The blood clots you are referring to are called deep vein thromboses. These are blood clots that form within a vein and do not form in the setting of bleeding
. What makes deep vein thromboses dangerous is that if the clot in the vein dislodges and moves towards the heart, it could end up getting stuck in the lungs. This is what is called a pulmonary embolism, and is one of the feared complications of a blood clot in one of the deep veins. One contributor to the development of blood clots is immobility. We see them form frequently in patients that are stuck in a hospital bed for many days, and those that are on an airplane for many hours. Certainly sitting down for an extended period of time is the risk factor that makes long flights a risk factor. However, we don't see this occur frequently in office workers. This is probably because you move around a lot more at the office than you would sitting in the middle seat of an airplane.
If you are worried about the possibility that you have a clot, or if a close family member recently developed a clot, then you can talk to you primary care physician
about getting tested for a clotting disorder that could predispose you to getting a thromboses. Good luck.