Lead

Lead 207.2(1) +2 +4   Pb 82 1s22s22p63s23p63d104s24p6 4d105s25p64f145d106s26p2 14,6,p [ ? ] Post-Transition Metal: Properties: corrosion-resistant, dense, ductile, and malleable blue-gray transition metal Hazard: toxic

Lead is a chemical element, typically found as a solid in its elemental form. It has the chemical symbol Pb (from the Latin plumbum), atomic number (number of protons) Z = 82, and a standard atomic weight of 207.2 g/mol.

Lead is in the class of heavy metals, and is abundant in nature. Lead is a very corrosion-resistant, dense, ductile, and malleable blue-gray metal that has been used for at least 5,000 years.

Early uses of lead included building materials, pigments for glazing ceramics, and pipes for transporting water. Prior to the early 1900s, uses of lead were primarily for ammunition, brass, burial vault liners, ceramic glazes, leaded glass and crystal, paints or other protective coatings, pewter, and water lines and pipes. The advent of the electrical age and communications, which were accelerated by technological developments in World War I, resulted in the addition of bearing metals, cable covering, caulking lead, solders, and type metal to the list of lead uses. With the growth in production of public and private motorized vehicles and the associated use of starting-lighting-ignition (SLI) lead-acid storage batteries and terne metal for gas tanks after World War I, demand for lead increased. Later, radiation shielding in medical analysis and video display equipment and as an additive in gasoline also increased usage.

History

Long known, mentioned in Exodus. The ancients regarded lead as the father of all metals, but the deity they associated with the substance was Saturn, the ghoulish titan who devoured his own young. The very word "saturnine," in its most specific meaning, applies to an individual whose temperament has become uniformly gloomy, cynical, and taciturn as the results of lead intoxication.

In the rigidly hierarchical world of the ancients, lead was the plebeian metal deemed suitable for a vast variety of everyday uses. Lead products were, to a certain degree, accessible even to the poorest proletarian. But only the chosen few were at the top of the social totem pole were able to regularly indulge their insatiable craving for lead-containing products.

Lead was a key component in face powders, rouges, and mascaras; the pigment in many paints ("crazy as a painter" was an ancient catch phrase rooted in the demented behavior of lead-poisoned painters); a nifty spermicide for informal birth control; the ideal "cold" metal for use in the manufacture of chastity belts; a sweet and sour condiment popular for seasoning and adulterating food; a wine preservative perfect for stopping fermentation or disguising inferior vintages; the malleable and inexpensive ingredient in pewter cups, plates, pitchers, pots and pans, and other household artifacts; the basic component of lead coins; and a partial ingredient in debased bronze or brass coins as well as counterfeit silver and gold coins.

Most important of all was lead's suitability as inexpensive and reliable piping for the vast network plumbing that kept Rome and the provincial cities of the Roman Empire supplied with water. Indeed, the very word "plumbing" comes from the Latin word for lead, plumbum. The lead pipes that were the vital arteries of ancient Rome were forged by smithies whose patron saint, Vulcan, exhibited several of the symptoms of advanced lead poisoning: lameness, pallor, and wizened expression.

Addicted to lead

The Romans were aware that lead could cause serious health problems, even madness and death, but they were fond of its diverse uses and so minimized the hazards it posed. They equated limited exposure to lead with limited risk. What they did not realize was that their everyday low-level exposure to the metal rendered them vulnerable to chronic lead poisoning, even while it spared them the full horrors of acute lead poisoning.

The symptoms of acute lead intoxication appeared most vividly among miners who were thrown into unhealthy intimacy with the metal on a daily basis. Romans reserved such debilitating and backbreaking labor for slaves. Some of these unfortunates were forced to spend all of their brief and blighted lives underground, out of sight and out of mind. The unpleasantness of lead mining was further neutralized late in the Empire when the practice was prohibited in Italy and consigned completely to the provinces.

Lead smelting, which had once been commonplace in every Roman city and town, eventually followed mining operations to the provinces. Italy, the heart of imperial Rome, grew tired of the noxious fumes emanating from lead smelting forges. The obvious damage to the health of smithies and their families was a matter of little or no concern.

Roman aristocrats, who regarded labor of any sort as beneath their dignity, lived oblivious to the human wreckage on which their ruinous diet of lead depended. They would never dream of drinking wine except from a golden cup, but they thought nothing of washing down platters of lead-seasoned food with gallons of lead-adulterated wine.

The result, according to many modern scholars, was the death by slow poisoning of the greatest empire the world has ever known. Symptoms of "plumbism" or lead poisoning were already apparent as early as the first century B.C. Julius Caesar for all his sexual ramblings was unable to beget more than one known offspring. Caesar Augustus, his successor, displayed not only total sterility but also a cold indifference to sex.

The first century A.D. was a time of unbridled gluttony and drunkenness among the ruling oligarchs of Rome. The lead concealed in the food and wine they devoured undoubtedly had a great deal to do with the outbreak of unprecedented epidemics of saturnine gout and sterility among aristocratic males and the alarming rate of infertility and stillbirths among aristocratic women.

Still more alarming was the conspicuous pattern of mental incompetence that came to be synonymous with the Roman elite. This creeping cretinism manifested itself most frighteningly in such clearly degenerate emperors as Caligula, Nero, and Commodus. It is said that Nero wore a breastplate of lead, ostensibly to strengthen his voice, as he fiddled and sang while Rome burned. Domitian, the last of the Flavian emperors, actually had a fountain installed in his palace from which he could drink a never-ending stream of leaded wine.

