The toxicity of heavy metals depends on a number of factors. Specific symptomatology varies according to the metal in question, the total dose absorbed, and whether the exposure was acute or chronic. The age of the person can also influence toxicity. For example, young children are more susceptible to the effects of lead exposure because they absorb several times the percent ingested compared with adults and because their brains are more plastic and even brief exposures may influence developmental processes. The route of exposure is also important. Elemental mercury is relatively inert in the gastrointestinal tract and also poorly absorbed through intact skin, yet inhaled or injected elemental mercury may have disastrous effects.
Some elements may have very different toxic profiles depending on their chemical form. For example, barium sulfate is basically nontoxic, whereas barium salts are rapidly absorbed and cause profound, potentially fatal hypokalemia. The toxicity of radioactive metals like polonium, which was discovered by Marie Curie but only recently brought to public attention after the 2006 murder of Russian dissident Alexander Litvinenko, relates more to their ability to emit particles than to their ability to bind cell proteins.
Exposure to metals may occur through the diet, from medications, from the environment, or in the course of work or play. Where heavy metal toxicity is suspected, time taken to perform a thorough dietary, occupational, and recreational history is time well spent, since identification and removal of the source of exposure is frequently the only therapy required.
A full dietary and lifestyle history may reveal hidden sources of metal exposure. Metals may be contaminants in dietary supplements, or they may leech into food and drink stores in metal containers like lead decanters. Persons intentionally taking colloidal metals for their purported health benefits may ultimately develop toxicity. Metal toxicity may complicate some forms of drug abuse. Beer drinker’s cardiomyopathy was diagnosed in alcoholics in Quebec, and later Minnesota, during a brief period in the 1970s when cobalt was added to beer on tap to stabilize the head. More recently, a parkinsonian syndrome among Latvian injection drug users of methcathinone has been linked to manganese toxicity.
Classic examples of environmental contamination include the Minimata Bay disaster and the current epidemic of arsenic poisoning in South East Asia. In the 1950s, industrial effluent was consistently dumped into Japan’s Minimata Bay, and mercury bioaccumulated to exceedingly high concentrations in local fish. Although some adults did develop signs and symptoms of toxicity, the greatest impact was on the next generation, into which many were born with severe neurologic deficits.
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