young men in a chamber simulating cabin altitude and measured coagulation factors hour by hour. He found significant increases in coagulation factors in all the twenty young men.
www.airhealth.org/formation.html

US goverment Website
DVT occurs when a blood clot forms in the leg and blocks blood flow. These clots can lead to potentially life-threatening pulmonary embolisms, which is when the clot breaks free, travels north through the heart and lodges in a lung. The conditions affect some 250,000 to 2 million Americans every year and may be responsible for as many as 10 percent of all hospital deaths.
www.healthfinder.gov/news/newsstory.asp?docid=514254

Preventive Blood Clot Therapy Underused
According to the American Heart Association, more than 2,000,000 Americans develop deep vein thrombosis annually. An estimated 600,000 of these develop pulmonary embolism, a potentially fatal complication where the blood clots break off and form pulmonary emboli, plugs that block the lung arteries. 60,000 people die of pulmonary embolism each year.
www.healthatoz.com/healthatoz/Atoz/ency/deep_vein_thrombosis.html

New Scientist
"More than 2,000 people may die from flight-related deep vein thrombosis in Britain each year." "most air passengers who go on to die from DVT will develop symptoms days or weeks after flying"
www.newscientist.com/news/news.jsp?id=ns9999314