Chemical Agents Hydragyrum

Mercury has a bad reputation. Is it deserved? An answer lies ahead. Mercury has a venerable paternity, as it is one of the eight ancient metals along with gold, silver, iron, tin, copper, lead, and zinc. The Romans called it hydragyrum, liquid silver, from which its chemical symbol, Hg, derives. Mercury is found naturally in the earth's crust as mercuric sulfide, in the mineral cinnabar; it has been used for over 3000 years as the red-brown pigment vermilion (also called Chinese Red), which was used to make the red-brown ink of the Dead Sea scrolls. The Romans called the pigment minium, and because this red-brown color (depending on how it was ground) was the dominant color in small paintings, they were referred to as miniatures, and manuscript titles in red were called rubrics, from the Latin, ruber, for red.

The early use of mercury was likely as an alloy metal with gold and silver to form amalgams. The Romans burned their used garments that contained gold thread and extracted the gold from the ash with mercury.

Mercury exists in three forms: elemental mercury, inorganic mercury salts, and organic mercury. Elemental mercury is a silver-gray liquid, the only metal liquid at room temperature, and that vaporizes readily when heated. Commonly referred to as "quicksilver," it is used in thermometers, thermostats, switches, barometers, and batteries. Elemental mercury accounts for most occupational exposures. It forms a number of mercuric and mercurous salts, most of which readily dissociate into ions in the body. Mercury can also bind to carbon to form organomercury compounds; methyl mercury is the most important in human exposure.

The major sources of atmospheric mercury are the burning of coal, smelting, and mining, but it is dispersed naturally through weathering of rocks, erosion, volcanic emissions, and offgassing of the earth's crust. Incineration of garbage, including medical, agricultural and municipal wastes also release mercury into the environment. Figure 8.5 diagrams mercury cycling in the marine environment. Once Hgi+ enters the aquatic environment, it settles into sediments where methanogenic bacteria convert it via biomethylation into organic methyl mercury, which fish absorb via their gills while feeding on aquatic plants.

While mercury's reputation was tarnishing almost as it was worked, archeo-logical data clearly indicate human exposure, where it was never worked. Hair mercury levels from tenth-century arctic area mummies are as high as 4 ppm. As for those who worked closely with mercury, the danger was evident. Bernardino Ramazzini, professor and physician at the University of Padua, and the author of Diseases of Workmen (DeMorbis Artificum Diatriba, 1700), described the muscle degeneration and skin inflammation of the mirrormakers on Murano, an island off the coast of Venice, as well as the illness in surgeons who treated syphilis with mercury. Also, results for the madhatters of the eighteenth and nineteenth centuries, who worked with mercurous nitrate used in turning fur into felt for women's hats, are extensively documented. Inhaling the fumes, due to poor ventilation, these victims developed severe and uncontrolled muscle tremors and twitching limbs, called the "hatters' shakes."

However, it wasn ' i until the mid-iwentieth century that robust evidence from large epidemiologic studies and two major human disasters pinned down mercury's affinity for the central nervous system. Any mercury compound released into the environment becomes available for potential methylation to CH3Hg by soil and marine microbes. The mercury concentration in fish at the

Figure 8.5. The cycling of mercury in its several forms in an aquatic environment. CH3Hg+, methylmercury ion; CH3HgCH3, dimethylmercury; Hg(II), mercuric mercury; HgO, elemental mercury; H2S, hydrogen sulfide; HgS, cinnabar. (Figure adapted from Risks in Perspective—Mercury, The National Research Council, 2000.)

Figure 8.5. The cycling of mercury in its several forms in an aquatic environment. CH3Hg+, methylmercury ion; CH3HgCH3, dimethylmercury; Hg(II), mercuric mercury; HgO, elemental mercury; H2S, hydrogen sulfide; HgS, cinnabar. (Figure adapted from Risks in Perspective—Mercury, The National Research Council, 2000.)

top of the food chain is typically biomagnified up to 100,000 times the concentration in surrounding waters. Thus it was in May 1956, that Minimata disease became a household name worldwide. People living along the shores of Minimata Bay, Kyushu, Japan, were consuming substantial amounts of mercury-contaminated fish and shellfish. The source of the methyl mercury was the effluent from the Chisso Chemical Company using mercury as a catalyst in the production of acetaldehyde. Although the level of mercury in the bay was not all that high, it was concentrated as it ascended the food chain from shellfish to finfish, the staple diet of the villagers [56]. Abnormal gait, impaired speech, loss of muscular coordination, deafness, and blindness were the primary symptoms. Infants were born with cerebral palsy-like symptoms. Cats eating fish lost muscle function; some ran about madly, jumping into the bay and drowning. It was later realized that the floating fish and shellfish seen earlier were also mercury-related fatalities.

