What is mercury and where does it come from?

Mercury is an element that is found in rocks in the earth’s crust. Through mining and industrial processes, mercury is brought to the earth’s surface and used in manufacturing, electricity generation and consumer products (such as lamps, thermometers and dental material). Eventually, the mercury is emitted to the air or discharged to water as a byproduct of combustion or improper waste disposal. Once in air and water, mercury presents a risk to ecological and human health.

The mercury cycle describes the sources and movement of mercury through the environment. The modernday sources of mercur y can be broken down into airborne sources and water sources. In the United States, the major sources of airborne mercury include coal-fired power plants, industrial boilers, incinerators and chlorine manufacturing plants. Major water sources include wastewater treatment plants, gold mining operations, landfills and some manufacturing facilities.

The northeastern U.S. and eastern Canada receive mercury from local, regional and global emissions. However, most estimates show that U.S. emissions constitute the largest source of mercury that is deposited to the Northeast (approximately 60 percent) (NYSERDA 2002). Regulations to address mercury emissions from incinerators and ot her sources have been successful and, as a result, total U.S. emissions have declined 40 percent since 1990 (Figure 3) (EPA 2003).

Mercury is emitted to the atmosphere in several different forms, or species. As described in Box 1, the characteristics of these species determine the ultimate fate of mercury in the environment. To complicate matters, once these different species of mercury are emitted to the air they may change into a different species before being deposited.

What happens to mercury in the landscape?

After mercury is emitted from a smokestack (such as from a coal-burning power plant or incinerator) and travels through the atmosphere, it deposits on land and water. Together with mercur y from surface water discharges and ot her large sources, it makes its way through a watershed and ultimately to a nearby lake or stream. The extent to which mercury poses a human health or ecological risk depends, in part, on whether or not it is converted into the bioavailable toxic form known as methylmercury.

If the mercury is converted to methylmercury, it can be consumed by organisms and move up the food chain. An unfortunate characteristic of methylmercury is its ability to build up in the body over time (bioaccumulation) and increase in concentration as one organism eats another (biomagnification). Consequently, a very low level of methylmercury in the environment can produce extremely high body burdens in animals at the tops of food chains. In the case of mercury, a little bit goes a long way.

Why is mercury a problem to fish, wildlife and people?

Once mercury enters the body of an animal or a person, it can have a wide range of effects — from sublethal to lethal. Birds are particularly at risk for mercury poisoning because many species exclusively eat mercury-laden fish. They are also long-lived animals and therefore accumulate mercury in their bodies over a long period of time. For these reasons, birds such as loons are one of the most intensively studied animals in mercury research. From past research it is known that mercury can have adverse effects on individual birds, as well as on the population as a whole through changes in their behavior, reproduction and body chemistry. The mercury effects that have been documented in fish and wildlife are summarized in Table 1.

Although this report does not focus on human health, it is important to mention that fish consumption is the primar y mode of human exposure to mercury. Children under 12
and people who frequently eat fish with mercury are the most likely to be at risk for mercury exposure. In July 2000, the National Academy of Sciences completed a review of the latest scientific evidence regarding t he human health effects of methylmercury. They concluded that children of women who consume large amounts of fish and seafood are at highest risk (NAS 2000). A recent report estimates that over 600,000 children born each year are at risk for nervous system effects due to methylmercur y exposure in the womb (Mahaffey 2004).

Fig. 2 The mercury cycle

In 2003, the Centers for Disease Control and Prevention found that eight percent of American women of childbearing age had blood mercury levels above those deemed safe by the EPA (Shober 2003). To address this significant public health risk, fish consumption advisories have been posted by the EPA in 44 states due to mercur y contamination (EPA 2004).

What has been done to reduce mercury? (Papers 2 & 3)

Mercury is difficult to remove from the environment, but a variety of programs and policies have been proposed or adopted to reduce mercury use and pollution. Northeastern North America is a leader in mercury reduction and has implemented several important initiatives, including the New England Governors and Eastern Canadian Premiers Mercury Action Plan. The basic elements of mercury reduction efforts at t he national, regional and state level are outlined in Table 2.

Given the global circulation of mercury in the atmosphere, the problem must be addressed worldwide. The United Nations Environment Program has established a program to focus attention on the problem globally. This effort is supported by the ratification of the United Nation’s Convention on Long-Range Transboundary Air Pollution in 2003. Despite policy efforts, the research presented here demonstrates that mercury remains ubiquitous and persistent in the environment and that more work is needed to reduce ecological and human health risks associated with mercury pollution.