There are trace amounts of mercury vapor released by silver fillings when you chew or brush your teeth.  As you can imagine, when one of these fillings needs to be replaced, removing it with a high speed drill can release dangerous amounts of mercury vapor due to the extreme friction and heat generated by a dental drill.  For this reason, we use a very careful technique to minimize this exposure as much as possible.  Information can be found here by the International Academy of Oral Medicine and Toxicology.