Mercury
Mercury (Hg) is a naturally occurring metal that is mined chiefly as HgS in cinnabar ore. It is converted to three primary forms, each with a distinct toxicology: elemental (metallic) mercury (Hg0), inorganic mercury salts (eg, mercuric chloride [HgCl2]), and organic (alkyl and aryl) mercury (eg, methylmercury). Approximately one-half to one-third of commercial mercury use is in the manufacture of chlorine and caustic soda, one-half to one-third in electric equipment, and the remainder in various applications, such as dental amalgam, fluorescent lamps, switches, thermostats, and artisanal gold production. In the United States, mercury use in batteries and paints has been discontinued. Previous use in pharmaceuticals and biocides has declined sharply, although mercuric chloride is still used as a stool fixative, and some organomercury compounds (such as mercurochrome, phenylmercuric acetate, and thimerosal) are still used as topical antiseptics or preservatives. Some folk medicines contain inorganic mercury compounds, and some Latin American and Caribbean communities have used elemental mercury in religious or cultural rituals. Hazardous exposure has resulted from dermal use of imported skin lightening creams formulated with inorganic mercury salts. Aquatic organisms can convert inorganic mercury into methylmercury, with resulting bioaccumulation in large carnivorous fish such as swordfish. Mercury is released to the environment from the burning of coal and from fugitive emissions during the large-scale mining of gold. In an effort to curtail the use of elemental mercury in artisanal gold mining and other pathways of environmental mercury pollution, the European Union has enacted a ban on the export of most inorganic mercury effective 2011; a US ban on export of elemental mercury takes effect in 2013.
Mechanism of toxicity. Mercury reacts with sulfhydryl (SH) groups, resulting in enzyme inhibition and pathologic alteration of cellular membranes.
Elemental mercury and methylmercury are particularly toxic to the CNS. Metallic mercury vapor is also a pulmonary irritant. Methylmercury is associated with neurodevelopmental disorders.
Inorganic mercuric salts are corrosive to the skin, eyes, and GI tract and are nephrotoxic.
Inorganic and organic mercury compounds may cause contact dermatitis.
Toxic dose. The pattern and severity of toxicity are highly dependent on the form of mercury and the route of exposure, mostly because of different pharmacokinetic profiles. Chronic exposure to any form may result in toxicity (see Table II–34 for a summary of absorption and toxicity).
Table II-34 Mercury Compounds
Table II-34 Mercury Compounds
Absorption
Toxicity
Form
Oral
Inhalation
Neurologic
Renal
Elemental (metallic) mercury
Hg0 liquid
Poor
N/Aa
Rare
Rare
Hg0 vapor
N/Aa
Good
Likely
Possible
Inorganic mercuric salts
Hg2+
Good
Rare but possible
Rare
Likely
Organic (alkyl) mercury
RHg+
Good
Rare but possible
Likely
Possible
Elemental (metallic) mercury is a volatile liquid at room temperature.
Hg0 vapor is absorbed rapidly by the lungs and distributed to the CNS. Airborne exposure to 10 mg/m3 is considered immediately dangerous to life or health (IDLH), and chemical pneumonitis may occur at levels in excess of 1 mg/m3. In occupational settings, overt signs and symptoms of elemental mercury intoxication generally have required months to years of sustained daily exposure to airborne mercury levels of 0.05–0.2 mg/m3. The recommended workplace limit (ACGIH TLV-TWA) is 0.025 mg/m3 as an 8-hour time-weighted average; however, some studies suggest that subclinical effects on the CNS and kidneys may occur below this level.
Liquid metallic mercury is poorly absorbed from the GI tract, and acute ingestion has been associated with poisoning only in the presence of abnormal gut motility that markedly delays normal fecal elimination or after peritoneal contamination.
Inorganic mercuric salts. The acute lethal oral dose of mercuric chloride is approximately 1–4 g. Severe toxicity and death have been reported after use of peritoneal lavage solutions containing mercuric chloride in concentrations of 0.2–0.8%.
Organic mercury
Mercury-containing antiseptics such as mercurochrome have limited skin penetration; however, in rare cases, such as topical application to an infected omphalocele, intoxication has resulted. Oral absorption is significant and may also pose a hazard.
Methylmercury is well absorbed after inhalation, ingestion, and probably dermal exposure. Ingestion of 10–60 mg/kg may be lethal, and chronic daily ingestion of 10 mcg/kg may be associated with adverse neurologic and reproductive effects. The US Environmental Protection Agency reference dose (RfD), the daily lifetime dose believed to be without potential hazard, is 0.1 mcg/kg/d. The RfD was derived from studies of neuropsychological deficits arising from in utero exposure in humans. To minimize neurodevelopmental risk, the US EPA and FDA have advised pregnant women, women who may become pregnant, nursing mothers, and young children to avoid consumption of fish with high levels of mercury (eg, swordfish) and to limit consumption of fish and shellfish with lower mercury levels to no more than 12 oz (two average meals) per week (www.epa.gov/waterscience/fish/advice).
Dimethylmercury, a highly toxic synthetic liquid used in analytic chemistry, is well absorbed through the skin, and cutaneous exposure to only a few drops has resulted in a delayed but fatal encephalopathy.
Clinical presentation
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