Methylmercury and Dimethylmercury

The toxic effects of mercury depend on its chemical form and the route of exposure. 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. (reference)

Dimethylmercury can be EXTREMELY TOXIC has was unfortunately experienced recently following a simple manipulation by a professional chemist. However, the exposure to methylmercury is usually by ingestion, and it is absorbed more readily and excreted more slowly than other forms of mercury. Elemental mercury, Hg(0), the form released from broken thermometers, causes tremors, gingivitis, and excitability when vapors are inhaled over a long period of time.

Although it is less toxic than methylmercury, elemental mercury may be found in higher concentrations in environments such as gold mine sites, where it has been used to extract gold. If elemental mercury is ingested, it is absorbed relatively slowly and may pass through the digestive system without causing damage. Ingestion of other common forms of mercury, such as the salt HgCl₂, which damages the gastrointestinal tract and causes kidney failure, is unlikely from environmental sources.

Wild life mortality of seed-eating birds and birds of prey caused by seed dressed with Hg compounds, alerted the Swedes and Finns in the 1960's about the risks of using Hg and also initiated studies revealing alarmingly high Hg content in fish caught close to paper and chlor-alkali industries. Acquired knowledge about environmental transformations of inorganic Hg to far more toxic organic forms along initially largely unknown transfer pathways focused the attention on the large quantities of Hg used in paper and chlor-alkali plants. Swedish authorities were first to act and enforced legislation on the use of Hg in Sweden, to avoid a tragedy such as that in Minamata, Japan. Consequently, seed dressing with methyl-Hg was prohibited in 1966, and Hg was banned from all pesticides in 1988.(reference)

Methylmercury is produced by methanogenic (TK) bacteria (that produce methane), some of the oldest living cells known, says Tom Clarkson, a toxicologist at the University of Rochester. When mercury is methylated through ingestion by microorganisms, a carbon atom is added on to the mercury atom. This additional atom is what changes mercury's properties, allowing it to be readily accumulated in fish. Once released from microorganisms, methylmercury rapidly diffuses, binding to proteins in aquatic biota. From there it marches up the food chain in a process known as biomagnification.

Simply put, smaller fish absorb the methylmercury from water as it passes over their gills and as they feed on methylmercury-tainted flora and fauna. In turn, these fish are eaten by bigger fish, which is why the highest concentrations of methylmercury are found in fish at the top of the aquatic food chain. (graphic courtesy: reference 67)

Large, predatory species like tuna, swordfish, and shark in ocean waters and trout, pike, walleye, and bass in fresh waters contain more methylmercury in their tissues than smaller, non-predatory fish. Also, the older the fish, the more time methylmercury has to accumulate. (reference)

Karen E. Wetterhahn was a professor of chemistry at Dartmouth College and the founding director of Dartmouth's Toxic Metals Research Program. An expert in the mechanisms of metal toxicity, Professor Wetterhahn was best known for her research on chromium. She became ill and died in 1997, at the age of 48, as a result of a tragic laboratory accident involving a highly toxic mercury compound. (photo courtesy)

Wetterhahn's research work involved understanding how elevated levels of heavy metals interfere with such processes as cell metabolism and the transfer of genetic information. That work was the direct cause of her death.

"Karen was the acknowledged international expert in chromium carcinogenicity," noted John S. Winn, chairman of the Dartmouth chemistry department. She began a sabbatical at Harvard in the fall of '95. The work involved doing some model compound studies involving mercury chemistry with Steve Lippard's group at MIT.

That work led to mercury NMR characterization of the model compounds with the use of dimethylmercury as this element's NMR standard. Winn relates that while preparing the mercury NMR standard in a fume hood on August 14, 1996, Dr. Wetterhahn spilled one to a few drops of dimethylmercury onto her latex glove near her thumb. Knowing that dimethylmercury was very toxic, she quickly cleaned it up.

What she did not know was that dimethylemercury was so soluble that it permeated the glove instantly and penetrated her skin and was absorbed into the blood-stream. It took five months until her gait began to falter and her words slur. By the time Dr. Wetterhahn connected that laboratory spill with the damage spreading in her brain, nothing could help her. Tests showed that her body contained more than 80 times the lethal dose of mercury. Her vision narrowed to a pencil's thinness and winked out. She lost her hearing and speech and she faded into a long coma.

Dr. Wetterhahn was a most meticulous scientist, her colleagues said, taking what would have seemed to be appropriate precautions. It is an accident that could occur to any experienced chemist. This, however, is not the first fatality from work with dimethylmercury.

Dartmouth suggests that when handling dimethylmercury, a combination of gloves, a highly resistant laminate underneath a heavy duty, chemically resistant outer glove, should be used. Dr. Wetterhahn was the first tenured female professor at Dartmouth. She received the largest single grant to a faculty member at the college. It amounted to a seven million dollar grant to work on toxic heavy metals.