Is mercury a quicksilver bullet or a slow poison?
The name quicksilver is a good characterization for mercury in many respects. Apart from its physical appearance and effortless motility as a liquid metal, it is at the same time a semi-noble metal (suggesting non-reactive behaviour), but present in many chemical forms and it can participate in many chemical reactions. A broad classification would be to divide it into three forms: metallic mercury, inorganic salts, and organic mercury.
Mercury is ubiquitous, it is present in soil, it is released into atmosphere in the burning of coal, it is easily amalgamated with other metals making it useful for dentistry as well as for many technical uses, and we can still find old mercury thermometers in the medicine cabinet or blood pressure meters. Even though human activities release several thousand tons into the environment yearly, natural sources still predominate.
This essay concentrates on methyl mercury which is the most important form from the environmental point of view. However, one should not forget the mad hatter of Alice in Wonderland, a victim of chronic poisoning of the mercury used in felt preparation. The occupational exposure to metallic mercury vapour typically causes increased excitability and tremors along with a variety of other symptoms of the central nervous system and peripheral organs. A “new” source is energy-saving lamps in homes, if they break; they may contain 2–5 mg metallic mercury. At high concentrations, all forms of mercury are toxic and cause a myriad of symptoms. The differences are mainly kinetic; methyl mercury is absorbed better, and concentrates better in the brain than the other forms of mercury.
Mercury is a natural component of the bedrock. Therefore weathering of rock releases mercury into the environment, as do volcanic eruptions. The most important anthropogenic source is power plants burning coal; this accounts for about two thirds of all man-made release. Some industries have also released considerable amounts of mercury; they include chloralkali plants and metal and cement production. Mercury releases have decreased in Europe and North America, but are increasing rapidly in Asia.
One lethal use of mercury has been in the separation of gold from rocks and sand by dissolving it to mercury which is then evaporated. This causes terrible occupational risks to the gold miners already working in primitive conditions, and it also contaminates the nearby land and rivers. It has been estimated that 130 tons of mercury are being released each year into the Amazon basin alone.
Methyl mercury is formed in the aqueous environment by anaerobic microbial transformation. This means that a lack of oxygen promotes its formation, and thus methyl mercury levels can vary extensively in even quite close water sources – depending on how well they are oxygenated. High concentrations of oxygen-depleting organic material such as humus promote this transformation, and therefore certain fish e.g. pike which live in the brown-coloured lakes of the coniferous forest and marshlands, may contain very high concentrations of methyl mercury even when there are no industrial plants upstream.
Exposure to methyl mercury
The methyl mercury intake of Europeans originates mainly from fish, and the adult intake in most countries is 10–30% of the tolerable weekly intake value, which was revised to 1.6 micrograms/kg/week in 2003. In children, the data are rather sketchy, but it seems to be around the same range. Variations are great between countries and even within countries, and in many individuals these recommendations are exceeded.
The reason for fish being the most important source is bioaccumulation and biomagnification of methyl mercury within the aquatic food chains. Other forms of mercury do not accumulate into organisms, because metallic mercury is not reactive, and inorganic salts are so water-soluble that they are unable to not pass easily through lipid (fatty) membranes of living cells. Different fish accumulate methyl mercury very differently. The highest concentrations are found in large, old predatory fish such as pike, perch, swordfish, shark, and tuna (lower in canned tuna). The maximum concentrations set by the authorities in different countries are 0.5–1 mg/kg fresh weight, but in some species, concentrations of 3–4 mg/kg can be detected.
Fate in the human organism
The lipid solubility of methyl mercury helps it to accumulate in living organisms, but that is the end of similarities with the POPs. Methyl mercury does not concentrate in fat but in proteins. This is because it has a special attraction for the sulphur groups of one amino acid, cysteine and can also bind to cysteine-containing peptide glutathione, and proteins containing cysteine. This is both a blessing and a curse, because cysteine complexes help to make methyl mercury water soluble and allow it to be pumped out of cells and the central nervous system. On the other hand, binding to protein cysteines disturbs the functions of the proteins, and this is the reason for the diverse but specific forms of toxicity.
