Metals

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Metals are unique toxins. They are neither created nor destroyed (neglecting nuclear reactions), yet they readily change form and activity. A subgroup in which we are most interested is the heavy metals. Heavy metals are those with atomic number 22 to 34 and the elements below them in the periodic table, plus the lanthanides and actinides. Metals are widely and unevenly dispersed in the environment. Human activities that result in exposure to significant amounts of metals include mining and smelting operations, metal plating, fuel combustion, leather tanning, and their use in products from pigments to pipes.

Some metals are essential nutrients but are toxic at higher levels. Examples include chromium, cobalt, copper, iron, selenium, and zinc. Even arsenic may be required. There are three main mechanisms of toxicity for metals:

1. Complexation with proteins, especially with sulfhydryl groups of cysteine, resulting in enzyme disruption

2. The same complexation mechanism as (1), but affecting other proteins, such as those embedded in membranes

3. Competition with essential metals, such as enzyme cofactors

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman Copyright © 2006 John Wiley & Sons, Inc.

Metals are excreted by the kidney, by reversing absorption in the intestines, and by enterohepatic circulation. Because methylmercury concentrates in the latter pathway, the mercury can be removed by oral administration of adsorbents such as activated carbon.

Some aquatic plant and animal species growing in contaminated areas have been observed to develop tolerance for copper and zinc. Animals respond to cadmium, copper, and mercury by producing the low-molar-mass protein metallothionein in the liver. The metallothionein binds to these metals and to zinc, and transports them to the kidney for storage. Additional dosages of cadmium or mercury can displace the less toxic zinc or copper, resulting in less harm than the cadmium or mercury would otherwise cause.

Copper has many sources, including its use in drinking water pipes and its deliberate addition to surface water as an algicide. Copper is essential for many enzymes. However, higher animals have mechanisms to conserve it and other essential trace metals during periods of deficiency, and increase excretion at higher dosages. Copper is also critical to plant life, playing a role in electron transport in photosynthesis. It is also very toxic to aquatic plants, accounting for its use as an algicide in recreational lakes and pools.

Cadmium is a particularly toxic metal. Sources of exposure include wastewater produced in metal plating, sewage sludge applied to plants, smelting, and other occupational exposures. Cigarette smoking is an important source and may double the average body burden. Cadmium becomes complexed with metallothionein and stored in the kidney, where it accumulates over a lifetime. It is excreted very slowly, having a half-life of about 20 years. Damage occurs when a "critical concentration" of about 200 to 300 mg/g occurs in the kidney. It affects proximal tubules, reducing resorption of glucose and amino acids, and causing hypertension (high blood pressure). Inhalation can cause fibrosis, chronic bronchitis, and emphysema. Prostate cancer has been reported in occupational exposures. Calcium is lost, causing bone disorders, including pain, osteoporosis, and deformities. In the over 200 enzymes that require zinc as a cofactor, cadmium can displace the zinc competitively. Zinc is also a metabolic antagonist of cadmium. Therefore, high zinc intakes give some protection against cadmium toxicity.

Mercury is released to the environment by the burning of fossil fuel and refuse. It was formerly a major pollutant generated by the chloro-alkali industry. Food has about 5 to 20 mg/kg, except fish: Tuna and swordfish range from 200 to 1000 mg/kg. In the environment, mercury (Hg) is oxidized to inorganic ions. Anaerobic bacteria convert it to organic mercury, especially methylmercury or dimethylmercury. Organic mercury is much more toxic, since it is more readily absorbed. Dimethylmercury is volatile and can escape to the atmosphere from contaminated water and sediment. In the body, methylmercury is converted to the divalent inorganic form. Mercury has a very high affinity for sulfhydryl groups and thus can affect almost any enzyme and membrane-bound proteins. The effect on membranes is to increase their permeability to sodium and potassium, affecting cell osmolarity. Inorganic mercury is toxic to the kidney. Organic forms (methyl- or ethylmercury) target the nervous system. Divalent mercury is corrosive. If ingested, it causes abdominal cramps and bloody diarrhea, followed by renal damage. Calomel (monovalent or mercurous chloride) is less toxic but is associated with skin reactions. Elemental mercury vapor is an occupational hazard affecting the central nervous system. Neurological symptoms appear at exposure to concentrations as low as 0.05 mg Hg/m3. The half-life for elimination of ingested mercury in seals, fish, and crabs has been measured to range from 267 to 700 days.

