The human body can tolerate quite large amounts of mercury, although if the total exceeds 4 g there is serious risk of death. (A fatal dose of methyl mercury is probably around 200 mg.) About one person in ten has a level of mercury in their body that would make them unsuitable as food for any cannibals who followed the nutritional guidelines regarding excess mercury levels in meat, that it should not exceed 0.05 ppm. Today most mercury comes from our food, but in former times the use of mercury in various household products such as floor polishes, laundry aids, and paint served to increase absorption of the metal especially as it can easily penetrate the skin. Absorption this way has led to severe poisoning, and sometimes children with ringworm died when they were treated with antifungal cream containing mercury. A 9-year-old girl died five days after a solution of mercury(II) chloride in alcohol had been applied to a patch of ringworm on her scalp.
Mercury can also enter the body through the stomach wall, the lungs, and in certain cases has even been absorbed through vaginal douching and via the rectum as an enema - more of this in Chapter 4. The result of excessive exposure to mercury is violent vomiting and diarrhoea, and these may begin within 15 minutes and persists for hours. This reaction happens by whichever route the mercury has gained access to the body. Every microgram of mercury that enters our blood stream will interfere with some part of our body until we excrete it, and that may be a slow process. In 1960 a study on rats, using the radioactive isotope mercury-203, showed that mercury collected first in the liver from where it passed to the kidneys before being slowly excreted. Meanwhile it can disperse to other organs and will accumulate in the muscles, liver, kidneys, and bone. It is rapidly lost from the muscles and kidneys, but only slowly from the liver and bone. Mercury can collect in the skeleton and there are reports of drops of mercury being found in exhumed bones of people who have been treated with medicines containing metallic mercury during their lifetime.
The toxicity of mercury varies according to the form in which it comes. Methyl mercury is particularly dangerous and this will be dealt with in more detail below. Liquid mercury metal is the least toxic; mercury vapour is more dangerous, while the poisonous nature of mercury salts depends on their oxidation state. Compounds of the lower state, mercury(I), are generally much less toxic than compounds of the higher state, mercury(II), partly because the latter tend to be more soluble and can pass through the gut wall within minutes of being swallowed. The upper limit for a medical dose of mercury(I) chloride, calomel, was 30 times higher than that of mercury(II) chloride, also known as corrosive sublimate, and consequently calomel became the ingredient that doctors preferred to prescribe.
Acute mercury poisoning from a large dose of a soluble mercury salt causes damage to the kidneys, intestines, and mouth, and the symptoms are vomiting, stomach pains, weak pulse, and difficulty in breathing. If a fatal dose has been taken, death may intervene before any but the most obvious symptoms manifest themselves, but usually death takes about a week, although some people have been known to last three weeks before dying.
Mercury has a particular attraction to sulphur and it will attach itself to the sulphur atoms of certain amino acids. When these amino acids are part of an enzyme's protein it may result in that enzyme being rendered inactive. The enzyme Na/K-ATPase, which is essential to the working of the central nervous system, is particularly sensitive to mercury and this leads to the most noticeable symptom of mercury poisoning, the 'shakes'.
The mouth is also affected by mercury which stimulates the flow of saliva, a reaction that can appear within hours of taking the poison, although in some individuals it may never occur. If the poisoning continues, the odour from the mouth gradually worsens and the lips, gums, and teeth become inflamed and later are covered with a grey film. The condition may become so bad that the teeth become loose and drop out and parts of the jawbone may eventually be exposed.
Mercury targets the kidneys, and the first effect is to increase the rate of urine flow, thereby acting as a diuretic. In 1886 calomel was first prescribed for this purpose and was replaced in 1919 by another mercury-based drug called novasurol; this had been designed as a treatment for syphilis but which was noted to have a powerful diuretic effect. Later, novasurol gave way to other mercury compounds. Although a little mercury may have a stimulating effect on the kidneys, a lot has a disastrous effect; eventually they cease to function and urine flow stops. The body then becomes poisoned by its own waste products because these cannot be removed, and even if the kidneys begin to work again, it may be too late to save that individual.
The most distressing action of mercury is on the nervous system and the brain. The mental deterioration seen in cases of chronic poisoning by mercury is known as erethism and is characterized by symptoms such as timidity alternating with anger and aggression, lack of concentration, loss of memory, depression, insomnia, listlessness, and irritability. All forms of mercury can penetrate to the brain, methyl mercury particularly so, but the metal and mercury salt have the least ability to cross the blood-brain barrier.
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