Mercury (Hg - hydrargyrum - literally - liquid silver or watery silver) is a heavy metal, its atomic number is 80, therefore it is under number 80 in the Mendeleev’s periodic table. Its standard atomic mass is 200.592. Its atoms are heavier than atoms of gold. For comparison, the atomic number of gold (Au) is 79 and the standard atomic mass is 196.966569.

And yet, mercury is liquid at room temperature, and its atoms evaporate from the surface of the liquid. Its freezing point is –38.83 degrees C, and boiling point is 356.73 degrees C. It has many relatively stable isotopes with atomic masses from 196 to 204, some of which decay into gold, others – into platinum. The mass of one liter of mercury is over 13.5 kg.

In ancient China and India mercury was known and used before 2000 B.C., over 4000 years ago. In medieval alchemy it was believed to be the source of all other metals.

Mercury can be found virtually in every household around the world as well as in many scientific laboratories and industrial plants. It is widely used as a sensitive element in medical and laboratory thermometers and manometers. Its vapor is present in energy saving luminescent lamps and UV (ultraviolet) lamps. It is used in powerful electric switches as one of the contacts. Many people still have fillings in their teeth made of amalgams of mercury with other metals (now such fillings are not used). Its unique property to dissolve and absorb gold has been used and is used for mining gold.

In nature, mercury is present in pure form, but in very small quantities. Mainly it is produced out of several kinds of ore, the most widely used is cinnabar – a substance of bright red color. Cinnabar ground into powder is used as red pigment for paints and varnishes and is added into official wax seals for important documents.

And yet it is a very dangerous substance. If it comes into a living organism, it remains there until the organism dies. In small quantities it does not do much harm, but it accumulates over the lifetime, affects the nerve system, causes systemic poisoning of the whole organism and eventually can cause death. The toxic properties of mercury were discovered rather recently because there is no immediate link between exposure to mercury and death. Typically it takes years or even decades before people exposed to vapors of mercury become seriously ill or die.

We receive mercury in the form of vapor from the air, water and from food. And our food receives it also from the air or from water, where vapors in the air dissolve. Algae accumulate mercury from water, then are eaten by fish, than we catch that fish. In many countries it is now a standard procedure to test food products, especially sea food products, for presence of mercury. It is always there, but it is important not to let food with too much mercury get to our tables.

Vapors of mercury may come into the air naturally, during volcanic eruptions. Small quantities of mercury are present in coal that is burned by electric power plants. Their emission of mercury is considerable because millions of tons of coal are burned every year. Luminescent lamps contain up to several milligrams of mercury. If they are not recycled properly, eventually they get broken and release their poisonous content into the atmosphere.

Local contaminations can be caused by broken measurement instruments, like medical thermometers, for instance, or by spilled mercury in scientific or industrial laboratories. One milliliter of mercury has a relatively small surface of evaporation, and it is not very dangerous, though, still it is better to have proper ventilation if you use it in a laboratory. But when it is spilled, it splits into thousands of small and tiny droplets, and the surface of evaporation increases by many thousand times. This is a serious and very dangerous contamination, and it is very difficult to clean it up.

But the biggest source of contamination of the atmosphere with mercury, even bigger than coal power plants, is the small gold mines, especially illegal ones, around the world. They use mercury for extracting gold from soil. Then they boil out mercury to get pure gold and dump mercury vapors into the air. Workers of those gold mines (often illegal slave workers) get sick with mercury poisoning rather quickly, within a year or two. Vapors of mercury from those factories are spread by winds thousands kilometers around and poison oceans and fish in them.

Developed countries have adopted a convention, according to which, the use of mercury in households should be eliminated completely by 2030, and it should be substantially reduced in industries. The first step to this goal is elimination of production of new mercury. Now only recycled mercury is produced and used in countries that signed this convention.

This text is based on the information presented in the "Business Daily" BBC radio programme.
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