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Mercury


Mercury is a metal that exists in three chemical forms. Each has its own unique toxicology and is usually considered separately.

Although uncommon, acute mercury intoxication may be potentially life threatening. Occupational mercury exposures are now less common than in the past, but may potentially occur in a wide range of industries. Environmental exposures to mercury have led to a number of human health disasters. Clinical Toxicologists should be familiar with the disasters in Japan (Minimata Bay) and Iraq in the 1950s.

Sources
Elemental Mercury: dental amalgam, thermometers, paints, pigments, gold mining
Inorganic Mercury: mercurochrome, disinfectants, fireworks and explosives, processing fur and leather, photographic plates
Organic Mercury: embalming fluid, fungicides, pesticides, wood preservatives, seafood

Elemental mercury (quicksilver; the only metal to exist as a liquid at room temperature) is minimally absorbed across the skin or intact gut. However, inhalation of vapour following heating or aerosol following vacuuming may cause life threatening acute intoxication. Target organs include the lungs (pneumonitis and acute non-cardiogenic pulmonary oedema), kidneys, and central nervous system. Within hours there is an abrupt onset of headache, nausea, vomiting, chills, dyspnoea and dry cough with progression to respiratory failure over the next few days from pneumonitis.

Inorganic mercury (salts) cause very severe haemorrhagic gastrointestinal effects if ingested, followed by acute nephrotoxic, neurotoxic effects and shock. Potential lethal dose is 30-50 mg/kg.

Organic mercury compounds are mercury with a covalent bond to a carbon atom. They include short chain alkyl compounds (methyl, ethyl, propyl), long chain compounds and aryl (phenyl) compounds. Exposure by ingestion, inhalation or dermal application can lead to permanent neurological injury over weeks or months from initial exposure. The neurological sequelae include memory loss, emotional lability, cerebellar ataxia, sensory and motor loss.

Accidental paediatric oral exposures to mercurochrome (merbromin; a polyaromatic organic mercury compound C20H8Br2HgNa2O6) is regarded as benign. Deliberate self-poisoning with mercurochrome has been associated with elevated mercury levels but the significance is unknown.

Exposure to mercury in dental amalgams has been the subject of controversy for many years. The presence of dental amalgams containing mercury is associated with slightly increased urinary excretion of mercury that is considered clinically insignificant. However, despite the lack of robust evidence of health effects, some practitioners still advocate amalgam replacement and or chelation.

Injection of elemental mercury S/C or IV can lead to pulmonary embolism creating depots from which distribution of mercury to the brain may occur but the risk is undefined.
The incidence of systemic features is variable and often prolonged in onset, ranging from no adverse effects to the development of stomatitis, anaemia, peripheral neuropathy, renal failure, pneumonitis and interstitial lung fibrosis.

Whole blood, spot urinary and 24-hour urinary mercury levels may be obtained. The utility of each will depend on the clinical setting. Routine screening of mercury levels in patients with non-specific presentations is not recommended and incidental discovery of elevated levels is usually benign.

Normal whole blood mercury levels in an unexposed adult would be regarded as less than 10 ug/L (50 nmol/L) with a range from 2 to 15 ug/L (10 to 75 nmol/L). Children may have normal levels closer to 2 ug/L (10 nmol/L).
Whole blood mercury levels >20 ug/L (100 nmol/L) may confirm a recent exposure if measured soon after the exposure, but are not a reliable indicator of clinical outcome.

Urinary mercury levels less than 20 to 25 ug/L (100 to 125 nmol/L) are considered normal, although in occupationally unexposed persons, levels greater than 10 ug/L are seldom found.Toxic urine levels are > 50-100 ug/L (250-500 nmol/L) and associated with neuropsychiatric disturbance.
Chelation is generally considered for:
  • Significantly symptomatic patients after acute inhalation exposure
  • Symptomatic patients with urinary mercury greater than 50 ug/L (250 nmol/L)
  • Symptomatic patients with blood concentration greater than 100 ug/L (500 nmol/L)