Mercury Hazards to Living Organisms - Chapter 13 (end) pps

291 C HAPTER 13 Concluding Remarks Mercury has been used by humans for at least 2300 years, most recently as a fungicide in agriculture, in the manufacture of chlorine and sodium hydroxide, as a slime control agent in the pulp and paper industry, in the production of plastics and electrical apparatus, and in mining and smelting operations. Mercury burdens in some environmental compartments are estimated to have increased up to 5 times precultural levels, primarily as a result of human activities. The construction of artificial reservoirs, for example, which release mercury from flooded soils, has contributed to the observed elevation of mercury concentrations in fish tissues from these localities. Elevated levels of mercury in living organisms in mercury-contaminated areas may persist for as long as 100 years after the source of pollution has been discontinued. One major consequence of increased mercury use, coupled with careless waste disposal practices, has been a sharp increase in the number of epidemics of fatal mercury poisonings in humans, wildlife, and aquatic organisms. Most authorities agree on six points: 1. Mercury and its compounds have no known biological function, and the presence of the metal in the cells of living organisms is undesirable and potentially hazardous. 2. Forms of mercury with relatively low toxicity can be transformed into forms of very high toxicity, such as methylmercury, through biological and other processes. 3. Mercury can be bioconcentrated in organisms and biomagnified through food chains. 4. Mercury is a mutagen, teratogen, and carcinogen, and causes embryocidal, cytochemical, and histopathological effects. 5. Some species of fish and wildlife contain high concentrations of mercury that are not attributable to human activities. 6. Anthropogenic use of mercury should be curtailed, as the difference between tolerable natural background levels of mercury and harmful effects in the environment is exceptionally small. Most authorities also agree on another six points regarding anthropogenic use: 1. The sale, manufacture, or use of methylmercury compounds should be prohibited through legis- lation and enforcement. 2. The use of all other mercurials should be sharply curtailed. 3. Alternate technologies should be developed to replace mercury in agriculture, industry, and elsewhere. 4. Mercury should be removed from contaminated waterways and other ecosystems impacted through human activities. 5. New monitoring guidelines should be developed — and existing guidelines refined — to evaluate the success of mercury mitigation programs. 6. Mercury emissions from coal-fired power plants should be reduced using best-available technologies. © 2006 by Taylor & Francis Group, LLC 292 MERCURY HAZARDS TO LIVING ORGANISMS Additional research is needed to more fully assess the potential hazards of mercury and its com- pounds to living organisms. Some of the more pressing needs are listed in alphabetical order. AMALGAMATION CONTAMINANT PROBLEMS The use of mercury to recover gold has resulted in extensive and persistent contamination of the biosphere, with direct — and often fatal — consequences to all members of the immediate biosphere, including humans. The use of mercury for this purpose, which accounts for about 10.0% of all anthro- pogenic mercury emissions, should be abandoned, and improved remediation methodologies developed for mercury-contaminated environments using physical, chemical, and biological technologies. ANTHROPOGENIC EMISSIONS Combustion of mercury-containing fossil fuels accounts for up to 60.0% of the global mercury burden contributed by human activities. In the United States, coal-fired power plants are now considered the only significant source that continues to be unregulated, although available technol- ogies may reduce mercury emissions by up to 98.0%. Implementation of these technologies is recommended at installations now producing unhealthy depositions at calculated target sites. Removal of the source of anthropogenic mercury, such as closure of chloralkali plants, usually results in a slow decrease in the mercury content of sediments and biota of the mercury-receiving channel. However, the mercury loss rate depends, in part, on the initial degree of contamination, the chemical form of mercury, the physical and chemical conditions of the ecosystem, and the hydraulic turnover rate. The need to quantitate these variables is necessary for implementation of effective remediation measures. ATMOSPHERIC TRANSPORT Atmospheric transport of anthropogenic mercury may contaminate remote ecosystems hundreds of kilometers distant. Additional data are needed to predict deposition rates and deposition sites from distant mercury sources. CHEMICAL ANALYSIS There is a need for less expensive and more sensitive instrumentation to accurately measure mercury species in abiotic materials and biological tissues. The current procedure