Cycling of mercury is a complex process. It is important to understand that mercury has 1) sources (e.g., mercury is usually bound in ores such as cinnabar), 2) sinks which absorb it (e.g., sediments, soils, and living organisms), and 3) flows (e.g., gaseous and soluble forms, wet and dry deposition (rain and dust), and chemical and biological transformations that facilitate flows to different compartments in an ecosystem).
 
Once emitted, mercury can remain in the atmosphere for days to years depending upon its form, its reactivity with other components of the atmosphere, and local, regional or global weather patterns. The shorter the mercury remains in the air, the greater the rate of mercury deposition to aquatic and terrestrial ecosystems. Not coincidently, the trend for increasing environmental mercury burdens in the United States is from west to east, which follows the same trends for increasing acid rain. Increasing acidity of aquatic ecosystems enhances metal solubility.
 
Once mercury dissolved in rain or adsorbed to dust particles is deposited, it can undergo a variety of chemical reactions that can potentially make it more bioavailable for living organisms. Environments that are wet, acidic, and/or anoxic (low oxygen) have a greater ability to support the bacterial and chemical actions required to create forms of mercury, such as methylmercury, that are more bioavailable to animals. Thus, marshes (e.g., the Everglades) are among the key global mercury “hot spots” because they are wet, acidic, and anoxic. Scientists are just beginning to understand the complexities of mercury cycling in forest, desert, and tundra ecosystems; to date most research has been on aquatic habitats.
 
Today, scientists are now becoming aware of non-mercury factors that can influence mercury fluxes in the environment. It is possible that as global climate change warms the tundra’s permafrost, the mercury cycling in these environments will result in increased bioavailability of mercury that is currently stored in the organic layers of the soil. Increases in atmospheric ozone levels, such as that created in polluted urban regions, can increase the transformation of mercury into forms that are more readily accumulated by plants and animals. Changes in surface wind speed, decreases in sea-ice cover, and altered vegetation patterns (especially due to human disturbances) also influence mercury cycling rates and long-term trends.
 

Health Risks and Toxicological Effects of Mercury

There is an extensive literature on the biological effects of methylmercury exposure, the most common form accumulated by animals. Methylmercury is one of the most potent neurotoxins in the environment. Among the observed effects is altered structure and function of neurons, sensory impairments (often visual impairment is one of the first signs of mercury poisoning), muscle weakness, deficits in learning and memory, and cardiovascular dysfunction.
 
Mercury is of particular concern because it becomes more concentrated at each succeeding level of the food (or trophic) web. Thus, the higher a species is in this structure, i.e., top carnivores, the greater the risk is for mercury accumulation and display of its toxic effects. We humans are also top carnivores and, therefore, can bioaccumulate large concentrations of mercury in our bodies, with toxic results. Present day exposures are generally lower than the well-documented epidemics of methylmercury poisoning that occurred in Japan in the 1950s-1960s and in Iraq in the 1950s. However, even at these lower levels altered neurological, cardiovascular, and hormonal function is evident. These effects are long lasting, perhaps permanent.
 
While fish species that are top carnivores, e.g., walleye and pike, have much greater mercury burdens than such species as bluegill or crappie, it is becoming clear from worldwide studies of human populations that rely on aquatic species, that eating fish with high mercury content does not necessarily lead to similar toxic outcomes. Scientists now are beginning to understand that other components of a fish diet or foods commonly eaten with a fish meal, e.g., wild rice, are key to whether or not mercurialism (mercury poisoning) will be evident. Levels of omega-3-fatty acids, selenium, or vitamin E all have profound effects on reducing mercury’s toxic effects, especially on nerve and muscle function; it is still not clear if these items can assist the body in reducing the toxic effects of mercury on such complex behaviors as learning. While the exact details as to why this is true are not fully understood, it is clear that one must balance risk with benefits when deciding to eat fish. Due to the ability of mercury to pass to the fetus, this decision making process is particularly critical for pregnant women.
 

Recovery of Mercury-Contaminated Fisheries

The story for wildlife health is also a mix of good and bad news. It is well established that when mercury emissions, especially from coal-powered electrical generating plants, decrease, mercury burdens in local or regional fish populations also decrease. However, in fish and bird populations further from these sources and in more remote regions, current trends indicate that the level of mercury is increasing in some fish and wildlife that consume fish or aquatic insects as a key part of their diet, e.g., mink, loons, tree swallows, and herons. In these populations, reduced reproductive success (fewer eggs, smaller gonads, behavioral disruptions), general impairment of hormonal function (especially thyroid), suppressed immune function, and birth defects are still observed.
 
It is currently hard to predict these trends, however, due to the complexities of form of mercury, magnitude and type of mercury loading, as well as climate and weather factors. For example, studies on the effects of closing a chlor-alkali plant[3] resulted in decreased mercury loading to an aquatic system.
 
Scientists do not yet know the long-term trends for population-level mercury burdens or the biological effects on individuals or populations of fish if those fish and their offspring carry any burden of mercury. There is a need for long-term monitoring, research into long-term studies and long-term responses of watersheds to decreased loading. As for trends in oceans, there is still insufficient data to make any strong conclusions.
 

