Epidemiology and toxicity of mercury

INTRODUCTION

Mercury exists in metallic, inorganic and organic forms which are characterized by different toxic properties. Metallic and organic forms, the latter found as methylated mercury in contaminated fish, are toxic to the central nervous system and the fetus whereas inorganic forms of mercury may affect the kidneys. Although mercury is a silvery white liquid, the metal is volatile at room temperature because of its high vapor pressure. Mercury also exists in different oxidation states and can form a number of organomercuric compounds. These physical properties contribute to the considerable toxicity observed with mercury. In order of importance, the principal organ systems affected by mercury poisoning are the central nervous system and the kidneys [1-3].

USES OF MERCURY

The unique physical properties of mercury have led to its use from prehistoric times to the present day.

• Colored cave drawings from more than 10,000 years ago have been found to contain the red stone of mercury ore (cinnabar, mercury sulfide, HgS).
• In the 19th century, epidemics of occupational mercury poisoning resulted from heavy exposure to mercury in the mirror and felt hat industries.
• Mercury is still used in the manufacture of many technical and medical instruments, including sphygmomanometers, manometers, thermometers, and barometers.
• Liquid metallic mercury can be used to concentrate gold from crushed ore or sediments. This technique is immediately dangerous to miners; it was commonly performed during the California gold rush and is still used in several countries. It has been estimated, for example, that approximately 500,000 gold miners in Brazil currently use liquid mercury to concentrate gold from sediments [4,5]; this may cause considerable environmental contamination [6].
• During the Middle Ages, mercury salts were used for the treatment of various diseases such as syphilis. The use of mercury in the treatment of psoriasis and, as a potent diuretic, for congestive heart failure persisted far into the 20th century.
• Certain mercury compounds still have limited use in human medicine (eg, vaccines, antiseptics, and skin ointments). In addition, human exposure may result from the use of certain types of mercury-containing Chinese traditional medicines and skin lightening creams, or in hair dyes.
• Amalgam tooth fillings are widely used in dentistry and can be found in hundreds of millions of people around the world. Amalgam or dental silver is composed of a mixture of 50 percent metallic mercury and metal powder (which is usually composed of silver, tin, copper, and zinc in certain proportions).

HUMAN EXPOSURE

Humans are exposed to mercury via many different routes and in different forms. The general population is primarily exposed to the metal from dental amalgam and the diet. As a rule, amalgam fillings are the most important source of inorganic mercury and fish are the most important source of methylated or organic mercury [1,2,7].

Amalgam fillings — The release of mercury from amalgam fillings is proportional to the number of fillings and the total amalgam surface area. It has been difficult to accurately estimate the release from amalgam fillings; however, an expert committee from the World Health Organization believes that the average exposure from dental amalgam is approximately 10 mcg/day [1]. Measurements of urinary excretion of mercury have revealed that persons with a habit of tooth grinding release considerably more mercury from their dental fillings than those without this habit [8].