It’s easy to think of mercury as a single substance. However, there are several different forms of mercury, which exhibit very different behaviour in the environment and in the human body. Today I’m going to briefly introduce some of the different mercury compounds, where they come from and what happens to them; future posts in this series will focus on the effects of some of the key mercury compounds.
The “simplest” form of mercury is elemental mercury (chemical symbol Hg). This is what most people probably think of when they hear the word mercury and is what you find in mercury thermometers and gauges, as well as in dental amalgam (mixed with silver and other metals). Unique among metals it is a liquid at room temperature. Some of it can also volatilize and turn into a gas; this mercury vapour is found in fluorescent light bulbs. Elemental mercury is an inorganic form of mercury.
Mercury can also be found as several inorganic salts. Examples include mercury (II) sulphide (HgS), a naturally occurring mineral also known as cinnabar; and mercury (I) chloride (Hg2Cl2), used in electrochemistry, but historically used for medical treatments. Many of these substances are naturally occurring, but also result from industrial processes. Mercury in water may be in the form of dissolved mercury salts.
Mercury is also found in several organic compounds. Perhaps the best known of these is methylmercury [CH3Hg]+, which is actually a positively charged ion often combined with an anion such as chloride. It can be formed from inorganic mercury by certain bacteria and other organisms, particularly in water, as well as from the burning of waste containing mercury. Methylmercury accumulates in organisms, and can biomagnify up the food chain (i.e. predators have higher concentrations than their prey). This is the type of mercury that is of most concern from fish, particularly fish high up the food chain due to this biomagnification.
Ethylmercury is another organic ion [C2H5Hg]+; unlike methylmercury, this compound does not really bioaccumulate. Its main significance is as a metabolite of thiomersal (thimerosal), best known as a preservative in certain vaccines (mainly flu vaccines) but with other antiseptic and preservative uses as well.
Phenylmercuric acetate (C8H8HgO2) is an organic mercury compound used in some fungicides, and was historically used in latex paints and at golf courses; it isn’t used anymore in most developed countries. Methylmercury and some inorganic mercury compounds have also been used in fungicides.
Over the next few weeks (as time allows and interspersed with other posts) I’ll go into more depth on at least some of the more important mercury compounds, including what happens to them in the body and what potential effects they may have (and in what amounts).