Blood clots can form in the deep veins of the leg. This is termed deep vein thrombosis or DVT . If they are large enought, they can break loose and travel up through your veins into the right side of your heart and on into the artery to your lung.  This is called a pulmonary embolus (PE).  (An embolus is simgular; one. Emboli is plural.) If the blood clot is big enough it can block circulation to your lung.  This causes shortness of breath, pain in your chest, and pain with breathing.  A sufficiently large PE can be fatal.

Predisposing factor for the development of deep vein thrombosis (DVT) and PE include prolonged immobilization such as riding in a car or plane; major hip, knee, abdominal, or cancer surgery; underlying abnormalities of your coagulation system (thrombophilia); and previous DVT. Virchow's triad includes abnormalities of the blood, the blood vessel, and slow flow.

Blood clots can also form in the superficial veins of your legs, so-called "phlebitis," or "thrombophlebitis."  To avoid confusion with DVT, we like to use the term superficial venous thrombosis or SVT.   If the blood clot extends up to the junction with the common femoral vein, or extends into the common femoral vein, there is a risk of PE. In some cases DVT occurs in conjunction with SVT.

Predisposing factors for the development of superficial venous thrombosis (SVT) include enlarged varicose veins, prolonged sitting, known or unknown malignancies and underlying hyper-coagulation tendencies (thrombophilia).  A recent study showed a 15% incidence of underlying thrombophilia in patients with SVT.

Recent articles have reported on the incidence of DVT in people who make long plane flights.  In one study of 231 patients, 10% of patients developed DVT in the calf; none of these wore compression hose.  None of the patients who wore class I compression stockings developed DVT.  Conclusion: Wear class I compression hose on long-haul flights.