When you think of mercury, what comes to mind? Do you think about the small, hot planet in our solar system nearest the sun? Perhaps the Roman god with winged feet?

I know some people who would surely think about the car. But how about heavy metal?

No, not rock music; the transition metal with the atomic number 80. Mercury is one of only five elements that is a liquid at room temperature.1 The periodic table symbol for mercury, Hg, comes from the Greek name hydrargyrum, a combination of the words for water (hydro) and silver (argyros) forming a word meaning “watery silver.’’ It’s an apt physical description of elemental mercury. The other common name for mercury is quicksilver.

Mercury, an element with a storied past, still presents dangers hospitalists must be aware of.

Organic compounds of mercury especially are toxic at even low levels. Human exposure must be limited when possible. Many elements and heavy metals are used by living organisms in trace amounts. Not mercury. Mercury is a toxin at any dose. Pure elemental mercury is less toxic than the organic compounds and salts. Exposed to water, mercury is quickly converted to the more toxic form, methylmercury. Because of its volatility, any open container of mercury is a biohazard and presents a risk of poisoning from mercury vapor. Inhaled vaporized mercury is readily absorbed through the alveoli.2

Mercury—especially organic compounds of mercury—are toxic at even low levels, and efforts should be continued to limit human exposure when possible

Historical Uses

Mankind has used mercury-containing products for a long time. Some of the red color in ancient cave drawings is cinnabar, or mercury sulfide (HgS). Cinnabar has also been used as the red pigment in tattoos. When I was in the Navy, we were told any skin rash that spared the red color in a tattoo was sure to be syphilis; according to the information we were given, the red pigment in a tattoo was created by mercury. Actually, tattoo parlors use several forms of red pigment, but mercury sulfide is certainly one of the choices.

Historically, many cultures used mercury in a variety of ways. More than two millennia ago, Greeks used mercury in medicines; Romans incorporated the element in their cosmetics.3 Medicinal uses of mercury have included the treatment of syphilis and use as a diuretic, antiseptic, and laxative.4 In the mid-20th century, mercury-containing compounds were popular as a remedy for infant-teething pain.

From the 17th to 19th centuries, mercury was used in the process of making felt for hats; hat makers were subjected to excessive exposure to the element. Mercury toxicity manifested as psychiatric illness and was widely recognized as an occupational hazard of hat-making.4,5 Reflecting this observation, “mad as a hatter” became a figure of speech and even found its way into the famous book “Through the Looking Glass (And What Alice Found There)’’ by Lewis Carroll.

Gold mining presents another occupational exposure to mercury. During 19th-century gold mining in California, mercury was used to extract gold from ore. Today, much controversy and ecological protest is focused on the South American gold mining industry, particularly in Brazil. In the past 20 years or so, it is estimated that 2,000 tons of mercury have been released into the environment in the Brazilian Amazon.6 Watchdog groups have been sounding the alarm for people who live in the villages along the Amazon River and consume a diet of fish caught from the river. Hair samples of those villagers revealed high levels of mercury.7 To date, clinical disease has not been reported there.

Infamous Occurrences

The most noted occurrence of clinical mercury poisoning was in Minamata, Japan. Starting before World War II and continuing until 1968, industrial mercury-containing waste was dumped into the sea near Minamata. The townspeople’s diet consisted of locally caught fish; the fish were heavily contaminated with mercury. Many people suffered permanent disability, and more than a thousand died.8 The effect of mercury poisoning was so great the disease was called Minamata’s disease.

Another epidemic occurred in the early 1970s in Iraq. Grain intended for use as seed for planting was treated with a mercury-containing fungicide. Accidentally, the local population used the grain for making bread, resulting in many cases of clinical mercury poisoning.8

One of the most historically interesting stories of mercury poisoning involves the first emperor of the Qin Dynasty in China, Qin Shi Huang Di, who lived from 259 to 210 B.C. At age 12 when Qin Shi Huang ascended to the throne, China was divided into warring feudal states. By asserting ruthless power, he succeeded in conquering the last warring province and unified China in 221 B.C. Although his rule was brutal and tyrannical, his accomplishments included standardization of the systems of measurement and weights, currency, and Chinese script. He was also a builder, conscripting labor for the development of roads, canals, and—most famously—his own mausoleum. The tomb of Qin Shi Huang at Xian is famous for its size and contents, including the army of more than 8,000 life-size, terracotta warriors.9

Qin Shi Huang Di (Di means supreme god) set out on a tour in 210 B.C. seeking eternal life in the “Islands of the Immortals.” It is not clear if his entourage found the site, but he was given a potion or pills, made by his court scientists, to make him immortal. The medication contained a toxic compound of mercury, and he died.9 The mausoleum was completed just in time to accommodate the emperor.