involves sample prepa- ration using graphite furnace and measurement using atomic absorption spectrometry, which can detect methylmercury concentrations greater than 10.0 µg/kg in muscle of marine fishes. CRITERIA Mercury criteria for protection of living organisms are numerous, disparate, and — in many cases — unsatisfactory. For example, mercury criteria proposed by the U.S. Environmental Protection Agency for the protection of freshwater aquatic life are 0.012 µg/L medium (4-day average), not to exceed 2.4 µg/L on an hourly average; however, these criteria offer only limited protection to freshwater ecosystems. The saltwater criteria of 0.025 µg Hg/L medium (4-day average), not to exceed 2.1 µg/L hourly, are unsatisfactory for the protection of marine life. It is emphasized that the establishment of satisfactory mercury criteria over large geographic areas frequently involves a © 2006 by Taylor & Francis Group, LLC CONCLUDING REMARKS 293 number of social, political, and economic concerns that currently override sound scientific data — to the dismay of the environmental and health sciences community. Improved communication between scientists and information transfer specialists, including the press and elected officials, is highly recommended. DENTAL AMALGAMS Further evaluations of the safety profiles of dental amalgams and other types of materials used intraorally are needed to establish credible guidelines for the use of mercury-containing dental materials during pregnancy. The use of mercury in preparation of dental amalgams should be reconsidered because it constitutes the main source of mercury exposure in some populations. EMISSION CONTROLS Although natural sources of mercury to the environment, such as volcanic eruptions, continue to outweigh anthropogenic sources such as mercury from gold mining, industrialization, arms man- ufacturing, and fossil fuel power plants, many anthropogenic sources can be controlled or eliminated with available technologies. For example, mercury concentrations in surface soils near coal-fired power plants in the United Kingdom can be reduced up to 40.0% using flue gas sulphurization technology. Although legally banned in most countries, the use of mercury compounds as slimicides in pulp and paper mills and as catalysts in the production of chlorine and sodium hydroxide continues; in both cases, alternatives are available and should be implemented. ENVIRONMENTAL LEGISLATION Reduction of mercury concentrations in mercury-contaminated invertebrates as a result of human activities may be possible through environmental legislation and subsequent enforcement. In one case, mercury concentrations in mussels from Bergen Harbor, Norway, were reduced 60.0% in 10 years following passage of environmental legislation restricting mercury discharges into the harbor accompanied by vigorous enforcement. Future environmental legislation should incorporate funds for enforcement as well as research and monitoring studies. FETOTOXICITY Intrauterine methylmercury exposure studies comprise one of the most important research areas now under investigation. Specifically, these investigations focus on quantification of prenatal expo- sure to mercurials from maternal consumption of marine fishes and mammals, and subsequent neurobehavioral risks in resultant children on reaching school age. Animal surrogate models should be established and long-term studies initiated using radiolabeled and stable inorganic and organic mercury compounds in combination with known dietary contaminants. MECHANISMS OF LETHAL ACTION Concentrations of total mercury lethal to sensitive, representative, nonhuman species range from 0.1 to 2.0 µg/L of medium for aquatic organisms; from 2200.0 to 31,000.0 µg/kg body weight (acute oral) and 4000.0 to 40,000.0 µg/kg (dietary) for birds; and from 100.0 to 500.0 µg/kg body weight (daily dose) and 1000.0 to 5000.0 µg/kg diet for mammals. Organomercury compounds, © 2006 by Taylor & Francis Group, LLC 294 MERCURY HAZARDS TO LIVING ORGANISMS especially methylmercury, are always more toxic than inorganic mercury compounds. Numerous biological and abiotic factors modify the toxicity of mercury compounds — sometimes by an order of magnitude or more — but the mechanisms of action are not clear and require elucidation. MERCURY POISONING Because neurobehavioral disturbances were observed in Hg o vapor poisoning cases 20 to 35 years after exposure, and because Hg o vaporizes readily and mercury vapor is comparatively toxic via inhalation, it is recommended that the use of elemental mercury should be discontinued wherever possible. More rapid development is recommended of mercury-antagonistic drugs — such as thiols — administered in cases of acute inorganic mercury poisoning, and mercury-protectant drugs — including thiamin, selenium-, sulfur-, tellurium-, and other Group VI derivatives — to guard against organo- mercurial intoxication. MITIGATION Four courses of action now seem warranted. First, toxic