About Mercury and Methylmercury

Much has been written about mercury in the environment and specifically, about concentrations of methylmercury in fish. The following is a primer that describes the various forms of mercury and where they are found.

Mercury

Mercury is a naturally occurring element and a part of the earth’s crust, oceans and atmosphere. According to the Environmental Protection Agency, “the same amount has existed on the planet since the earth was formed” (Mercury Study Report to Congress, 1997). It cycles in various forms through the environment, undergoing complex chemical and physical changes along the way.

Mercury is found in several forms including metallic or elemental mercury -- the heavy, silvery liquid that vaporizes quickly. Elemental mercury is used to produce chlorine gas and caustic soda, and is also used in thermometers, dental fillings and batteries. Most of the mercury in the environment is elemental mercury, which tends to stay airborne.
But most mercury in nature is attached to other elements, such as chlorine, sulfur, or oxygen, and form inorganic mercury compounds. Inorganic mercury can produce "salts," which are usually white powders or crystals. Mercury salts are sometimes used in skin lightening creams and as antiseptic creams and ointments. Inorganic mercury compounds tend to be more reactive and soluble, and can turn into airborne particles or gases that fall to the ground or into water. In most cases, these particles are rapidly absorbed into the soil or sediment at the bottom of bodies of water, where they may stay for a long period, sometimes hundreds of years. The rest eventually vaporizes and diffuses into the atmosphere.

When inorganic mercury combines with carbon, the result is an organic mercury compound. The most common one, methylmercury, is produced mainly by microscopic organisms in the water and soil.

Most of the mercury that is released into the atmosphere comes from natural processes such as surface volcanic eruptions, deep-sea vents and underwater volcanoes, hot springs, evaporation from bodies of water and the soil, and erosion. According to the Smithsonian Institution, there are more than 5,000 surface and underwater volcanoes in the world erupting approximately 50 to 60 times a month. As a result, volcanic degassing is thought to be the single largest source of ocean and atmospheric mercury.

In addition to these natural sources of mercury, the element is released into the environment through industrial processes, such as power plant emissions, burning municipal and hazardous waste, pulp and paper milling, and mining. Current studies indicate that these “man-made” mercury emissions account for approximately 30 percent of the mercury released into the environment.

Methylmercury

The form of mercury that is produced in bodies of water is methylmercury, an organic mercury compound produced by methane-generating bacteria in the water. As these bacteria ingest the mercury, they add a carbon atom to the hydrogens through a natural process called methylation. Once methylated, the mercury more easily binds to proteins and accumulates in fish.

Although how methylmercury is formed and accumulates in fish is still poorly understood, it is believed that the methylation occurs deep in the ocean where mercury resides in the sediment. For this reason, many experts studying methylmercury concentrations in fish report that these levels have remained consistent over time and are not correlated with mercury levels in the air or atmosphere. Supporting this scientific position are a number of published studies, including the following:

• A Princeton University study, funded by the Environmental Protection Agency and published in Environmental Science and Technology in 2003, compared the methylmercury levels in Yellowfin tuna caught off Hawaii in 1971 and the concentrations of the tuna caught in 1998. The researchers found that the mercury levels were unchanged;
• A study reported in the journal Science in 1972 found that fish in the Smithsonian Museum that were caught in the late 1800s had average methylmercury levels of 0.38 parts per million (ppm). In comparison, fish samples analyzed by the Food and Drug Administration (FDA) in 1978 contained an average of 0.16 ppm of methylmercury while more recent tests conducted by FDA found an average of 0.12 pmm in the 25 most commonly consumed fish;
• A study of hair samples on 550-year-old Alaskan mummies found a mean methylmercury level of 1.2 ppm, whereas the methylmercury concentration in today’s Alaska population is 0.6 ppm.

At the same time, scientists have been looking at how methylmercury accumulates in fish and have determined that freshwater fish tend to accumulate more mercury than saltwater varieties. This is because the characteristics and sulfate-reducing bacteria in many freshwater bodies stimulate mercury production. It is also well known that as larger fish eat smaller ones in the aquatic food chain, methylmercury levels in their systems rise. This is why large, older fish in both oceans and freshwater have higher mercury levels than their smaller and younger counterparts.

Based on an extensive review of the scientific literature and ongoing monitoring studies around the world, all of the major governments and public health organizations agree that in small amounts, methylmercury is not harmful to humans. In fact, in the November 5, 2004 issue of Morbidity and Mortality Weekly Report, the Centers for Disease Control and Prevention (CDC) published the findings of a new study confirming that mercury levels from fish consumption for women and young children in the United States are well below any level of concern. Specifically, CDC researchers used the ongoing National Health and Nutrition Examination Survey (NHANES) to analyze the amount of mercury in the blood of more than 3,600 women of childbearing age and 1,500 children aged one to five years and found that all had mercury levels significantly below the threshold for any known risk.

However, at high levels, methylmercury is toxic to humans. There have been four cases worldwide where people have been poisoned by consuming foods containing very high concentrations of methylmercury. During the 1950s, 111 people from Minamata City, Japan died or experienced neurological disorders from eating fish contaminated with very high concentrations of methylmercury, up to 40 parts per million (compared to FDA’s limit of 1 ppm). In this case, an industrial facility was releasing manufactured methylmercury directly into Minamata Bay. A second incidence in 1965 occurred in Niigata, Japan where 120 people were similarly poisoned.

The other two incidents involved people in Iraq who ate bread from grain that was contaminated with a fungicide containing mercury. Here, the people were exposed to even higher levels of mercury than in Japan and thousands were hospitalized.
The key concern is that methylmercury may harm an unborn baby’s or young child’s developing nervous system if pregnant and nursing women consume large amounts of fish containing high levels of methylmercury.

Because of these incidents, the major governments and health agencies around the world have developed strict standards to regulate the amount of methylmercury in fish that people consume. Of key concern is ensuring that the amount of methylmercury consumed by pregnant and nursing women will not harm the unborn baby’s or young child’s developing nervous system.

In the United States, the basis for regulatory policy is the “reference dose” developed by the U.S. Environmental Protection Agency (EPA). According to EPA, the reference dose (RfD) is an estimate of the highest daily dose of a chemical that the most sensitive in the population can be exposed to over a lifetime of exposure without experiencing an adverse effect. EPA’s reference dose for methylmercury in fish is 0.1 microgram per kilogram of body weight per day (µg/ kg-day) or 0.7 µg/kg bw/week, which builds in a ten-fold factor of safety for the most vulnerable populations.

EPA’s reference dose is also applied in developing seafood consumption advice for the U.S. population. The most recent advice was issued jointly by FDA and EPA in March 2004 and updates previous advisories issued separately by the two agencies. Focusing specifically on pregnant and nursing women, women who might become pregnant and young children, the advisory offers these major recommendations:

1. Do not eat shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury.
2. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
3. Another commonly eaten fish, albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces of albacore tuna per week.
4. Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week.

By following this advice, pregnant and nursing women, women who may become pregnant and young children will consume the concentrations of methylmercury in fish that are at least ten times lower than the lowest level for any known risk. However, these special populations will also receive the many health benefits of eating fish.