MEDICAL NEWS

Mercury Toxicity (I)

Mercury is a widespread heavy metal whose toxic effects have been well documented. Mercury exists in three chemical forms: elemental, inorganic and organic. All three forms are toxic in different ways. Elemental mercury (Hg0 ), is absorbed as vapour and is highly toxic. Mercury readily vaporises even at room temperature and exposure to it can lead to damage in the central nervous system. Mercury in this form is present in scientific instruments such as thermometers, fluorescent tubes and batteries. Dental amalgam fillings contain elemental mercury in concentrations up to 50%. Inorganic mercury (Hg+ and Hg2+ ) which can be found in mercury salts and some dermatological preparations, cannot cross the blood-brain barrier but reaches the kidney, and it is this organ particularly that is damaged. Organic mercury compounds (R-Hg+) derived from fish, seafood, fungicides, herbicides, and wood preservatives are readily absorbed by living organisms and are considered more hazardous than inorganic mercury. Environmental Pollutant Exposure to mercury used to be mainly an occupational hazard rather than an environmental one. More recently, mercury has also become an environmental pollutant. This has occurred through industrial use of mercury in the manufacture of plastics, paper and batteries with the resultant discharge of the contaminated effluents into lakes and rivers. According to a three-year study (1996-1998) by The Open University in Hong Kong, tests of water samples from the East River - the source of 80 percent of the SAR's drinking water - found the levels of heavy metals are much higher than international safety standards. Water quality in the East River has been worsening due to increasing urbanisation in southern Guangdong. Recent Headline: The consumer council recently conducted a study on the heavy metals concentration of 12 different brands of "water melon extract"- a traditional Chinese drug for mouth ulcers and gum inflammation. Two brand types were found to have levels of mercury exceeding the existing limit 13 times. With no clear prescription printed on the package, consumers often assess it's medical credibility by personal experience alone. A person's health can be seriously affected consuming excessive levels of mercury. A study by the World Health Organisation (WHO) in 1995 showed that a major source of mercury accumulation in the human body comes from food. Concentrations of mercury in meat can be as high as those in fish and seafood but because meat stores inorganic mercury, only 10 percent of it stay in the body. Fish and seafood retain methylmercury, of which up to 90 per cent can be stored in the body's tissues. A tragic and now infamous event which occurred in Minamata Bay - Japan in the 1953, highlighted the dangers of inorganic mercury as a water pollutant when it was methylated by microorganisms in anaerobic sludge lying at the bottom of the bay. In 1960, organic methylmercury was finally detected in seafood eaten by the local population. The poisoning affected 397 villagers resulting in 68 people deaths and 22 severe birth defects. Mercury poisoning is still sometimes referred to as Minimata disease. Mass poisonings have also occurred in various parts of the world where organomercury compounds have been used as fungicides to treat seed grain. The treated grain should not be used as food but if it is used to feed livestock, the meat becomes contaminated. One such large-scale poisoning incident occurred in Iraq in 1971-1972 when alkylmercury fungicides were used to treat cereal grain. This involved 6000 people and resulted 500 deaths. In 1998, M.D. Dickman et al. studied the relationship between Hong Kong male fertility and mercury in seafood. Hair mercury concentrations in fertile males were shown to increase with age (ages 25-72). Men with higher levels were found to be twice as likely to be subfertile. Individuals that ate fish or shellfish more than four times per week had significantly higher mercury concentrations in their hair than those who ate fish and shellfish less frequently. Hong Kong vegetarians who had consumed no seafood for the previous five years had the lowest hair mercury levels (1.21 versus 3.33 mg/kg for Hong Kong non-vegetarians - equivalent to 0.12 versus 0.33 mg%). Symptoms of Mercury Poisoning The symptoms of methylmercury poisoning reflect the entry of the compound into the central nervous system beginning with memory loss, paresthesias, ataxia, narrowing of the visual field, and progressing to loss of muscle co-ordination and emotional instability and eventually cerebral palsy. The latter was the most distressing effect seen in Minamata. Children and newborn infants with severe cerebral palsy even when the mothers