mercurials in agriculture and industry should be replaced by less toxic substitutes. Second, controls should be applied at the point of origin to prevent the discharge of potentially harmful mercury wastes. Third, continued periodic monitoring of mercury in fish and wildlife is needed for identification of potential problem areas, and for the evaluation of ongoing mercury curtailment programs. And fourth, additional research is merited on mechanisms of mercury accumulation and detoxification in comparatively pristine ecosystems. MOBILIZATION Mobilization rates of mercury from major global reservoirs are needed to effectively predict risk, especially rates from oceanic sediments that contain 98.75% of the estimated 334 billion metric tons of mercury now in the environment, and from oceanic waters with an estimated 1.24%. MONITORING Continued monitoring of mercury content in abiotic materials, in crops and other vegetation, in aquatic and terrestrial invertebrates, and in human and vertebrate animal tissues is necessary to effectively determine compliance with existing mercury criteria for protection of health of humans and natural resources, establish mercury fluxes in different ecosystems, aid in prediction of future problem areas and species, and as the basis of mercury remedial actions. Monitoring emphasis should be on key geographic areas and species, short sampling periods — to be repeated in the future if results warrant — and little deviation in collection methods, sample preservation, and chemical analytical methodologies. Multidisciplinary research teams are recommended and should include ecologists, toxicologists, statisticians, and chemists, at a minimum. PRODUCTION Accurate data on mercury production and consumption are not known with certainty. These data would be useful in the calculation of local mercury emissions and in the prediction of environmental risk. © 2006 by Taylor & Francis Group, LLC CONCLUDING REMARKS 295 RESIDUES IN BIRDS Many species of birds, including endangered species, are at risk owing to the consumption of prey containing inordinately high concentrations of mercury. Recovery of the endangered wood stork in the southeastern United States, for example, is considered jeopardized owing to the high mercury content in the diet of nestling wood storks. Management plans to reduce mercury emissions in the southeastern states should give high priority to the welfare of endangered birds and other endangered species in their jurisdiction. Mercury concentrations in muscle and liver of many species of waterfowl commonly hunted are excessive and considered harmful to human consumers; advisories should be issued until the levels in tissues fall below the limits set by regulatory agencies. RESIDUES IN FISH AND ELASMOBRANCHS Older adults of commercial species of sharks, swordfish, tuna, marlin, and other long-lived predators routinely contain more than 2.0 mg Hg/kg FW muscle, almost all in the form of methylmercury, placing human consumers of these products at unreasonable risk. Fishery managers are advised to impose a maximum size regulation for capture and landing of older adults of long-lived piscivores containing elevated mercury loadings, possibly through adoption of selective capture gear, thereby reducing mercury availability to humans; at the same time, this move would improve the repro- ductive potential of many near-depleted stocks because larger females are considered more efficient producers of eggs. All information gathered by state and federal agencies on mercury concentrations in edible fishery products of commerce should be transmitted regularly to consumers to assist them in decision making about the risks from fish consumption. RESIDUES IN HUMANS Recently, the U.S. Environmental Protection Agency stated that concentrations greater than 1.0 mg total Hg/kg DW hair are indicative of mercury exposure and that women of child-bearing age should restrict consumption of fish with elevated mercury content. In view of the fact that many populations exceed this hair mercury concentration, namely 45.2% of New Yorkers and 34.2% of Floridians, it seems prudent to reexamine this conclusion. Consumption of high dietary mercury intake from seal meat and blubber daily by pregnant aboriginal women is associated with elevated mercury concentrations in maternal and fetal tissues, but with no measurable adverse effect on the fetus or resulting infant. This finding requires verification. Other investigators have stated that accumulation of mercury in tissues is associated with an excess risk of acute myocardial infarction and increased risk of death from coronary heart disease and other circulatory problems. This needs to be verified and, if true, appropriate counter- measures developed. RESIDUES IN MAMMALS High natural concentrations of mercury in livers of pinniped mammals, typically greater than 143.0 mg total mercury/kg FW, does not seem injurious to animal health owing, perhaps, to the biological