By Razak Hj. Lajis
The Sun, November 7, 1995
Mercury is one of the few heavy metals that serves no beneficial physiological function in the human body. Every known form of this toxic silvery liquid has to be handled with care. There are more than 100 known mercury compounds. Thus, the possibility of mercury intoxication is just as high.
Mercury occurs naturally in the environment. It arises from the degassing of the earth's crust through volcanic eruption and evaporation from the ocean. Various types of human activity can also contribute to higher concentration of mercury in the environment. The widespread use of mercury has significantly resulted in increased levels of mercury in rivers and lakes. Indiscriminate disposal of waste materials and the burning of fossil fuels contribute to the diffusion of mercury into the environment.
An important point that needs to be redressed is the displacement of this toxic element in ground water and air. Similarly, special considerations have to be emphasised, especially those relating to the transfer of mercury and related compounds into plants and its gaseous form into the atmosphere
Most people are probably oblivious to the fact that mercury is second to lead as a cause of heavy metal poisoning. It is used widely in the electrochemical and pharmaceutical industries.
Previously, it was extensively used in the production of skin and hair bleach, cathartics, antiseptics, diuretics and explosives. However, the importance of mercury in medicine has steadily diminished due to the advent of safer non-mercurial medicinal products in the market.
Mercury is also present in products such as electrical lamps, batteries, paints and tooth filling. Environmental mercury pollution arising from the disposal of these products remains a cause for concern.
In the case of poisoning case, the degree of toxicity depends on the chemical form in which mercury appears. This is because the absorption, distribution and excretion of mercury and its compounds vary considerably with the chemical form of the metal.
Mercury poisoning seems to mainly affect younger children. An infant is also more prone to retain higher percentages of absorbed mercury in the brain than an adult. A pregnant mother and her foetus may acquire mercury through occupational exposure or ingestion of contaminated food. However, depending upon the level of contamination, air and water can also contribute significantly to the daily exposure to mercury.
Types of mercury
Mercury generally occurs in two forms, organic and inorganic. Elemental mercury, mercurous and mercuric salts are often classified as inorganic while the alkyls and aryls mercury are grouped as organic mercury. The organic forms of mercury involve mostly short-chain alkyls, such as methyl or ethyl mercury.
People may possibly be exposed to mercury through both occupational and non-occupational activities. The most common source of occupational exposure can occur during the manufacturing of some chemical. Some of the products are described below:
| Inorganic mercury | Organic mercury |
|---|
- use as a liquid cathode in electrolytic production of chlorine and caustic soda from salt water
- preparation of amalgams in dentistry, chemical processing, jewelry making.
- manufacture of compounds for use as pesticides, antiseptics, germicides and skin preparations.
- manufacture of batteries, lamps (fluorescent and mercury ore), power tubes.
- manufacture of inorganic salts for use as catalysts in production of chemicals such as urethanes, vinyl chloride monomers, anthraquinone derivatives.
- use or manufacture of measuring instrument as in thermometers, barometers and manometers with mercury as a working fluid.
- roasting and smelting operations and extraction of silver and gold
- during use and manufacture of compounds for pulp and paper industry.
|
- mining, extraction and processing of mercury ore
- manufacturing of fungicides
- treatment and utilization of bulbs and seeds
- use as a wood, timber and paper preservative
- during use in medicines
Adapted from OSHA (Occupational Safety and Health Administration) document |
Poisoning from Elemental mercury
Elemental mercury is the most common form of mercury used in thermometers or barometers. At room temperature, it is either solid or gaseous in form. Elemental mercury gives rise to a vapour which dissolves only slightly in water. Among mercury compounds, it is known to be the most volatile.
There have been a few poisoning cases involving mercury spillage from broken thermometers at home. Unless it is properly cleaned up, it can cause intoxication in young children.
After inhalation, 70% to 80% of elemental mercury is retained and absorbed. It exists in the non-ionised form that is highly lipid soluble. It easily crosses the blood brain barrier. It is also well distributed to the liver and kidney. It can also be absorbed from the skin quite easily. This will sometimes cause poisoning that arises from topical application of mercury-containing products.
Acute inhalation of mercury vapour may be followed by chest pain, difficulty in breathing, coughing, chills, fever, weakness, salivation, nausea, vomiting and a metallic taste in the mouth. These symptoms can be observed within a few hours from the beginning of exposure. Following intense exposure to metallic mercury vapour, lung damage may occur. The effect of chronic exposure is cumulative. It may take weeks or sometimes years before any recognisable clinical symptoms are produced.
Poisoning from Organic Mercury
Organic mercury compounds appear to be the biggest environmental mercury contaminants. Methyl-mercury compounds, a constituent of organic mercury, are found in abundance, especially in seafoods. They are almost completely absorbed from the gastrointestinal tract and are distributed to most tissues. They pass into the foetus and are excreted in milk. They are lipid soluble and rapidly absorbed from the gastrointestinal tract.
Signs and symptoms of organic mercury ingestion include sensory loss, uncoordinated movement, tunnel vision, memory loss and dementia. In general, only exposure to organic mercury compounds or mercury vapour will lead to clinically evident central nervous system changes.
Initial symptoms differ widely but tremor seems to be the most characteristic change. Other central nervous system manifestations include depression, irritability, exaggerated response to stimulation, loss of confidence, insomnia, emotional stability, forgetfullness or confusion. The compounds have also been reported to induce adverse effects on human development.
Poisoning from Inorganic Mercury
The ingestion of inorganic mercury salts can be fatal as well. Following ingestion, symptoms such as nausea, vomiting (with or without blood), metallic taste in the mouth, stomach pain, erosion of intestinal mucosa and bloody diarrhea may occur. Cardiovascular collapse and psychological disturbances may ensue too. In chronic exposure, the symptoms are similar to those of chronic mercury vapour exposure.
Children exposed to inorganic sources have been reported to develope acrodynia or "pink" disease. The complex symptoms of this disease include redness of the palms and soles, oedema of the hands and feet, skin rashes, hypertension, insomnia, loss of appetite and constipation or diarrhea.
Hypersensitivity to mercury is often unrecognized. Reactions to mercury preparations, specifically inorganic products, may vary from slight skin rashes to severe cases of anaphylactic reaction. Allergic or auto-immune reactions to mercury therefore do not really need to follow the same dose-dependency, as seen in toxic reactions.
Having generally discussed the implication and effects of mercury intoxication in the human body, we will be focusing more on the unexpected turn of events and some controversial issues related to mercury in the environment in next week's article.
Among the most debated issues are those concerning the safety of mercury amalgams and mercury-contaminated seafood and matters arising from it.
The writer is a pharmacist at the National Poison Centre, Universiti Sains Malaysia, Penang.