In October 1971, Iraq imported more than 95,000 tons of methylmercury-treated grain seed: 73,201 tons of wheat seed and 22,262 tons of treated barley, shipped from Mexico to Basra, Iraq, and distributed free of charge to all 16 provinces, between September and November 1971—after the growing season. Rather then holding the seed for later planting, much of it was ground into floor and used to make home-baked bread.. The first case of CH3Hg poisoning appeared in late December. During the following 2 months 6530 people were hospitalized and 459 died. Many other victims were turned away from the hospitals [ 57 ].

In the final decades of the twentieth century, three vigorous epidemiologic studies were conducted to determine whether subtle neurological effects could be associated with chronic, low-level exposure to a fetus in utero.

One study was conducted in the Faroe Islands, in the North Atlantic between Iceland and Scotland; another, in the Seychelle Islands, in the Indian Ocean between Tanzania and India's southwest coast; and a third, in New Zealand.

In the Faroe Island study [58], neither neurologic nor clinical MeHg-related abnormalities were found in 917 children evaluated at 7 years of age. The median maternal hair mercury level was 4.5 ppm. Methylmercury levels in this cohort occurred via consumption pilot whale meat. Yet neurodevelopment test scores of highly exposed children were normal. Levels of CH3Hg in the whale meat had averaged 1.6 ppm.

In the Seychelles study, conducted jointly with pediatricians from the University of Rochester, no adverse neurodevelopmental effects were detected in 643 children, even though maternal hair mercury levels reached 66 ppm, far higher than the Faroe Island levels [59]. Mothers in the Seychelles reported eating 10-14 fish meals per week, yet fish mercury levels averaged 0.3 ppm. The New Zealand study found associations with MeHg similar to those of the Faroes [60]. However, in developing its reference dose (RFD) to be used for populations in the United States, the EPA chose the Faroe Island study as a baseline. In 2001, the EPA announced its RFD—-he safe dose that can be consumed every day over a 70-year lifetime without ill effects. This daily dose is 0.1 mg/kg. This decision over rode any known health benefit of fish consumption such as omega-3 fatty acids, believed to be protective of heart and stroke events. But bear in mind that the mercury warnings are meant primarily for women of childbearing ages, as diet is the primary route of exposure for the population. It is also worth considering that in Alaska, where fish consumption is both frequent and high, during the period 2002-2004 biomonitoring of hair of pregnant women revealed a 79% incidence of levels below 1 ppm; 92% of the women had levels below 2 ppm, and 83% had levels below 1.2 ppm—the level corresponding to EPA's reference dose. On the basis of these levels, the state of Alaska advised all Alaskans, including pregnant women, women who were breastfeeding, and children, to continue unrestricted consumption of fish from Alaskan waters.

In 2005, the Harvard Center for Risk Analysis convened an expert panel to evaluate whether altering fish consumption by women of childbearing age and other adults was or was not beneficial, given the concern for organic mercury. The panelists were concerned with balancing risk of mercury consumption and potential for harm to a developing fetus, versus the benefits of fish as a source of omega-3 polyunsaturated fatty acids that may confer health benefits. They concluded that men and women of childbearing age could easily switch to fish of low to no mercury content, as well as reducing the frequency of fish meals, but not totally abandon fish, given its proven benefits [61, 62]. Clear enough, but recently a female high school student, who we shall call

Erica Spiritos, in her junior year, told me that a teacher informed the class that if a woman planned to become pregnant, she should stop eating fish 3 years prior to the pregnancy to ensure that mercury would not adversely affect the fetus. Clearly, this is first-order nonsense. Nevertheless, if a high school teacher believes this, and feels so strongly about it that she passes it along to her students, there is ample reason to wonder where such nonsense was obtained, and whether it can be rectified. What do you think?

As for non-fish-eaters, the major source of mercury exposure is elemental mercury from mercury-containing dental amalgams. But this is minuscule, and as a recent study revealed, does not increase the risk of low- birth- weight infants [63]. Nevertheless, mercury's reputation is well deserved. The less of it in air, soil, and water, the better for all of us.

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