There are two forms of mercury: inorganic mercury and organic methylmercury (MeHg). When inorganic mercury evaporates, it is biotransformed into methylmercury through microbial methylation in saltwater and freshwater sediments. Methylmercury is a persistent environmental contaminant and itbioaccumulates once it enters the food chain, building up in larger organisms that eat smaller, contaminated organisms. This is why large predatory fish such as tuna and shark have notoriously high mercury levels. The process, known as Biomagnification, occurs with many hydrophobic toxins.

Mercury causes nervous system defects in children and impedes fetal development. Numerous poisonings have highlighted the harmful effects of mercury use and mercury pollution.

The neuropsychiatric effects of mercury (and lead) exposure are discussed by Mark Filidei, of the San Francisco Preventive Medical Group, in an article, Toxic Metals and Mental Health.

Mercury Cycle

Main Article: Mercury Cycle
The mercury cycle begins with the evaporation of mercury from land and sea sources. Volcanoes and the burning of fossil fuels, especially coal and wastes, account for a large proportion of mercury released. The vapor is absorbed and distributed evenly throughout the higher levels of the atmosphere, where it is oxidized to water-soluble ionic mercury and returned to the earth’s surface in rain (Clarkson, 2002). Because of this cycle, mercury is a worldwide pollutant and its levels do not vary from unpopulated regions to populated ones.

Health Effects

Mercury from coal fired power plants and other sources travels through the atmosphere and water. Some is changed to methylmercury, which can enter the food chain to be concentrated at each step on that chain. Large old predators like sharks and pike, or scavengers like halibut, hold the greatest concentrations of mercury. The mercury is particularly problematic during development, so these limits here are designed to protect women who might become pregnant and children 12 or younger.

Inorganic mercury causes the most harmful effects when its vapor is inhaled (Gilbert, 2004). When elemental mercury vapor enters the body, it is readily absorbed into the bloodstream and easily crosses the blood-brain barrier and the placenta. After entering the brain, mercury is oxidized and will not transfer back across the blood-brain barrier. When continuous exposure occurs, mercury accumulates in the nervous system, leading to potentially debilitating nervous system afflictions. Health effects include tremors, drowsiness, depression, and decreased performance on memory and verbal tests.

Adverse health effects from overexposure to methylmercury are much more common and have been observed through several tragic case studies (highlighted below in the History of Use section). The first case of widespread methylmercury poisoning was in Minamata, Japan, where an abnormally high number of children experienced symptoms similar to cerebral palsy.

Mercury is a known human developmental toxicant. The U.S. National Research Council states, “60,000 newborns annually may be at risk for adverse neurodevelopmental effects from in uteroexposure to methylmercury (MeHg)” (Gilbert, 2004). Autopsies of the developing brains of those affected in the Minamata, Japan mercury tragedy show widespread damage to all areas of the brain. Tissues from fetuses killed in the Iraq Mercury Poisoning episode showed disrupted cellular patterns and underdeveloped tissues (Clarkson, 2002).

Symptoms of exposure arise only after a latency period during which no effects are observed. The length of the latency is indirectly proportional to the level of exposure: the higher the exposure, the less time it takes for the symptoms to be observed.

Common Sources and Precautions

Common Sources:

  • Switches in gas furnaces, heaters, etc.
  • Major household appliances (tilt switches in freezers, dryers, etc.)
  • Irons (tilt switches)
  • Automobile switches
  • Bilge pumps, sump pumps, etc. (float switches)
  • Dental amalgam
  • Measuring devices and laboratory equipment, such as barometers, manometers, etc.
  • Medical equipment and supplies
  • Batteries
  • Computers
  • Novelty items
  • Film pack batteries
    Fish and Shellfish
    Fish and shellfish have a natural tendency to concentrate mercury in their bodies, often in the form of methylmercury. Species of fish that are high on the food chain, such as shark, swordfish, king mackerel, albacore tuna, and tilefish contain higher concentrations of mercury than other species.

One should eat the above fish in moderation. Women of childbearing age should limit the amount of canned tuna they eat to about one can (six ounces) per week. A woman who weighs less than 135 pounds should eat less than one can of tuna per week. Children under six should eat less than one half a can of tuna (three ounces) per week. Specific weekly limits for children under six range from one ounce for a twenty pound child, to three ounces for a child weighing about sixty pounds. Follow the guidelines in the Physicians for Social Responsibility’s fish consumption guide.

Household Devices
Compact fluorescent lights (CFLs) do contain a very small amount of mercury, but it is fully contained as long as the bulb is not broken. Fossil fuels release mercury into the environment when burned to produce energy, so CFLs help to reduce overall mercury emissions. (In 2004, 67 percent of CFL lamps sold contained 5 mg Hg or less per bulb, while 96 percent contained 10 mg or less. A voluntary commitment in 2007 by bulb manufacturers belonging to NEMA, the National Electrical Manufacturers Association, has capped mercury content to even lower levels).

If a CFL breaks in your home, close the doors leading to other areas of the house if you can, then immediately get out of the house for an hour or more to allow the mercury to dissipate. For guidelines on cleaning up spilled mercury, see this EPA website. Call the Poison Control Center for additional advice. Do not throw spent CFLs in the garbage: take them to a local household hazardous waste (HHW) drop-off site.

Old thermometers contain mercury and are prone to breakage. Switch to digital thermometers, and take mercury thermometers to the HHW site.

Flu Vaccine
Thimerosal is a mercury-containing preservative used in vaccines, especially in the flu vaccine. It has purportedly been removed from all child vaccines other than the flu vaccine. If you get the flu shot annually, request a mercury-free mist or a preservative-free flu vaccine (if available). From a mercury exposure standpoint, this is especially important for babies, children, and pregnant or nursing women. The mist vaccine is not approved by the Food and Drug Administration (FDA) for pregnant women, so if pregnant or nursing, check with your doctor for the preservative-free vaccine.

Paula Dorf Cake Mascara (raven) contains Thimerosal (the mercury-containing preservative) (CosmeticsDatabase). Avoid mascara, skin lightening creams, or any cosmetic product that lists mercury or Thimerosal in the ingredients. To find specific products to avoid, search the Environmental Working Group’s Cosmetics Database.

Hopefully these products are on the wane, but beware of cheap or imported silver jewelry, or cheap jewelry bought on a vacation. These may contain mercury or Lead.

Amalgam is about 50% mercury and the mercury is slowly released, causing health problems for some people. Go to a mercury-free dentist, or do not have any new mercury fillings put in.

Button Batteries
Keep toys with these batteries out of young children’s reach. Dispose of spent batteries at a household hazardous waste site. Consider avoiding any trinkets and gadgets with button batteries.

Arts and Crafts
Gilding may use a mercury-bearing alloy. Artist paints and papermaking intensifiers may contain mercury. Ask the manufacturer if necessary to find out if their materials are mercury-free.