Health issues related to methylmercury in fish—which are discussed in the “Chemical Risk” section of The Environmental Source— are only half of the mercury policy debate. Of equal concern to many is the source of that mercury. Some see the alleged consumer health risk from mercury exposure as a justification for restricting mercury emissions from coaland oil-fueled electric utility power plants. Because methylmercury in fish is unhealthy for consumers, critics argue, mercury power plant emissions must be significantly reduced in order to improve public health.
However, even if the amount of mercury in the American diet did pose some genuine health risk, it still is not clear that even sizable reductions in mercury emissions from U.S. power plants would have an appreciable effect on exposure to methylmercury. In contrast, the cost of complying with new power plant emissions regulations is estimated to have a large human impact.
Mercury is a naturally occurring element that appears in the environment in elemental form, as well as in organic and inorganic compounds. In its various forms, mercury cycles through the environment—in air, land, and water—and is circulated and modified by both natural and human (anthropogenic) activities.
Most of the mercury in power plant emissions is in either elemental or inorganic form. It is the organic compound methylmercury, however, that accumulates in fish and other animals. Methylmercury is created in two primary ways. First, elemental mercury can bind with dissolved organic carbon in oceans and other waterways. Second, certain microorganisms in soil and water can ingest inorganic mercury and add carbon atoms to the molecules in a process called methylation. The elemental and inorganic mercury in power plant emissions can be converted into methylmercury in each of these ways. However, extensive study of the mercury cycle shows that only a small portion of the mercury from anthropogenic sources is converted to methylmercury.
Organic compounds such as methylmercury readily bind to proteins, and methylmercury binds easily with fats in the tissues of living organisms. Once it begins to accumulate in aquatic organisms such as algae and plankton, methylmercury becomes more concentrated as it bioaccumulates up the food chain. Small fish eat the algae and plankton, amassing greater methylmercury levels, and larger fish accumulate still higher levels by eating small fish.
Historical records show that fish have always had trace amounts of methylmercury, however, and that the amounts have remained relatively stable throughout the years, despite large increases in mercury emissions in the latter half of the 20th century. French scientists, for example, recently found that methylmercury levels measured in Yellowfin tuna were the same in 1998 as they were in 1971, despite a prediction of a 9 to 26 percent increase that would have corresponded with increases in global mercury emissions. Fish caught during the late 19th and early 20th centuries—and preserved at the Smithsonian Institution—have average methylmercury levels more than three times higher than a similar sample of fish today. Similarly, the amount of methylmercury in human bodies today is within the same range as that in preserved human corpses from several centuries ago.6 Thus, ample evidence shows that the range of methylmercury to which humans are exposed has remained essentially constant or falling over the past century, despite steadily rising levels of anthropogenic mercury emissions during that time.