Mercury [Hg, 80]
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27.6 HUMANS
Mercury content in human body has been estimated at 6 mg, of which in the blood
8 μg/L, in the bone 0.5, and in the tissue 0.2–0.7 mg/kg (Emsley 2011). It accumulates in the thyroid and pituitary glands, in the brain, kidneys, liver, pancreas, testes,
ovaries, and the prostate gland. It is concentrated in the kidneys at 50%–90% of the
body burden. Its levels (mean and range, in μg/kg) are as follows: 6.7; 2.4–12.2 in the
occipital cortex; 49, 21–105 in kidneys; 6.7, 1.9–22.1 in the gray brain matter; and
3.8, 1.4–7.1 in the white brain matter (WHO 1991a).
Exposure to Hg is commonly evaluated using Hg concentrations in blood and
urine, although hair may also be used as a biomarker of Hg exposure. The concentration of total Hg in blood is accepted as a reasonable biomeasure of MeHg
exposure. The WHO has estimated that the average total blood Hg concentration,
for the general population is approximately 8 μg/L; however, individuals with high
fish consumption can have concentrations in blood as high as 200 μg/L. Typical total
Hg concentrations in urine have been reported to be in the 4–5 μg/L range (Health
Canada 2010b).
The health effects of Hg are diverse and depend on its chemical form, the dose,
the duration of exposure and the exposure route, and also dietary content of interacting elements, especially Se. There are three forms of Hg from a toxicological point
of view: metallic Hg, inorganic Hg salts, and organic Hg compounds. The most
common organic Hg compounds in nature are methylmercury (CH 3 Hg + , MeHg) and
dimethylmercury ([CH 3 ] 2 Hg). Metabolism of Hg compounds to other forms of Hg
can occur within the tissues of the body.
Amounts of Hg added to some pharmaceuticals, such as thiomersal (ethyl mercury), which is used as a preservative in some vaccines, are very small by comparison with its other sources. Claims have been made that thimerosal in vaccines
may be a cause of autism and related disorders; however, this theory has yet to be
confirmed.
All forms of Hg have adverse effects on health at high doses. However, the evidence that exposure to very low doses of Hg from fish consumption, the receipt of
dental amalgams, or thimerosal in vaccines has adverse effects is open to wide interpretation (Clarkson et al. 2003).
There are substantial differences in the toxicity of elemental Hg metal, inorganic
Hg salts, and organometallic Hg compounds. Less than 0.01% of Hg 0 , approximately
10% of inorganic Hg and 95% of organic Hg, is absorbed from the gastrointestinal
tract following oral ingestion (Langford and Ferner 1999).
Methylmercury is the most toxic form. It affects the immune system, alters
genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight. Methylmercury is particularly damaging to developing embryos, which are five to ten times more sensitive than adults.
Dimethylmercury can be extremely toxic. However, the exposure to MeHg is usually by ingestion, and it is absorbed more readily and excreted more slowly than
other forms of Hg. Elemental Hg(0), the form released from broken thermometers,
causes tremors, gingivitis, and excitability when vapors are inhaled over a long
period of time.