In the previous chapter we looked at the effects of mercury as the metal, as its vapour, and as its inorganic compounds. However, its most insidious form is organic mercury, better known as organo-mercury, which is a technical term meaning that one or two carbon atoms are bonded directly to the metal, as in methyl mercury chloride or dimethyl mercury. As we shall see, methyl mercury is worryingly mobile in its ability to circulate through the environment, move up food chains, and even to travel unchecked around the human body. (A methyl group consists of a single carbon atom and three hydrogens: CH3.)
The first recorded deaths from dimethyl mercury occurred in London in 1865. In January of that year, two laboratory assistants at the Royal Institution, in the laboratory of Professor Edward Frankland, were reacting sodium-mercury amalgam with methyl iodide with the objective of making dimethyl mercury. They were completely unaware of the dangers it posed. One of the men, a 30-year-old German, Dr Ulrich, was admitted to St Bartholomew's Hospital on 3 February and died ten days later, the other man, 23-year-old Mr T. Sloper, was admitted on 25 March but survived a year before expiring. These deaths were used as an excuse to attack the eminent Frankland in the press but it eventually transpired that the assistants had been working under the supervision of a Professor William Odling. Moreover, Ulrich had told a friend that his exposure had been an accident, caused by his breaking a sealed tube of dimethyl mercury and spilling the contents, with the result that he and Sloper had inhaled a lot of the vapour while clearing up the mess.
Not that this accident prevented others using dimethyl mercury as a possible medical treatment and in 1887 a series of tests began in which injections of it were tried as a cure for syphilis. The dosage was 1 ml of a 1% solution, but no patient received more than two injections because the experiment was stopped when tests on dogs showed how dangerous the substance was.
More recently, dimethyl mercury accidentally caused the death of a woman chemist, Karen Wetterhahn, who was a professor at Dartmouth College, New Hampshire. She died on 8 June 1997 aged 49. She had used dimethyl mercury when analysing compounds by nuclear magnetic resonance (NMR) - see Glossary. She was studying the way in which mercury ions interact with the proteins that repair DNA and had made up a solution of two materials that she was about to study using NMR. She had taken the usual precautions of wearing safety glasses and latex gloves, and handling her materials in a fume hood. However, on that fateful day of 14 August 1996, as she removed some dimethyl mercury from its vial with a pipette, drops of the liquid fell on to one of her gloves. Wetterman did not immediately remove the glove, thinking that it would protect her, but dimethyl mercury penetrates rapidly through latex and it then went through her skin.*
It was only in January 1997 that the first symptoms of mercury poisoning began to appear: tingling sensations in her fingers and toes, slurred speech, unsteady gait, and visual disturbances. Mercury poisoning was diagnosed on 28 January and blood analysis showed a level of 4 mg/litre, which is more than 50 times the toxic threshold. Two weeks later she slipped into a coma from which she never recovered. Analysis of her hair showed that her body had absorbed the methyl mercury in August. Why it takes so long to exert its toxic effect is still not known, but the body can only excrete about 1% a day of its methyl mercury burden and this is not fast enough to stop its insidious action.
* Ideally when handling dimethyl mercury a researcher should wear heavy-duty protective gloves over an inner latex pair.
Methyl mercury is particularly dangerous because it attaches itself to the sulphur atom of a cysteine amino acid and the molecule then appears to be indistinguishable from another amino acid, methionine, at least as far as gaining access to cells of the body which mistake it for the methionine they need. This deceit by mercury enables it to move freely throughout the body and even to cross the blood-brain barrier and that of the placenta. Once inside the brain, the methyl group may be removed from the mercury atom, thereby converting the mercury to inorganic mercury, but there is no mechanism by which this can then be transported out of the brain. When methyl mercury gets into the brain of a baby or a child, it stops cell division and blocks microtubule formation leading to permanent brain damage.
Although methyl mercury is dangerous, there are other organic groups that can be attached to mercury, such as the ethyl group which has two carbon atoms, and while they may be much less dangerous they can still kill. Such compounds began to be produced by the chemical industry in the 1920s for use as antiseptics, seed disinfectants, fungicides, and weed-killers. By the 1960s more than 150 proprietary products were on sale, based on ethyl or phenyl* mercury compounds. They were manufactured as antimicrobial agents, chiefly to control the spread of fungal disease in plants, and were thought to be much safer than methyl mercury compounds which were rarely used. One of the most effective seed dressings was Ceresan, which was a 2% solution of ethyl mercury chloride. It prevented all kinds of fungal diseases that could blight a crop and destroy a farmer's livelihood, and all over the world the treated seeds were eagerly bought because crop yields were greatly improved by their use. Wheat, barley, oats, and corn that are grown from treated seeds absorb very little of this mercury.
Sadly this form of crop protection led to several mass poisonings in developing countries when villagers made bread from the treated grain. In Iraq in 1956 and 1960 it found its way on to the open market and was used to make flour causing many people to be taken ill. A more serious outbreak occurred there in January and February of 1971 when more than 6500 people needed hospital treatment and 460 people died. Similar, but smaller, outbreaks of
* The phenyl group is the name given to a benzene ring when it is bonded to something else.
poisoning occurred in Pakistan and Guatemala. As long ago as 1942, two secretaries employed at a warehouse in Calgary, Canada, died because a consignment of 20 000 pounds (9000 kg) of diethyl mercury was stored near the desks where they worked. They died from mercury poisoning even though they never actually handled the material. The fumes from the drums were responsible, and later tests showed that the concentration of mercury in the air was almost 3 mg/m3. Warnings had already been issued about the dangers of such organo-mercury compounds, but they had been ignored.
Those warnings came in 1940 as a result of what had happened to a bright 16-year-old English boy Arthur H (the medical records do not reveal his surname) who had done well at the technical school he attended, and who got his first job with a company that made mercury seed dressings. Within five weeks of starting work he began to notice his fingers and toes were numb, and as the weeks went by Arthur became clumsy in his movements and unsteady on his feet. His personality changed so that he became confrontational and abusive, and his handwriting degenerated into a scrawl. Mercury poisoning was diagnosed and he was hospitalized, but doctors watched helplessly as he deteriorated, until he spent all his time lying listlessly in bed, unable to speak and barely able to eat. Eventually Arthur began to improve, but it was six months before he could walk again and nine months before he could communicate with people. Two years later he had recovered sufficiently to walk upstairs unaided, but his speech was still hesitant and his writing a mere scrawl. Fifteen years later he was still having trouble, being unsteady in his movements and with a tremor in hands, and this was still the case 25 years later. He never held a full-time job again.
Organo-mercury compounds work their malevolent way through the body only slowly and this was clearly demonstrated in April 1954, when a nurseryman used ethyl mercury phosphate to treat tomato plants that had become infested with stem rot fungus. His first symptoms only appeared the following December and consisted of headache and vomiting, progressing to a numbness in his limbs by the following May as his condition progressively deteriorated. He died in July 1955.
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