Why are tourniquets dangerous?

Tourniquets are considered dangerous for several reasons. However, the with proper training and application they can be one of the most effective life saving devices in an emergency care givers arsenal. Anyway here's a few answers to your question:

1) Immediately after the application of a tourniquet everything distal to the site is cut off from it's blood supply and is no longer being oxygenated. This WILL lead to cell death and nerve damage.

2) Without blood flowing through the injured extremity, the blood remaining stagnant within it will begin to clot. If these clots aren't removed or dissolved and the tourniquet isn't removed properly(preferably in a controlled hospital setting) they can enter the circulatory system and block vessels which can cause all kinds of problems(stroke, cardiac arrest, etc).

3) Toxins, caused by cell waste, build up in the blood below to the tourniquet. The patients natural waste disposal might be overwhelmed after the tourniquet is removed, leading to organ and respiratory failure.

These are the major complications with tourniquet use. I hoped it helped a little bit.