Mercury is a natural element, a toxic heavy metal, which is highly volatile, the vapor form having the ability to kill cells outright rather than merely do damage. The breakage of a mercury thermometer is a potential, although usually unrecognized, medical emergency. Once exposed to air, mercury vaporizes rapidly. If inhaled it makes its way into the tissues of the body in minutes. A large dose can be lethal.
Inorganic mercury is found in nature.
Organic mercury has passed through a living system of some sort and has come out in the chelated form. One particularly dangerous form of mercury is methylated mercury, which is produced by the chelating systems of certain bacteria. Only chelation therapy can fully and reliably remove this toxin from your body. An ounce of prevention is surely worth a pound of cure. It is much better to never have amalgams put in.