The glutathione complexes attempt to remove methyl mercury from the body by helping its excretion to bile, but unfortunately this complex becomes degraded in the gut and then the methyl mercury is reabsorbed back into the human body. Since excretion to urine is minimal, methyl mercury can be rather persistent in human body, and its half life is of the order of two months.
Methyl mercury concentrations are higher in the brain than in blood or most other tissues. It also readily passes through the placenta into the foetus, and levels in foetal brain are 5–7 times higher than those in maternal blood.
Methyl mercury binds avidly to hair, because hair keratin is rich in cysteine. The sulphur-containing cysteine is why hair smells so terrible if burned. The high levels make it possible to analyse mercury in hair, and with long hair it is even possible to calculate exposure at different times by cutting the hair to pieces and calculating the situation at the time when each piece was growing. Hair concentrations are about 250 times higher than the corresponding concentrations in blood.
The negative health effects of mercury are difficult to study, because the main intake comes from fish, and fish has been clearly shown to be beneficial to health in many ways. For example, fish consumption decreases the risk of acute cardiac deaths, it may prevent development of coronary disease, it has been shown to promote normal development of the nervous system and improve intellectual development in children.
There are two major health concerns associated with methyl mercury in fish: increased cardiovascular mortality in adults, and developmental deficiencies of the central nervous system after exposure during pregnancy or in early childhood. Most of the data on cardiac deaths are from a large 14-year cohort study in Finland. Abundant use of non-fatty predatory fish such as pike was associated with heart disease. If the hair concentration of mercury exceeded 2 microg/g, then the protective and beneficial effects of fatty acids of fish were lost.
Developmental problems are even more difficult to evaluate. It is clear from the incidences of frank poisoning that methyl mercury is highly damaging to the developing nervous system, especially the central nervous system. This was clearly shown in Minamata where industrial wastewater contaminated Minamata Bay with mercury, and the resulting methyl mercury poisoned virtually the whole population. Another poisoning incident was due to consumption of methyl mercury treated seeds during a famine in Iraq. In both cases, various developmental defects were seen when pregnant mothers were exposed to mercury; in particular developmental defects of the central nervous system.
The impact of low intake from fish is far more difficult to assess. There are three major series of studies addressing the issue, from the Faroe Islands, the Seychelles and New Zealand. Fish consumption of mothers was highest in the Seychelles, averaging 12 meals a week. No definite developmental defects were found. In the Faroe Islands, the intake was mainly from whale meat and blubber. Slight retardation was seen in children at 7 and 14 years of age, of the order of a 2-month delay in development when their mercury concentration doubled. All studies agree with the possibility that the risk level would be about 10 μg/g in maternal hair, confirming suspicions based on the Iraq incident. A few mothers exceeded that level in the Seychelles and Faroes, but in most countries where there is lower fish consumption, the advice is that fish consumption could be safely increased, even though the safety margins are not very high.
Mercury, in the form of methyl mercury, is presently the most important toxic heavy metal. The problem is that intake is mainly from fish, and no one can doubt that fish consumption confers clear benefits for health, in fact counteracting the same health outcomes worsened by mercury: fish oils prevent cardiovascular mortality, and are important for the development of the central nervous system. The safety margin is probably less than with any POP at the present levels. In many populations, some individuals are exceeding the safe weekly intake, not only the administrative tolerable intake estimates with their safety margins.
Mercury intake originates mostly from fish, and it is clearly advisable to avoid consuming too often the kinds of fish with high mercury content. These include shark, swordfish, large pike, and tuna. During pregnancy, these species are best avoided. One should never forget that fish is a healthy foodstuff, but a variety of species should be consumed.
Notes and references
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