Issue Date: September 1, 2008
Not All Mercury Is Equal
As I read the article on efforts to get rid of mercury, it occurred to me that when the toxic properties of mercury are discussed on television or in newspapers, often no mention is made of the enormous difference in toxicity between chemically different forms of mercury (C&EN, July 28, page 47). One hopes that most readers of C&EN understand this difference; however, it would have been worth stating the facts at the beginning of this article. Methylmercury and dimethylmercury are exceedingly poisonous, and extremely low concentrations are very dangerous. However, there are forms of mercury that are many orders of magnitude less toxic and might not be dangerous at all.
The principal case discussed in this article is the use of amalgam for filling teeth. Hundreds of millions of people have had teeth filled with dental amalgam, and large numbers of people have no dental amalgam fillings. If amalgam filling led to a substantial increase in any dangerous health condition, we would certainly know about it through the many studies comparing these groups. Dentists have effectively carried out a huge unplanned experiment, and the results are clear. Of course, there are still people who believe that there are harmful effects of dental amalgam, but there is no evidence of this at all.
A number of people have ingested pounds of liquid metallic mercury in the past to clear intestinal obstructions. I’m sure no one would currently be advised to do this; however, it’s clear that it does not produce serious immediate harmful effects. Mercurous chloride (calomel) has been used in the past as a laxative or purgative. Again, it would not be recommended now, but this shows that it is not exceedingly toxic.
In contrast with liquid mercury, we do know that mercury vapor is quite poisonous. Mercury is volatile enough that its vapor pressure at room temperature would produce very harmful levels of mercury vapor at equilibrium. This knowledge has caused people to become extremely worried when mercury is spilled, for example, from a broken mercury thermometer.
Two factors reduce the danger to a great degree. Mercury vapor is extremely heavy and would stay close to the floor. Also, the diffusion of mercury into air is extremely slow because of its high atomic weight. Spilled mercury might produce mercury vapor, but high levels would be very close to the ground. Unless one lay down and breathed air at ground level, the danger to any person would be small.
I have read of cases where a small amount of mercury was spilled and people were excluded from the room for a long time. I even read about speculation that a house would have to be torn down because of this problem. Although one hopes that chemists in general are aware of the facts, it would be a good idea in an article such as this to repeat them briefly.
New Haven, Conn.
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