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Here is an excerpt from: file:///C:/Users/user/Documents/My%20Documents/Instruction%20manuals/Mercury.htm :
"Mercury as a Poison

When considering the hazards of mercury, soluble mercury compounds and metallic mercury are very different. Mercury can enter the body through the lungs, through the skin, and via the digestive system. The absorption of mercury vapor by the lungs is an efficient process, but it is difficult to acquire dangerous amounts of metallic mercury by the other routes. Ingestion of soluble mercury compounds produces acute mercury poisoning, but is very easy to avoid and rare. Usually, mistaking mercuric chloride pills for something harmless is the cause (see below). The kidneys contain a protein, metallothionein, that binds mercury tightly until it is excreted. Mercury in the blood has a half-life of only three days, but tissue mercury has a half-life of perhaps 90 days. Small amounts of mercury, therefore, are efficiently excreted and cause no harm. This information is from the Handbook of Laboratory Safety (2000). Some earlier sources claim that mercury is a cumulative poison, but it apparently is not. The kidneys, however, can be overloaded and symptoms of mercury poisoning will then result. The mercuric ion Hg++ attacks the kidneys and can result in fatal kidney failure. Acute mercury poisoning is, however, a small risk and easily avoided. Mercury passes through the placenta in acute mercury poisioning, and so is a hazard to the fetus. Whether there is any danger in cases of the usual concentration of environmental mercury (as in fish) may be doubted.

One indication of acute mercury poisoning is the secretion of excess saliva, and the condition is called salivation. This often occurred among workers in mercury refining, and also in felt-making, where the soluble mercury compound mercuric nitrate, Hg(NO3)2·2H2O is (was) used. This was the reason for the "mad hatter." The Environmental Protection Agency named mercury a hazardous air pollutant in 1971, which it would be if there were any mercury in the air.

Mercury metal and insoluble compounds are little hazard, and can be handled occasionally with safety. Chronic exposure is a different matter. Mercury vapor is absorbed in the lungs, and chronic exposure even at low levels should be avoided. The toxic limit (PEL)is only 0.05 mg/m3, which is very low indeed, exceeded by the vapor pressure at room temperature. However, the accumulation of the equilibrium vapor pressure is extremely unlikely in most cases, where the mercury is present in visible droplets. Although much worry accompanines spills of liquid mercury, the hazard is probably negligible. T. G. Winter (see References) has recently demonstrated that mercury vapor due to small spills of metallic mercury is not hazardous. Notably, mercury from minor spills can be found in the cracks between vinyl tile flooring. Mercury should not be heated in the open; this can easily produce dangerous amounts of vapor.

A vacuum cleaner should not be used for mercury cleanup. Unless specially designed for this purpose, it only atomizes the mercury and makes it more hazardous than before the cleanup. Substances that amalgamate with the mercury can lock it up so it does no damage, and often these amalgam particles can be swept up easily.

However, the modern panic over environmental hazards coupled with faulty estimation of risks have led to something of a mindless crusade that has eliminated mercury in many common applications, such as in clinical and laboratory thermometers and in antiseptics (mercurochrome and merthiolate). At one time, the medical sciences erred in the other direction, administering mercurous chloride and liquid mercury as medicines. It was estimated that there were two broken clinical thermometers per bed per year, which would seem to indicate considerable clumsiness in nurses. Careless disposal of mercury batteries was said to represent 68% of mercury pollution.

Mercury itself, and most of its compounds, are very insoluble and so are not hazardous in themselves. It was discovered, however, that in anaerobic sediments where there was industrial mercury waste, slightly soluble dimethyl mercury, (CH3)2Hg, was produced. If fish containing dimethyl mercury are consumed, the often fatal Minamata Disease results. The disease is named after the Japanese bay in which the mercury was released that brought this to notice. Dimethyl mercury occurs in tuna fish in extremely tiny amounts that are probably only a testimony to how sensitive the detection methods can be. In larger amounts, it is reputed to cause fetal damage, but is very probably no general hazard at all, because it is so rare.

Recently (December 2003), news reports stated that tuna fish contained "large amounts of mercury" in an alarming tone. What they really should have said is that tuna fish contains only extremely small, scarcely detectable, amounts of methyl mercury. The extrapolated threat appears to be to the nerves of developing fetuses. I wonder how many cases have been observed of fetal mercury damage by normal tuna fish. I suspect that the number is zero. The authorities probably have no idea of the magnitude of the hazard, only that a hazard could be possible. Perhaps we have homeopaths at work here! Generally, they deal in extrapolations of inexact data to minute concentrations, and probably do not know the rate at which organisms take up and excrete such tiny quantities. The problem is that methyl mercury could be a threat, but I have no confidence that the authorities know enough about it to protect the public, only to enjoy the creation of panic. Tuna fish, indeed, also contains many nutrients valuable to fetuses.

The level of knowledge of many experts can be judged from statements like: "mercury vapor is very heavy and collects at low levels." Well, the molecular weight is large, but mercury vapor is usually mixed with air and obeys Dalton's Law like any other gas. Even aside from turbulent mixing, the scale height at ordinary temperatures is a kilometer or so, so no considerable increase in concentration can be expected in a room. Large amounts of vapor might not mix with air quickly, and so would indeed initially sink to the floor. However, the vapor these people are talking about is slowy emitted by droplets. You might as well say that water vapor collects near the ceiling."

Bill Morris
Pukenui
New Zealand

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