What can cause blood clots in the lungs or legs?

Smoking. Smoking is one cause, varicose veins, swelling in the lower legs, trauma, being stagnant i.E sitting on a plane for a long trip., and oral contraceptives are the most common causes for the clot to go to the leg. Once in the leg, they can dislodge and go to the lung then the clot would be called a pulmonary embolism. (pulmonary = lung, embolism = moving clot.).