Mercury

1. What is mercury?

• 1.1 In what forms does mercury exist?
• 1.2 How does mercury exist in the environment?
• 1.3 How can the form of mercury affect living organisms and the environment?

1.1 In what forms does mercury exist?

Mercury occurs naturally in the environment. Sometimes known as quicksilver, it is a heavy metal, like lead or cadmium, that exists in different chemical forms:

• Elemental mercury or metallic mercury is the element in its pure, ‘un-combined’ form. It is a shiny, silver-white metal that is liquid at room temperature, but is rarely found in this form in nature. If not sealed off, mercury slowly evaporates into the air, forming a vapour. The quantity of vapour formed increases as temperatures rise. Elemental mercury is traditionally used in thermometers and some electrical switches.
• Inorganic mercury compounds or mercury salts, more commonly found in nature, include mercuric sulphide (HgS), mercuric oxide (HgO) and mercuric chloride (HgCl₂). Most of these are white powders or crystals, except for mercuric sulphide which is red and turns black after exposure to light. Some mercury salts, such as mercury chloride, also form vapour, but they stay in the air for a shorter time than elemental mercury because they are more soluble in water and more reactive.
• Organic mercury is formed when mercury combines with carbon and other elements. Examples of organic mercury compounds are dimethylmercury, phenylmercuric acetate and methylmercuric chloride. The form most commonly found in the environment is methylmercury.

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1.2 How does mercury exist in the environment?

Several forms of mercury exist naturally in the environment, the most common being metallic mercury, mercuric sulphide, mercuric chloride, and methylmercury.

Natural processes can change the mercury from one form to another. For instance, chemical reactions in the atmosphere can transform elemental mercury into inorganic mercury.

Some micro-organisms can produce organic mercury, particularly methylmercury, from other mercury forms. Methylmercury can accumulate in living organisms and reach high levels in fish and marine mammals via a process called biomagnification (i.e. concentrations increase in the food chain).

Because mercury is one of the basic chemical elements, of which all things are made, it cannot be broken down or degraded into something else. Once released into the biosphere through natural events or human activities (see Question 4), mercury readily moves and cycles through the environment. Soil, water bodies and the sediments underneath them are believed to be the places where mercury comes to rest until it is ultimately removed from the biosphere again. More...

1.3 How can the form of mercury affect living organisms and the environment?

Different forms of mercury (see 1.1) affect living organisms and the environment differently.

For exposed living organisms, the form of mercury affects:

• how available it is to cause effects within the body;
• how it moves around inside the body;
• how toxic it is;
• how it accumulates, is transformed and leaves the body;
• how it biomagnifies (builds up) along the food chain.

For the environment, the form of mercury influences how readily it can move within and between the atmosphere and oceans, and how far it can travel in the air. For instance, elemental mercury vapour can stay in the atmosphere long enough to travel around the world, whereas other forms of mercury may fall back to earth relatively close to their source. 

Emissions of some forms of mercury into the air (for example from industry) can be controlled more easily than others. Inorganic mercury can be removed from air emissions reasonably well, while elemental mercury emissions are more difficult to capture and eliminate. More...