Heavy Metals and Heavy Metals Poisoning
Heavy metals are metallic elements that have a high atomic number and are poisonous to living organisms. Approximately 30 metals have been shown to be poisonous to humans. Examples of heavy metals that are poisonous include mercury, chromium, cadmium, arsenic, and lead. Because they are poisonous, heavy metals are sometimes referred to as toxic metals. Heavy metals may be poisonous on their own or as part of chemical compounds.
It has been known for centuries that certain metals are toxic. For example, Theophratus of Erebus (370-287 B.C.) and Pliny the Elder (A.D. 23-79) both described poisonings that resulted from arsenic and mercury. Other heavy metals, such as cadmium, were not recognized as poisonous until the early nineteenth century.
Heavy metals occur naturally in the environment in rocks and ores. They cycle through the environment by geological and biological means. The geological cycle begins when water slowly wears away rocks and dissolves the heavy metals. The heavy metals are carried into streams, rivers, lakes, and oceans. The heavy metals may be deposited in sediments at the bottom of the water body, or they may evaporate and be carried elsewhere as rainwater. The biological cycle includes accumulation in plants and animals and entry into the food web.
Sometimes these natural cycles can pose a hazard to human health, because the levels of heavy metals exceed the body's ability to cope with them. A further complication is the addition of heavy metals to the environment as a result of human activity. Activities such as mining and manufacturing greatly increase the release of heavy metals from rocks and ores. The activities also create situations in which the heavy metals are incorporated into new compounds and may be spread worldwide.
The health hazards presented by heavy metals depend on the level of exposure and the length of exposure. In general, exposures are divided into two classes: acute exposure and chronic exposure. Acute exposure refers to contact with a large amount of the heavy metal in a short period of time. In some cases, the health effects are immediately apparent; in others, the effects are delayed. Chronic exposure refers to contact with low levels of the heavy metal over a long period of time. A person may not even be aware that the exposure is occurring. Heavy metal poisoning can be insidious because health problems may develop very slowly.
Depending on their form, heavy metals and their compounds can be taken up through the skin, respiratory system, or the digestive system. Contaminated air, water, soil, food, and drugs yield potential heavy metal exposures. Once in the body, heavy metals may be distributed to all parts by the blood. In both acute and chronic exposures, the symptoms of poisoning depend on the identity of the heavy metal or the compound in which it occurs.
Usually, immediate symptoms are linked to an acute exposure; however, not all acute exposures are immediately followed by symptoms. For example, ingesting a large dose of arsenic can cause fever, loss of appetite, enlarged liver, and heart failure soon after ingestion. However, the nerve damage resulting from brief skin contact with dimethyl mercury, a mercury compound used in some laboratories, may not be apparent for months.
Health problems linked to chronic exposure may include respiratory problems, altered blood cells, cancer, nerve damage, and liver or kidney damage. Chronic exposure to heavy metals may also have reproductive side effects. Some heavy metals and their compounds can accumulate in the body's tissues, such as in the bones or in nerves. In pregnant women, heavy metals and their compounds may cross the placenta and harm an unborn child.
One of the most famous cases of mercury poisoning resulting from chronic exposure occurred in Minamata, Japan. The city's harbor was contaminated with industrial wastes that contained methylmercury. Fish in the harbor were likewise contaminated and people who ate these fish were gradually poisoned. The methylmercury accumulated in their brains and caused severe, irreversible damage. More than 120 people were poisoned; 22 of them prior to birth because their mothers had unknowingly eaten contaminated fish.
Children are especially susceptible to health problems caused by heavy metals, because their bodies are smaller and still developing. Exposure to lead before birth or during infancy and childhood may lead to birth defects, developmental delays, and decreased intelligence.
Environmental controls are in place in many countries to limit exposures to heavy metals. However, these controls may be inadequate where the technology to limit heavy metal emissions is not available or the rules are not enforced. Also, accidents have occasionally occurred in which many people are exposed. Naturally occurring heavy metal contamination is difficult to control.
Other exposures occur because current rules do not apply to past actions. For example, lead was once commonly used in house paints before it was apparent that low levels of lead could present a health problem. Lead is not used in modern house paints, but many houses were built and painted before the rules went into effect. Therefore, older houses often pose a potential lead poisoning threat, especially for young children.
Heavy metal poisoning is treated by a class of drugs called chelating agents. Although the details of how each chelating agent works differ, they all share the same basic function. They work by binding to heavy metals in the body and forming nontoxic compounds with them. These nontoxic compounds can be removed much more easily than the heavy metals alone. Common chelating agents include British anti-lewisite (BAL) and related compounds, ethylenediaminetetraacetic acid (EDTA), and penicillamine.
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