The serious toxicity of mercury has been known for many years. The expression "mad as a hatter" arose as a result of felt hat makers commonly being exposed to mercury and suffering the effects of mercury poisoning. This mercury exposure led to the common symptoms known as "erethism", and consisted of shakiness or tremor, anxiety or excitability, memory loss, headaches, insomnia, and mental decline.

Nowadays, mercury toxicity is thought to be a possible cause of Parkinson's disease, Alzheimer's-like brain failure, heart failure, heart arrhythmias and irregularities, and certain cancers. Unfortunately, unlike most poisons, which cause illness very soon after ingestion or exposure, mercury toxicity is a chronic, insidious and slowly progressive disorder. Often by the time symptoms occur, there is already a heavy mercury burden in the body with substantial and possibly irreversible damage.

Unfortunately, most physicians are not familiar enough with the harmful effects of mercury to consider this as a cause of many diseases. Mercury has a particular affinity to become deposited in vital organs such as the brain and nervous system, the heart, the liver, kidneys and bone marrow. In patients with heart failure, for example, mercury levels in the heart have been found to be several thousand times the normal amount. Mercury is also known to cause dementia (loss of mental capacity), peripheral neuropathy (damage to nerves in the body), Parkinson's disease, and cancer.

Mercury exposure and toxicity is far more common than most of us tend to think. Our environment has become highly contaminated with mercury as a result of the burning of fossil fuels, particularly coal, for energy. Mercury normally occurs in the Earth's crust, but is released either by burning coal, or through incineration of mercury-containing equipment such as thermometers, blood pressure cuffs, electrical switches, thermostats and computer chips. Coal-burning and medical waste incineration are the two biggest sources of mercury contamination in our environment.

When mercury is burned, it enters the atmosphere as mercury vapor, and then is carried downward with rain, contaminating our rivers, lakes, streams, groundwater, and eventually our oceans. A recent study in Massachusetts showed that the mercury content of rainwater was actually higher than the existing mercury levels in our lakes and ponds. Rather than cleaning our water supply, rain was actually adding to the mercury contamination.

To make matters worse, mercury is consumed by fish and concentrated as one moves up the food chain. Small fish eat plankton and algae contaminated with mercury; bigger fish eat the smaller fish, and even bigger fish eat the big fish. As a result, the largest predatory fish in the ocean have the highest mercury levels. The fish known to harbor the highest levels of mercury include the large, predatory marine fish such a tuna, swordfish, mahi-mahi, halibut, shark, tilefish and king mackerel.

Regular consumption of these fish is the greatest source of mercury exposure and toxicity in humans. The body also very readily absorbs mercury that comes in this form from fish, increasing the body's level and total burden of mercury. Unfortunately, mercury is very slowly and very poorly eliminated by our bodies. Over time, and with continued consumption of fish, our mercury levels climb steadily. Eventually, enough mercury is deposited in vital organs to cause organ failure and illness.

Another potential source of mercury contamination comes from silver amalgam dental fillings. One of the components of these fillings is mercury. Amalgam fillings may be a significant source of mercury exposure in someone who has many such fillings in their mouth. The likelihood of mercury toxicity from fillings seems to relate not to the number of fillings so much as the total surface area of these fillings. Elemental mercury in amalgam fillings is not absorbed by swallowing and absorbing from the stomach, but rather by small amounts of mercury vapor that is released from the surface of the fillings and then inhaled. This mercury vapor is readily absorbed by the lungs and promptly enters the vital organs of the body.

Chewing gum, as well as having mixed metals in the mouth (e.g. a combination of silver and gold fillings or crowns) may accelerate the release and exposure to mercury. It should be no surprise that recent studies have shown that dentists often suffer from declines in performance, memory, and coordination due to their occupational exposure to mercury. Thankfully, alternate materials are now available for fillings, and dentists are much more careful about their use and handling of mercury.

Mercury is known to present a particular hazard to children and infants. Mothers exposed to mercury in the 1950s disaster in Minamata Bay , Japan , gave birth to infants with mental retardation, cerebral palsy and other neurologic abnormalities. Because the developing brains of children and infants are much more susceptible to the harmful effects of mercury, both the American Academy of Pediatrics and the American College of Obstetrics and Gynecology have issued warnings about mercury exposure to children and infants. Parents are advised to turn in any mercury-containing thermometers. Pregnant women and children are advised not to consume tuna and other large marine fish more than once a week.

What can we do to prevent mercury toxicity?

Choose smaller fish instead of the large predatory marine fish. These safe and healthy fish include sardines, herring, smelt, mackerel, trout, salmon, pollock, tilapia, catfish, sole, and scrod.

Limit intake of large fish to once a week (or less if a significant mercury burden is already present). These fish include tuna, swordfish, shark, mahi-mahi, tilefish, King mackerel,. Medium-sized predators such as halibut and sea bass have moderate levels. Some shellfish, particularly Gulf coast shellfish may have moderate levels of mercury, also.

Ask your dentist about mercury-free fillings if any preventive dental work is needed. Be particularly careful about having existing fillings removed, since this can potentially cause more exposure and release of mercury. Any removal of existing mercury fillings must be done only by an experienced dentist.

Enhance the body's natural elimination of mercury by increasing your intake of fiber, clean water, dark green leafy vegetables, broccoli, and Vitamin C.