Table 2.
Toxicological features of mercury (Hg) species.
| Organic Mercury | Elemental Mercury (Hg0) | Inorganic Mercury (Mercurous Hg+; Mercuric Hg2+) |
|---|---|---|
| Source of exposure | ||
| - MeHg: diet (fish, rice) - EtHg: a topical antiseptic and antifungal agent in vaccines |
- Hg vapours released from maternal dental amalgam (50% is Hg0) [41] - Accidental exposure (from broken thermometers and other devices used in school laboratories) - Specific products (e.g., mercury-containing paints) |
Use of cosmetics containing salts (skin creams, soaps, medications); vaccines; breast milk for infants |
| Absorption | ||
| - Oral: MeHg from GI tract (95%) [4,29] - Transdermal: EtHg from vaccines (100%) [5] - Inhalation: only from vapours of MeHg |
- Oral: metallic Hg in GI tract is converted to mercuric sulfide [4] - Dermal: absorption of Hg0 through children’s skin - Inhalation: from Hg vapours (70–85%) [2] |
- Oral: absorption through the GI from breast milk (infants) or from water (children) - Dermal: absorption through children’s skin - Inhalation: aerosol from Hg salts |
| Distribution | ||
| - MeHg from the GI tract is distributed to the blood; in the body it is present as hydrophilic complexes attached to the sulfur atom of thiol ligands [46] - MeHg crosses the blood-brain or placental barrier via a MeHg-l-cysteine complex transported by neutral amino acid carrier [29,46] - After thimerosal injection, the EtHg–cysteine complex is exported from muscle cells by thiol-containing proteins. Then, it exchanges with generic plasma thiol proteins, like albumin [5] |
After absorption it crosses the lungs and, thus, into the bloodstream, where, due to its high lipophilicity, is distributed throughout the body, including the blood-brain and the placenta barrier [7] | - From the GI tract it is distributed to the blood and organs. Mercuric Hg has affinity for sulfhydryl groups in the RBCs and plasma [4,47] - Due to its ionic charge it does not cross the blood-brain or the placenta barrier |
| Biotransformation (metabolism) | ||
| - MeHg is stable in the body, but intestinal flora, tissue macrophages, and fetal liver are site of demethylation to inorganic Hg [4] - EtHg is much more less stable; it is rapidly degraded to mercuric Hg |
Elemental Hg is oxidized to mercuric Hg in the RBCs by catalase and hydrogen peroxide [4] | Mercuric Hg is unstable in vivo; it is converted to elemental Hg (rat study); only intestinal flora is site of methylation [46] |
| Excretion | ||
| - MeHg is secreted in bile and excreted in feces (~90% in feces occurs as inorganic Hg after demethylation) [46] - MeHg is excreted also in breast milk [2] - Approximately 1% of the human body burden of MeHg is excreted daily [2,46] - MeHg half-life elimination has been estimated at 45–90 days [4,46] - EtHg is excreted mainly by feces - From infant blood the EtHg excretion resulted more rapid than MeHg due to its rapid conversion to mercuric Hg (half-life 3–7 days) [5] |
- Hg vapour is excreted via sweat and saliva, or as mercuric Hg via feces and urine - Approximately more than 1% of the human body burden is excreted daily [46] - Half-life elimination has been estimated at 58 days [4,46] |
- Inorganic Hg is excreted mainly in urine; saliva, bile, sweat, exhalation, and breast milk are other routes of excretion - Half-life has been estimated at 49–96 days [4,46] |
| Target organs | ||
| MeHg and EtHg have the same target: fetal brain, CNS, other system (cardiovascular, reproductive, immune, etc.) | CNS, kidney, lungs, skin | Since mercuric Hg induces metallothionein production in the kidneys, the highest concentration is in this organ, however also CNS and skin are critical sites |
MeHg: methyl-mercury; EtHg: ethyl-mercury; CNS: central nervous system; GI: gastrointestinal; RBCs: red blood cells.