Exposure and Toxicity

Other than occupational exposure to mercury, the most important mercury exposures in the United States are dietary, the use of mercury amalgam in dentistry, and mercury used as a stabilizing agent in vaccines. The risk of toxicity depends on the form of mercury involved. Elemental mercury is poorly absorbed through the gastrointestinal tract; methylmercury, on the other hand, is readily absorbed if ingested. As previously noted, inhaled elemental mercury vapor is easily absorbed in the lungs.

Consensus supports the fact that dietary ingestion of methylmercury as found in large game fish—tuna, swordfish, shark, king mackerel, and tilefish—is the most important exposure to organic mercury. Small fish eat the mercury-contaminated plant life on the ocean bottom and are consumed by larger fish, and those fish are consumed by larger fish. Thus, the large game fish in the food chain accumulate high concentrations of mercury. It is recommended that children and pregnant women limit consumption of these types of fish.8

Dental silver for tooth fillings is a mixture of 50% mercury and 50% metal powder, such as silver, copper, or zinc. Because dental amalgam uses elemental mercury, gastrointestinal absorption is limited. Two studies examining more than 500 children each followed the neuro-psychiatric development of children with mercury amalgam fillings with a control group. No statistical differences were identified in IQ, memory, visual/motor, and renal function between the two groups over five- and seven-year follow-ups. Yet, doubts linger concerning long-term follow-up and questions raised with respect to the incidence of multiple sclerosis and Alzheimer’s disease.10 Overall, dental mercury amalgam is believed to be relatively safe with no evidence of acute toxicity.

Controversy has swirled around the use of thimerosal, a mercury-containing stabilizer and antibacterial agent used in vaccines. Used since the 1930s, thimerosal is metabolized to ethyl mercury in the body and has been implicated in a host of ailments, including the marked increase in incidences of autism. Although no dose-dependent toxicity has been established, questions concerning genetic vulnerability to mercury have been raised and considered significant.11-12 In 1999, the Department of Health and Human Services recommended thimerosal be decreased or eliminated from childhood vaccines.

Telltale Signs

Clinical symptoms of mercury toxicity are often insidious in onset and nonspecific, making diagnosis difficult without a high index of suspicion. Complaints may include gastrointestinal symptoms, headaches, insomnia, visual disturbances, peripheral neuropathy, or ataxia.2 Exposure to inorganic mercury from mercurial salves (merthiolate) or the chronic use of mercury-based cathartics may present with the constellation of symptoms known as acrodynia or pink disease, Feer disease, Feer’s disease, Swift syndrome, Swift’s disease, Swift disease, Swift-Feer disease, vegetative neurosis, dermatopolyneuritis, erythredema polyneuritis, and trophodermatoneurosis. This is characterized by a desquamating rash, hair loss, erythema of the palms and soles, anorexia, and gastrointestinal complaints. Elemental mercury does not readily cross the blood-brain barrier, so neurological complaints are not dominant.2

Organic mercury poisoning typically presents with neurological symptoms. In-utero exposure may result in spontaneous abortion; or the infant, if delivered, may suffer mental retardation. Adult toxicity presents with sensory and motor-neurological complaints, visual field loss, hearing loss, dysarthria, or cerebellar symptoms of ataxia. Severe toxicity results in movement disorders, paralysis, and seizures.2 Evidence of kidney damage and reproductive failure are also commonly associated.

In suspected cases, obtain blood and urine levels of mercury. Levels of mercury in hair may be helpful in some instances, but false-positive findings make this method of testing less reliable.

Treatment is supportive. Employ chelating agents if the patient is acutely symptomatic. The agent of choice is BAL (dimercaprol). Administer it as directed by consultation with a poison control team. BAL is not recommended for children. TH

References