were symptom-free, a classic characteristic of a tetragen. Methylmercury is able to cross the placenta and may consequently concentrate in the fat tissue and the brain of the embryo and fetus. In addition, fetal red blood cells concentrate methylmercury 30 percent more than the adult red blood cells. The damage caused by methylmercury is permanent. Other research confirms these findings, A.C. Barbosa et al. published "Concentration of mercury in hair of indigenous mothers and infants from the Amazon Basin". A subsampling of 30 mothers had segmented hair analysis that showed a mean decrease of 20% in body burden during pregnancy indicating a level of placental transference of mercury to fetuses. Clinical signs in uterine-exposed infants include delayed development and mild neurological disturbances. Mercury Exposure and TMA The determination of blood or urinary mercury body burden is only useful in cases of acute intoxication. As heavy metals are quickly deposited in body tissues, metal level determinations are poor indicators of the body burden in cases of chronic low level exposure. The extent of chronic exposure can be determined by Tissue Mineral Analysis (TMA). Mercury has been measured in human hair forensic studies, for dietary reasons, in toxic and healthcare work and to examine environmental concentrations in polluted and unpolluted regions. Once absorbed into the body, methylmercury accesses the hair follicle through the blood that bathes the hair root. The assumption that concentration of mercury in hair is proportional to the amount of mercury in blood, and should therefore reflect the body load of the metal, has been extensively studied and demonstrated. It is therefore not only possible to estimate the body burden of mercury, but also, knowing the rate of hair growth it is feasible to recapitulate the body burden in past months. Conclusion TMA has been shown in research and in clinical practice to be related to human systemic levels. As an invaluable screening tool, hair, according to many researchers and the U.S. Environmental Protection Agency (EPA), is a tissue of choice for determining toxic mercury exposure. "As an invaluable screening tool, hair, according to many researchers and the U.S. Environmental Protection Agency (EPA), is a tissue of choice for determining toxic mercury exposure." The increased concern about the health of persons exposed to very low environmental mercury concentrations is because mercury causes subclinical effects at low concentrations. The symptoms are difficult to detect and measure. Individuals with elevated toxic mercury levels do not always show clinical symptoms of overload. However, toxic minerals can have antagonistic effect on various essential minerals leading to disturbances in metabolic utilisation. The body uses a number of nutrients to keep itself free of toxic heavy metals. Some of these nutrients act as natural chelating agents such as vitamin C and sulfur containing amino acids (cysteine, methionine). Others work to restore proper mineral balance by acting against toxic metals such as iron, selenium, zinc, vanadium and other minerals. For example, selenium protects tissues from the oxidative damage caused by mercury. A low selenium-to mercury ratio may be indicative of increased free radical production. Sources of Mercury Deteriorating Mercury Amalgams Cosmetics (some) Fungicides (some) Bleaching Skin Creams (some) Medications (some) Polluted Water Diuretics (some) Battery Manufacturing Contaminated Seafood* * Even more toxic is dried delicacies, like shark's fin, scallop (added to many Cantonese dishes), squid and oyster. (30 times more mercury than fresh fish) Clinical Symptoms of Chronic Mercury Burden Behavioral Changes Depression Dermatitis Dizziness Fatigue Gum Disease Hair Loss Insomnia Allergic reactions or Asthma Muscle Weakness Reference: 1. Airey D.: Mercury in Human Hair Due to Environment and Diet: A Review. Environ Health Perspect 52:306-316, 1983. 2. Cernichiari E., Toribara T.Y., Liang L., Marsh D.O., Berlin M.W., Myers G.J., Cox C., Shamlaye C.F., Choisy O., Davidson P., Clarkson T.W.: The Biological Monitoring of Mercury in The Seychelles Study. Neuro Toxicol 16:613-628, 1995. 3. Barbosa A.C., Silva S.R.L., Dorea J.G.: Concentration of Mercury in Hair of Indigenous Mothers and Infants from the Amazon Basin. Arch Environ Cont Tox 34:100-105, 1998. 4. Gerhard I., Monga B., Waldbrenner A., Runnebaum B.: Heavy Metal and Fertility. J Tox Environ Health 54:593-611, 1998. 5. Dickman M.D., Leung K.M.C., Koo L.C.L.: Mercury in Human Hair and Fish: Is there a Hong Kong Male Subfertility Connection? Elsevier Sci 1150:1-5, 1999. 6. South China Morning Post July 03, 1999. 7. South China Morning Post - Postmagazine January 24, 1999 .