Mercury, especially as a vapor is toxic to the human body. Up to modern times it had been used as a medicine to treat various ailments. Even great men such as Isaac Newton swallowed doses of it. Fortunately, in its liquid state it will generally flow through the system without being significantly absorbed by the body. But breathed in it can cause cumulative poisoning of the system.
Mercury is a neuro-toxin, meaning that it adversely affects the nervous system. Symptoms of long-term mercury poisoning are personality change, nervousness, trembling and even dementia. A pregnant woman exposed to it is more prone to have a child with brain disabilities. At one time certain trades were subject to mercury poisoning as an occupational hazard. Most notoriously, hatters used mercury in the process of preparing felt for various hats. Their constant exposure to mercury literally drove them mad, and thus we have the saying, "mad as a hatter" made famous in Lewis Carroll's Mad Hatter. The outlawing of the use of mercury in the felting process did not occur until 1941.
There is a great deal of controversy over the sources of mercury poisoning. It is alleged by some groups that silver dental fillings (also called amalgams) could be a source of mercury in the human body. This claim is strongly denied by the American Dental Association. It is a fact that one of the major components of a silver filling is mercury, but the ADA says that it seldom takes on the dangerous gaseous form. Another source of mercury is coal fired electrical plants which allow mercury into the air as part of the coal burning process. Fish tend to accumulate mercury in their system. When consumed by humans this may or may not prove harmful. It is suspected that pregnant women and young children may be most vulnerable to this form of mercury poisoning.
There are hair, blood and urine tests to determine mercury levels within a person. Mercury can be leached out of the human system in a process called chelation. Anti-oxidants and selenium are also said to help reduce mercury levels in the body. It is also believed that the herb cilantro may aid in removing harmful metals from the body. There are groups that advocate the removal and replacement of amalgam fillings as well as the abandonment of coal as a source of electrical power. Even so, the properties of mercury continue to make it a useful metal especially in lighting applications and temperature measuring devices.