- Last Update On : 2013-01-26
Mercury is a worldwide pollutant that exists in a variety of forms, each of which has different degrees of absorption and toxicity.
- Elemental mercury, also known as quicksilver, is a dense, silver-colored liquid at room temperature. It is minimally absorbed by the GI tract and has minimal toxicity. If mercury becomes vaporized, inhaled fumes are absorbed by the lungs and distributed by the circulatory system throughout the body, causing damage to the CNS and kidneys.
- Mercurial salts are absorbed by the GI tract and have toxic effects similar to mercury vapor.
- Organic (methylated) mercury is readily absorbed through the skin or GI tract and is distributed to all tissues within 4 days. Almost all organic mercury is eliminated through the liver by bile excretion. Toxicity is dose related and may not be manifested for several weeks to months.
Mercury burden in the environment is largely from natural sources, e.g., soil erosion and degassing of the earth's crust. Additional amounts enter the environment though fossil fuel combustion, waste incineration, and smelting processes. Mercury vapor emitted into the atmosphere is eventually oxidized into inorganic forms, which are water-soluble. It is then distributed worldwide by rainfall. Most of the inorganic mercury settles in the sediment, where it reacts with sulfate to form insoluble precipitate. A small percentage is methylated by bacteria and becomes bioavailable. Methylated mercury is ingested and concentrated by living organisms. Concentration within animals is magnified up the food chain. As a result, the highest levels of mercury are found in larger and older predatory fish such as shark and swordfish.
People are primarily exposed to mercury through their diet. In most foods, except for fish, mercury exists in inorganic form. Fish is the primary source of methylated mercury in the diet. Other common sources of mercury exposure include mercury amalgam in tooth fillings and vaccines containing thimerosal. Dental amalgams, which also add to the mercury burden, do not pose a danger for most dental patients or dental personnel who practice good safety procedures in their environment. Vaccines are now formulated as single-use, eliminating the need for thimerosal preservative.
Mercury toxicity most frequently results from self-inflicted poisoning, accidents, or chronic workplace exposure. Industrial uses of mercury include electrolysis, electrical switches, and fungicides. Symptoms begin with paresthesia of extremities and proceed to visual and gait abnormalities, followed by difficulties with speech, hearing loss, muscle weakness and mental deterioration. sensitization. Other manifestations of toxicity include: abdominal cramps, diarrhea, dermatitis, proteinuria, and hepatic dysfunction.
In general, the amount of mercury detected in blood and urine correlates with the degree of toxicity. A 24 hour urine collection is the best specimen for evaluation of acute exposure to inorganic mercury. The reference range is <20 ug/L. Urine mercury levels up to 50 µg/L can result from seafood diets, primarily methylmercury, without any clinical evidence of toxicity. Urine mercury values greater than 150 µg/L are generally considered toxic. The ICP-MS method measures total mercury, including both organic and inorganic species.
Blood is the preferred specimen for detection of organic mercury. Whole blood should be collected in to a royal blue top tube. Unexposed individuals have mercury levels <10 ug/mL. The reference range is <60 ug/mL. Whole blood levels of >60 ug/L for organic mercury and >150 for inorganic mercury indicate significant exposure.
Hair analysis is useful to assess chronic exposure, because hair methyl mercury readily accumulates in growing scalp hair at levels ~250 times higher than blood. After it is incorporated into hair, the mercury concentration remains relatively unchanged. Longitudinal analysis of hair segments can provide months to years of insight into prior blood levels. Approximately 50 strands of hair are required.
Urine mercury levels for healthy subjects (reference population) are influenced by diet and environmental background exposure.