unavailability of the metal. Mercury metabolism in marine mammals should be clarified, with emphasis on potential risk to animal health and to human consumers of pinniped tissues. © 2006 by Taylor & Francis Group, LLC 296 MERCURY HAZARDS TO LIVING ORGANISMS RESIDUES IN REPTILES Mercury contamination of the Florida Everglades — primarily from anthropogenic activities — resulted in high levels of mercury, mostly methylmercury, in tissues of alligators. Alligator muscle, which is sometimes consumed locally, contains more than 6.0 mg Hg/kg FW, a level considered unsafe for human consumption over extended periods, especially to the developing human fetus. This situation must be resolved through public education, passage of appropriate legislation, and strict enforcement of existing laws. SIGNIFICANCE OF MERCURY RESIDUES The significance of mercury concentrations in field collections of plants and animals on organism health and on the health of consumers is not known with certainty, despite a relatively large database. The mechanisms of mercury accumulation and retention in the biosphere need clarification. Addi- tional data are required on the chemical form of mercury in the organism and on factors that affect mercury bioavailability. For the protection of sensitive species of mammals and birds that regularly consume fish and other aquatic organisms, total mercury concentrations in these prey items should probably not exceed 0.1 mg/kg fresh weight for birds and 1.1 mg/kg for small mammals. The significance of elevated mercury levels in tissues of fish and wildlife is not fully understood; some species of marine pinnipeds, for example, normally contain high concentrations of mercury in various tissues without apparent adverse effects. Usually, however, concentrations in excess of 1.1 mg/kg fresh weight of tissue (liver, kidney, blood, brain, hair) should be considered as pre- sumptive evidence of an environmental mercury problem, but this requires verification. SUBLETHAL EFFECTS Significant adverse sublethal effects were observed among selected aquatic species at water concen- trations of 0.03 to 0.1 µg Hg/L. For some birds, adverse effects — predominantly on reproduction — have been associated with total mercury concentrations (in mg/kg fresh weight) of 5.0 in feather, 0.9 in egg, and 0.05 to 0.10 in diet; and with daily intakes of 0.64 mg/kg body weight. Sensitive non- human mammals showed significant adverse effects of mercury when daily intakes were 0.25 mg/kg body weight, when dietary levels were 1.1 mg/kg, or when tissue concentrations exceeded 1.1 mg/kg. Future research needs should emphasize sublethal interaction effects of mercury with other envi- ronmental contaminants, emphasizing effects on reproduction and metabolism. TRANSFORMATIONS Mathematical models are needed that can accurately predict the transformation rates of inorganic and organic mercury species in aquatic, terrestrial, and atmospheric environments; flux rates of different mercury species between compartments; and ultimately, bioavailability and accumulation of individual mercury species — particularly methylmercury species — by living organisms. Additional research is recommended on rates of inorganic mercury-ligand formation in water and runoff and its effects on methylmercury formation in soils, quantification of the sources and transport characteristics of mercury species in terrestrial environments, and identification of the mercury species in terrestrial watershed runoff in dissolved and particulate fractions. © 2006 by Taylor & Francis Group, LLC CONCLUDING REMARKS 297 TRANSPORT BY PLANTS The role of terrestrial vegetation in expediting transport of elemental mercury from the geosphere to the atmosphere is considerable in areas heavily contaminated with mercury from historical gold mining activities; however, mechanisms of action affecting flux rates are poorly defined. The role of macrophytes, such as Spartina, in mercury cycling in saltmarsh environments is considerable. The role of higher plants in mercury cycling in other ecosystems should be evaluated. VACCINE PRESERVATIVE The use of ethylmercury thiosalicylate in Mexico and other countries to preserve medical vaccines must be evaluated, especially where a fetus may be subjected to this organomercurial from exposure of the expectant mother. © 2006 by Taylor & Francis Group, LLC . Francis Group, LLC 292 MERCURY HAZARDS TO LIVING ORGANISMS Additional research is needed to more fully assess the potential hazards of mercury and its com- pounds to living organisms. Some of the. inorganic mercury poisoning, and mercury- protectant drugs — including thiamin, selenium-, sulfur-, tellurium-, and other Group VI derivatives — to guard against organo- mercurial intoxication. MITIGATION Four. 100.0 to 500.0 µg/kg body weight (daily dose) and 1000.0 to 5000.0 µg/kg diet for mammals. Organomercury compounds, © 2006 by Taylor & Francis Group, LLC 294 MERCURY HAZARDS TO LIVING ORGANISMS especially