Module 3: Toxicology - Section 12: Heavy Metals

OHM12.X: Mercury - Part 1

MERCURY:

A. Production and Uses:

Mercury is mined primarily as cinnabar (HgS) and, in very rich ores, as metallic mercury. The United States is the world’s third largest producer, and California accounts for 60 percent of the U.S. production. Both underground and surface mines exist. The ore is usually processed at the mine site by heating it to about 700 degrees C and allowing oxygen to combine with HgS, producing SO2 and Hg vapor. The vapor is then condensed. The metal may also be recovered in a retorting process in which the heated ore is combined with limestone, which extracts the sulfur, yielding Hg vapor. Secondary production of mercury from dental amalgams, scrap batteries, sludges, discarded instruments and electrical devices, and dismantled chloralkali plants is also a significant industry.

The largest consumption of mercury is for electrical apparatus and in chlorine and caustic soda, (chloralkali), production facilities. Mercurials are used as catalysts in polyurethane foams. Mercury is used extensively in dentistry and in industrial and control instruments (thermometers, barometers, manometers). In the past it has been used in paints to inhibit mildew, and in marine paints to prevent the growth of barnacles, worms, algae, and seaweed. Its use for these purposes was banned by the EPA in 19712. Mercurial pharmaceuticals likewise are no longer used. Mercury fulminate (Hg(NCO)₂) is an explosive material used in detonators and blasting caps.

Elemental mercury poisonings are being increasingly reported in gold panning and precious metals reclamation processes. Laboratories may have significant mercury exposures because of careless handling and improper cleanup of spills. Mercuric oxide is used in making dry batteries for hearing aids and other electronic products. Solders which contain mercury are also used in making these batteries and their use carries a significant risk because of the temperatures to which they must be heated during soldering. In the case of a mercury spliil, sulfur powder or metallic dust particulate should be used to amalgamate mercury.

B. Physical and chemical properties

Mercury is the only metal which is a liquid at ambient temperatures (MP = -39 degrees C, BP = 357 degrees C). It has a significant vapor pressure and consequently it is hazardous because of its potential for generating mercury vapor. Chemically pure mercury is stable at ordinary temperatures but at elevated temperatures it combines spontaneously with oxygen to form mercuric oxide (HgO). Mercuric oxide and the mercury salts which are commonly used (mercuric chloride, mercurous sulfate, mercuric acatate) are soluble in water.

A large number of organic mercury compounds have been synthesized. The salts of some of these are also soluble in water. Toxicity of mercury depends greatly on the chemical species. Elemental mercury exposure is most hazardous because of the high vapor pressure and small particle size of mercury fume and thus ready absorbtion across the alveoli. Mercury salt exposure is more likely to occur via the gastro-intestinal track and may result acutely in severe gastrointestinal inflammation. Organic mercurials such as methyl mercury or dimethyl mercury are notoriously toxic to the central nervous system.

Several environmental disasters have occurred from organic mercurials. Epidemic poisoning occurred after consumption of mercury fungicide contaminated seed grains eaten by starving farmers during famines both in Iraq and Turkey . Another famous case occurred in Minimata, Japan, from the bioaccumulation of inorganic mercury salt wastes emitted from a chlor-alkali industry into a bay with resultant methylation by anaerobic bacteriae in the bay and subsequent incorporation into seafood.

Postgraduate Diploma in Occupational Health (DOH) - Modules 3: Occupational Medicine & Toxicology (Basic) by Profs Mohamed Jeebhay and Rodney Ehrlich, Health Sciences UCT is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License. Major contributors: Mohamed Jeebhay, Rodney Ehrlich, Jonny Myers, Leslie London, Sophie Kisting, Rajen Naidoo, Saloshni Naidoo. Source available from here. For any updates to the material, or more permissions beyond the scope of this license, please email healthoer@uct.ac.za or visit www.healthedu.uct.ac.za. Last updated Jan 2007.
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