See also occupational disease.

Sources

Many of the traditional sources of lead in the United States have been minimized by a variety of federal laws, enacted from 1978 on, banning lead paint and glazes and leaded gasolines, and prohibiting the use of lead pipes in construction and the use of lead solder in food and soda cans. Workplace exposure has been regulated by laws requiring the use of respirators, dust suppressors, and proper ventilation, and lead waste disposal guidelines have been developed. Continuing sources of environmental lead include water that has passed through old lead pipes, paint in older buildings, lead improperly disposed of in public landfills, and industrial sources such as mining, smelting, and recycling processes necessary to produce lead for batteries and other products.

Young children are usually exposed by ingesting paint chips containing lead. This source is most prevalent in poor areas where old, peeling lead-containing paint and plaster in rundown housing is common. Inadequately nourished or emotionally deprived children who resort to chewing inedible things (a condition known as pica) are most susceptible.

Effects and Treatment

Acute lead poisoning can result in abdominal discomfort, nervous system damage, and encephalitis. Chronic exposure is characterized by a blue line on the gums and can lead to damage to the brain, kidneys, nervous system, and red blood cells. Even low levels can contribute to hypertension in older people or to "silent lead poisoning" in exposed children, which affects the developing brain and leads to visual-motor problems and lowered intelligence. Lower doses may be treated by altering the diet to counteract lead's effects and and cleaning the person's environment to reduce intake. Higher doses are treated with chelating agents, drugs that remove lead from the body. Symptoms recur upon subsequent exposure.

Properties and Isotopes

Lead is a dense, relatively soft, malleable metal with low tensile strength. It is a poor conductor of electricity and heat. Lead has a face-centered cubic crystalline structure. It is below tin in Group 14 of the periodic table. Although lead has a lustrous silver-blue appearance when freshly cut, it darkens upon exposure to moist air because of the rapid formation of an oxide film; the film protects the metal from further oxidation or corrosion. All lead compounds are poisonous (see lead poisoning). Lead resists reaction with cold concentrated sulfuric acid but reacts slowly with hydrochloric acid and readily with nitric acid.

The element has four naturally occurring stable isotopes, three of which result from the decay of naturally occurring radioactive elements (thorium and uranium). Since this decay takes place at a constant rate, it is possible to predict either the maximum age of a lead-containing rock or its composition at some earlier date, as long as the rock has not been chemically altered. There are 25 known radioactive isotopes of lead, some of which occur naturally in small amounts.

Natural Occurrence and Processing

Although lead is seldom found uncombined in nature, its compounds are widely distributed throughout the world, principally in the ores galena, cerussite, and anglesite. Australia, the United States, Canada, and Russia are among the chief producers of lead. In the United States galena (a lead sulfide ore) is mined in southern Missouri, with some ore coming from the western states. The ore is concentrated by the flotation process and is then refined by electrolysis or by smelting. About one third of the lead used in the United States is so-called secondary lead, i.e., lead and lead alloys reclaimed chiefly from automobile batteries.

Uses

The single most important commercial use of lead is in the manufacture of lead-acid storage batteries (see battery, electric). It is also used in alloys such as fusible metals, antifriction metals, solder, and type metal. Shot lead is an alloy of lead, antimony, and arsenic. Lead foil is made with lead alloys. Lead is used for covering cables and as a lining for laboratory sinks, tanks, and the "chambers" in the lead-chamber process for the manufacture of sulfuric acid. It is used extensively in plumbing. Because it has excellent vibration-dampening characteristics, lead is often used to support heavy machinery and was used in the foundations of the Pan Am Building built over Grand Central Station in New York City. Lead is also employed as protective shielding against X rays and radiation from nuclear reactors.