Lead is found in batteries, gasoline additives, paint, lead pipe, and brass fixtures. Many uses have been phased out recently, particularly indoor paint, gasoline, and pipe solder. Even without the lead pipe and pipe solder sources, brass remains an important source in plumbing systems. This is because brass fixtures are made with about 8% lead to improve machinability. Inorganic lead affects heme; organic lead affects the nervous system. Anemia is found at a blood lead level of 50 mg/dL; enzyme effects are found at as low as 10. Above 80 mg/dL (70 in children) encephalophathy can occur with symptoms including vomiting, lethargy, irritability, and so on, and possibly proceeding to coma and death. In children, a moderate exposure can produce measurable mental defects; 40 to 50 mg/dL can result in hyperactivity, a decrease in attention span, and a slight reduction in IQ score. In occupational exposures, 40 mg/dL can cause peripheral neuropathy, nerve damage causing numbness and the classic symptoms "footdrop" and "wristdrop." A decreased calcium intake causes lead to redistribute from storage in the bone, where it is harmless, to the more vulnerable kidney. The organic lead in auto exhaust from burning leaded fuel degrades rapidly but becomes a source of inorganic lead.

Organotin compounds are used in metal coatings to prevent biological growth on ship hulls and in cooling towers. Tributyltin is one of the common compounds used in this way. However, it has been found to exert significant toxic effects in estuaries after leaching from ship hulls.

Chromium is used in leather tanning, paints, and pigments. It is found in valences from +II to +VI, but the trivalent and hexavalent are the most important biologically. Trivalent chromium is the most common form. It is an essential nutrient, being needed as a cofactor for insulin. Hexavalent chromium is corrosive and causes allergic skin reactions on dermal exposure and cancer by inhalation. The toxicity of hexavalent chromium may be caused by reactions involving the reduction of the hexavalent to the trivalent form within cells.

Zinc is commonly used in galvanic protection of iron. The mechanism of protection involves the oxidation of the zinc to soluble ionic forms. Zinc toxicity causes anemia in mammals by interfering with absorption and utilization of copper and iron. The zinc level that causes toxic effects depends strongly on the ratio of zinc to copper.

Arsenic is found as high as 5 mg/kg in seafood. Trivalent arsenite is the most toxic form, followed by pentavalent. It combines reversibly with thiol groups in tissues and enzymes and can substitute for phosphorus in biochemical reactions such as oxidative phosphorylation. Occupational exposure has been associated with lung cancer but has not been shown to cause cancer in lab animals. Ingestion has been related to cancer of the skin, liver, bladder, and lung. Noncarcinogenic effects range from loss of appetite to irritation of epithelial tissues to paralysis of the hands and feet. The U.S. drinking water standard is 10 mg/L as of January 2006. Natural levels of arsenic as high as 280 mg/L are found in drinking waters in New Hampshire. Some 20 to 60 million people in Bangladesh use water above 50 mg/L, some as high as 2000 mg/L. Another effect commonly found in Bangladesh is thickening and discoloration of the skin. As a result, victims there have been socially ostracized. Twenty to thirty years after exposure to 500 mg/L, about 10% of people develop cancer; 60 mg/L in drinking water is rapidly fatal.

Aluminum has been found to be concentrated in the brains of people with Alzheimer's disease (AD). However, it is not known if this is a cause or a result of the disease. There are a number of epidemiological studies linking dementia to aluminum in drinking water. However, other evidence contradicts this, such as the fact that studies of humans and animals exposed to large amounts of aluminum do not show development of AD. Aluminum has also been found to cause skeletal problems in adults and infants who received large doses as part of a medical therapy.

Beryllium causes pulmonary fibrosis when inhaled, hypersensitivity reactions on dermal contact, and is a probable human carcinogen. Inhaled nickel is an occupational

TABLE 21.1 Acute Toxicity: 48- to 96-h LC50 or EC50 of Several Aquatic and Marine Phyla3

Organism

Cadmium

Chromium

Copper

Mercury

Lead

Zinc

Arthropods

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