Socioeconomic Consequences of Mercury Use and Pollution

Why is mercury an economic issue? The answer lies in the versatility of mercury as a useful commodity and the fact that people believe that the benefits of those uses far outweigh the risks to health. It is used in batteries (18% of the world’s consumption), as a dental amalgam (8%), a catalyst for plastics production (8%), and in chlor-alkali manufacturing (21%). Defining risks, however, is a tricky business. Changes in IQ due to childhood mercury exposure may result in future economic loss to the individual but translating that into regional, national, or global losses is difficult. One example can illustrate this difficulty. For an equal level of exposure, economic losses over a lifetime for an Amazonian Indian may not be similar to those of an American. Consider, for example, the differential in potential educational and economic opportunities for the two populations and one begins to understand the reason why economists use ranges of monetary losses, ranges that may be quite large.
 
There are also clear cultural and social disparities in levels of exposure. Policies of developed countries in which mercury use from virgin ore is reduced and recycling encouraged have the unexpected result of altering regional mercury supplies and demand. In turn, these policies and practices decrease global mercury costs. Unintentionally, the result has been to encourage greater use of mercury in those poorer countries whose citizens participate in small-scale gold mining activities, also called artisanal mining, which accounts for 30% of the mercury consumed today. In this process, mercury is used to amalgamate gold to separate it from the ore. The miners (10-15 million people worldwide) and the communities dependant on this activity (approx. 50 million people) are then exposed to mercury vapors at levels often exceeding 50 micrograms per cubic meter of air, a value that is 50 times the maximum acceptable level proposed by the World Health Organization. Amazingly enough, this process, accomplished largely by the world’s poor, contributes more than 10% of the modern human loading of mercury to the atmosphere. This is of critical importance since the body may retain up to 80% of inhaled mercury vapor.
 
Another example of cultural and economic disparities in use of and exposure to mercury occurs with subsistence fishing communities worldwide. A number of these communities, in response to mercury contamination of their food source, have switched to less nutritious foods and more sedentary lifestyles. More important, however, is the loss of social cohesion because fishing, and the use of specific species, forms the basis for their cultural and religious identity.
 
To change the course of mercury use and abuse in the developing world will require education. If information transfer, i.e., education, is the issue, if education and risk communication are required, then developed countries may need to take the lead in assisting the less wealthy nations of the world. Risk communication can be a significant challenge due to cultural and linguistic boundaries. One such conflict is the role of diet. If fish have increased mercury burdens, then it is also of paramount importance for the richer nations of this planet to understand that fish is not merely a food for native populations but an intrinsic part of their culture. While the intentions may be excellent, there is the danger of upsetting community social cohesion. Part of risk communication must be sensitivity to those local norms, cultural requirements, and social expectations.
 

Concluding Remarks: One Scientist’s Thoughts

While there is little debate that mercury, even in minute quantities, is highly toxic to living organisms (e.g., less than 1 molecule of methylmercury in 1 million molecules of water is sufficient to kill fish eggs or cause permanent neurological damage to a fetus), why it is toxic, the permanency of those effects, and how that toxicity can be prevented, reduced, or reversed are still matters of intense debate in the scientific and public health communities. What is needed, therefore, is a sane policy not only from our elected officials at all levels and the policy makers in our government agencies, but a great deal of education and proactive action on the part of individuals. Enough is understood today to suggest life style changes are appropriate. For more personal suggestions, see the action article below.
 
Putting pressure on our elected officials certainly can be useful but ultimately, we, by our collective power in the economic choices we make, will force the economies of the world to adopt an environmental ethic that is both good for the economy and for ecology. Remember it is no accident that both terms have the same root-oikos (or “eco”), meaning home.
 

Take Actions to Prevent Mercury Pollution:

Putting pressure on our elected officials certainly can be useful but ultimately, we, by our collective power in the economic choices we make, will force the economies of the world to adopt an environmental ethic that is both good for the economy and for ecology. Here is what you can do to prevent mercury pollution: 
 

1. Use less energy.

2. Buy products that use minimal or no mercury.

3. Recycle electronic equipment that has mercury in them, e.g., computers and fluorescent lights.

4. Eat a diet rich in selenium, omega-3-fatty acids, vitamin E, zinc, and iron.

5. Be involved in educating our children about mercury risks.

 
Dr. Weber is a fish neurobehavioral toxicologist at the University of Wisconsin at Milwaukee. In that capacity, he conducts studies that evaluate the behavioral effects of toxic chemicals that act primarily on the nervous system, including the brain. Over the years he has studied the effects of lead, mercury and other metals, as well as stormwater effluents in urban streams.
 

Notes:

[1]   For more information see the conference findings.

[2]   The primary natural sources of mercury include volcanic eruptions and erosion of native rock.

[3]   Chlor-alkali production uses several technologies to separate hydrogen, chlorine, and caustic soda from brine, one of which uses a mercury cell. While the World Bank no longer funds the technology and its use is decreasing, it still utilizes 21% of the world’s mercury. In Europe 53% of the chlor-alkali production still uses mercury cells, while the mercury use is only 10% in the US.

 
Originally posted in “On Eagles’  Wings” September 16th 2006