Lead has many commonly used compounds. Commercially important are the lead oxides, which have many uses. Litharge is lead monoxide, PbO; red lead is lead tetroxide, Pb₃O₄; lead peroxide or dioxide, PbO₂, is used in matches, as a mordant in dyeing, and as an oxidizing agent. White lead, 2PbCO₃·Pb(OH)₂ (basic lead carbonate), is an important pigment used in paints, putty, and ceramics. Chrome yellow, PbCrO₄, is a bright yellow pigment. "Sublimed white lead," PbSO₄·Pb(OH)₂ (basic lead sulfate), is also used as a pigment. Lead acetate (sugar of lead) is used as a mordant, and lead azide, Pb(N₃)₂, is employed as a detonator for explosives. Lead arsenate is used as an insecticide. Tetraethyl lead, used as a antiknock compound in gasoline, is now banned for environmental reasons in the United States and other countries.

Although lead and most of its compounds are only slightly soluble in water, the use of lead pipe to carry drinking water is dangerous, since lead is a cumulative poison that is not excreted from the body (see lead poisoning). The "lead" of lead pencils does not contain lead; it is a mixture of graphite and clay.

Method of dating very old rocks by means of the amount of common lead they contain. Common lead is any lead from a rock or mineral that contains a large amount of lead and a small amount of the radioactive precursors of lead (i.e., the isotopes uranium-235, uranium-238, and thorium-232). By this method, the age of the Earth has been estimated to be circa 4.6 billion years. This figure is in good agreement with the age of meteorites and the age of the Moon as determined independently.

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Poisoning by accumulation of lead in the body. Large doses cause gastroenteritis in adults and brain disorders in children. Anemia, constipation and abdominal spasm, confusion, a progressive paralysis, and sometimes brain cancer result from chronic exposure. Children are particularly susceptible to nerve and brain damage; sensitive tests show that even low levels of lead can harm children and are linked to behavioral problems. Sources in the home include lead-based paint, lead drinking-water pipes, and lead-glazed tableware. Babies, who put things in their mouths, are at highest risk. Working where lead is used and exposure to some insecticides are other risk factors. The U.S. phaseout of lead in gasoline was completed in 1996; similar bans are being implemented worldwide. Treatment involves giving antidotes that bind (see chelate) the lead in the tissues.

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Gray lead sulfide (PbS), the chief ore mineral of lead. One of the most widely distributed sulfide minerals, it occurs in many types of deposits and in many localities. In the U.S., galena is mined principally in the Mississippi River Valley. Galena often contains silver and so is often mined for that metal as well as for lead. Other commercially important minerals that frequently occur in close association with galena are antimony, copper, and zinc.

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Metallic chemical element, chemical symbol Pb, atomic number 82. Lead is a soft, silvery white or grayish, malleable, ductile, dense metal that conducts electricity poorly. Its stable isotopes are all end products of radioactive decay of uranium and other heavy elements. Known since ancient times, lead is so durable and resistant to corrosion that Roman lead pipes are still usable. Lead is used in roofing, as cable coverings, and in pipes, conduits, and structures. Other uses are in storage batteries, ammunition, and low-melting-point alloys (e.g., solder, pewter) and as shielding against sound, vibrations, and radiation. Lead is rarely found free in nature; its major ore is the sulfide galena (PbS). Because it and its compounds are poisons (see lead poisoning), lead-based paints and gasoline additives have been phased out in many countries. Lead in compounds has valence 2 and 4; an oxide (litharge, PbO) is the most widely used. Lead compounds are added to lead crystal (see glass), glazes, and ceramics and are used as pigments, drying agents for paints and varnishes, insecticides and herbicides, and fireproofing agents and in matches, explosives, and pyrotechnics. Almost half of all lead is recovered from recycled scrap. The “lead” in pencils is graphite.

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Mineral allotrope of carbon. It is dark gray to black, opaque, and very soft. Its layered structure, with rings of six atoms arranged in widely spaced parallel sheets, gives it its slippery quality. It occurs in nature and is used (mixed with clay) as the “lead” in pencils. It is also used in lubricants, crucibles, polishes, arc lamps, batteries, brushes for electric motors, and nuclear reactor cores.

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