Mercury is probably the toxic metal that most people are aware of, after lead. Mercury is present in the environment naturally and is used in industrial settings that incorporate electrolysis. It is also used in the manufacturing of paper.
In its elemental form (such as is present in some thermometers), mercury is non-toxic and can be ingested without significant effects. Inhalation exposure of elemental mercury is considered insignificant due to its low vapor pressure. It is only when mercury becomes oxidized and ionized to Hg2+ that it can have toxic effects. Once ionized, it can form organic molecules which are highly bioavailable and toxic. Methylmercury, which can be created by microorganisms in the environment or the gut, is very toxic and lipophilic. It accumulates in the CNS and elicits neurotoxicity.
Ionized mercury reacts with sulfhydryl groups on proteins, altering their tertiary structure, and rendering them dysfunctional. By altering proteins, mercury can also induce new antibodies to be made which can induce an autoimmune response in tissues. This often occurs in the kidneys and presents as nephritis. The CNS is also very susceptible to mercury toxicity when organic forms of mercury bind to neuronal proteins such as myelin.
Because of the often high levels of mercury found in rivers and lakes, fish and shellfish are the most common dietary sources of mercury. Other sources of mercury include dental fillings and the preservative thimerosal which is used in many vaccines. Although there have been multiple studies on these latter two sources of mercury, they have not been definitively associated with clinical toxicity.
Absorption:
Elemental mercury is poorly absorbed through the skin, although it can cause skin and eye irritation. Essentially, no elemental mercury is absorbed through the gastrointestinal tract. If elemental mercury does enter the body, it will not readily cross the blood–brain barrier or placental barrier.
Inorganic mercury is excreted in the urine and feces. Organic forms of mercury (most notably methylmercury), are easily absorbed from the GI tract with an efficiency of about 95%. Organic forms of mercury can accumulate in the brain and kidneys.
