What is mercury poisoning?
Mercury is a metallic element once used in a wide variety of applications, ranging from industrial use in processing ore to serving as the indicator fluid in thermometers. In its pure form, mercury is a soft, silver-colored metal with a melting point of –40 degrees Celsius. Consequently, mercury is liquid at room temperature. Mercury and mercury compounds such as methylmercury are known neurotoxins. Prolonged exposure to mercury, either through inhaling the vapors or through ingesting it as a contaminant in foodstuffs, will lead to permanent neurological damage. Ingested mercury can also irritate the gastrointestinal tract and has been known to damage kidneys.
Occupational neuropathy has long been anecdotally associated with mercury exposure. The phrase “mad as a hatter,” for example, became common in the nineteenth century when workers in the hat industry developed tremors, slurred speech, and problems thinking clearly following exposure to fumes produced by the felting process. A mercury nitrate compound was used to remove animal hair from hides in hat factories. In some cases of long-term exposure, neuropathy progressed to the point where the sufferers experienced hallucinations.
Despite the widespread prevalence of occupational illness among hatters, the nineteenth-century medical community did not recognize the dangers of mercury exposure. In fact, mercury was used for a variety of medical applications, ranging from the treatment of syphilis to its use in the topical antiseptic Mercurochrome. Mercurochrome continued to be used in the United States as late as the 1990s, although the Food and Drug Administration (FDA) ruled in 1998 that it would be treated as a “new drug,” meaning that any company wishing to manufacture it for nationwide distribution would have to submit it for FDA review first.
The mining industry also used mercury extensively. Mercury amalgamates readily with gold dust, making it heavier and causing it to sink into the bottom of rocker boxes as ore is washed as part of placer mining. During the California gold rush, at least 7,600 tons of mercury were deposited into Sierra Nevada streams. Similar amounts were used elsewhere globally, leading to large quantities of mercury being deposited on streambeds and ocean floors. That mercury is now making its way back into the environment in the form of methylmercury, the organic form of mercury that causes most concern among health-care professionals today.
Methylmercury is a mercury-carbon compound formed through bacterial action. Manufactured methylmercury and dimethylmercury were common fungicides until the 1970s, when they were banned due to their toxicity.
With mercury poisoning, prevention is the best form of treatment. Occupational exposure to mercury rarely takes place, and the biggest risk of mercury poisoning today comes from its ingestion within the food chain. Methylmercury bioaccumulates readily and has been found in large concentrations in seafood such as tuna, swordfish, and shark. Numerous studies have shown that when pregnant women consume fish containing high levels of methylmercury, the resulting mercury poisoning can cause permanent neurological damage to the developing fetus. Young children are also at risk of developing neuropathy if they eat fish with high mercury levels. This problem was first recognized in the 1970s but did not become widely publicized until the early twenty-first century. Pregnant women, nursing mothers, and young children are all now advised to avoid some varieties of fish completely and limit their consumption of others.
Although the use of mercury was discontinued for many applications as evidence mounted regarding the dangers of mercury exposure, thimerosal, a mercury compound, continued to be used as a preservative in vaccines into the twenty-first century. In the 1990s, many parents became convinced that thimerosal, which metabolizes readily to ethylmercury, was responsible for the rising rate of autism in American children. Some health activists argued that the increase in autism in the late twentieth century correlated with the increase in the number of vaccines that young children received in the first few years of life. The use of thimerosal as a preservative was discontinued by manufacturers for most vaccines, with the exception of influenza, despite research showing that vaccines were not linked to autism in any way.
One use of mercury that does continue despite several decades of debate over its safety is the application of mercury amalgam tooth fillings in dentistry. The typical “silver” tooth filling is actually 50 percent mercury. Health activists argue that the practice should be stopped, as mercury leaching from the filling into the body could lead to mercury poisoning. In 2002, the FDA concluded that research to date had shown no evidence of ill effects other than rare cases of allergic reactions.
Given the widespread dispersion of mercury into the environment through pollution from numerous industries, mercury exposure will continue to be a health issue for many generations to come. Marine estuaries, ocean floors, and inland lakes and rivers remain contaminated, and methylmercury will continue to work its way up the food chain.
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