Medieval and Renaissance lead

During the Middle Ages, lead was widely used by alchemists as a key component in procedures thought to be capable of generating gold from baser metals. Lead served an even more lofty function when leaded type launched Gutenberg's galaxy late in the fifteenth century. Mass printing was crucial to the eradication of ignorance that led to the upheavals of the Reformation and the Enlightenment. Lead could even be found in considerable quantities in decorative fixtures, roofs, pipes, and windows in the castles and cathedrals of Europe

Kinkier and more destructive uses of lead never lagged far behind. The advantages of the metal as an invisible and slow-acting poison were not lost on the Lucrezia Borgias and Catherine de Medicis of Renaissance Europe. Lead was known to be extremely convenient for eliminating inconvenient relatives. In fact, the world-weary French jokingly referred to the metal as poudre de la succession -- or succession powder. Another sinister latter-day use of lead was, of course, in the mass production of pistols, rifles, and cannons and the ammunition designed to blaze a bloody trail from their barrels.

Lead mining and smelting began in the New World almost as soon as the first colonists were settled. By 1621 the metal was being mined and forged in Virginia. The low melting temperature of lead made it highly malleable, even at the most primitive forges. Furthermore, lead's resistance to corrosion greatly enhanced its strength and durability. Technological progress in the American colonies and the American republic was to owe a great deal to this useful and abundant metal.

Lead in modern times

By the twentieth century, the U.S. had emerged as the world's leading producer and consumer of refined lead. According to the National Academy of Science's report on Lead in the Human Environment, the United States of America was by 1980 consuming about 1.3 million tons of lead per year. This quantity, which represents roughly 40 percent of the world's supply, translates into a usage rate of 5,221 grams of lead per American per annum: a rate of dependence on lead and lead-containing products nearly ten times greater than that of the ancient Romans! According to Jerome O. Nriagu, the world's leading authority on lead poisoning in antiquity, the comparable Roman rate of lead usage was approximately 550 grams per person per year.

Sources and properties

Lead is obtained chiefly from an ore called galena, which is primarily lead sulfide (PbS), by a roasting process. Anglesite, cerussite, and minim are other common lead minerals.

Properties

Lead is a bluish-white metal of bright luster. It is very soft, highly malleable, ductile, and a poor conductor of electricity. It is very resistant to corrosion; lead pipes bearing the insignia of Roman emperors, used as drains from the baths, are still in service. It is used in containers for corrosive liquids (such as sulfuric acid) and may be toughened by the addition of a small percentage of antimony or other metals.

Forms

Natural lead is a mixture of four stable isotopes: ²⁰⁴Pb (1.48%), ²⁰⁶Pb (23.6%), ²⁰⁷Pb (22.6%), and ²⁰⁸Pb (52.3%). Lead isotopes are the end products of each of the three series of naturally occurring radioactive elements: ²⁰⁶Pb for the uranium series, ²⁰⁷Pb for the actinium series, and ²⁰⁸Pb for the thorium series. Twenty seven other isotopes of lead, all of which are radioactive, are recognized.

Uses

Its alloys include solder, type metal, and various antifriction metals. Great quantities of lead, both as the metal and as the dioxide, are used in storage batteries. Much metal also goes into cable covering, plumbing, ammunition, and in the manufacture of tetraethyl lead, (CH₃CH₂)₄Pb.

The metal is very effective as a sound absorber, is used as a radiation shield around X-ray equipment and nuclear reactors, and is used to absorb vibration. White lead, the basic carbonate, sublimed white lead, chrome yellow, and other lead compounds are used extensively in paints, although in recent years the use of lead in paints has been drastically curtailed to eliminate or reduce health hazards.

Lead oxide is used in producing fine "crystal glass" and "flint glass" of a high index of refraction for achromatic lenses. The nitrate and the acetate are soluble salts. Lead salts such as lead arsenate have been used as insecticides, but their use in recent years has been practically eliminated in favor of less harmful organic compounds.

Lead paint

In more recent years lead was widely used to extend the protective properties of paints, helped automobiles attain better fuel efficiency, protected occupation ally exposed workers from harmful radiation and provided a suitably dense material for ammunition and fishing weights. Even though it is no longer used in many of these applications, millions of homes remain painted with lead paint. It's been estimated that (as of 2008) a large percentage of the residential housing in San Francisco which was built prior to 1978 probably has lead-based paint. Lead-based paint chips, as well as soil and household dust contaminated with lead are the primary sources of childhood lead poisoning.

Tetra-ethyl lead as a gasoline additive

Tetra-ethyl lead, commonly referred to as TEL, is a viscous liquid with the chemical formula (CH₃CH₂)₄Pb. Once widely used (circa 1925 to 1990) to increase the octane rating of gasoline (petrol), TEL usage in gasoline has been largely phased out by most nations primarily because of the toxicity of the lead emissions from internal combustion engines burning gasoline containing TEL. Another reason for discontinuing TEL usage was that it degraded the efficiency of the catalytic converters installed in automotive vehicles to reduce their emissions of air pollutants.

Ammunition

Lead, due to its density, has long been used as a military projectile, including as a sling bullet prior to the invention of firearms. While there are some reports of lead poisoning from bullets not removed from a living body, the kinetic effects of being shot usually far outweigh any chemical toxicity.

Lead poisoning from a retained bullet or missile is rare and is usually

dependent on the location of the missile in a bone or immediately adjacent to a joint. A review of the literature revealed only 14 cases in which there was adequate laboratory documentation of plumbism caused by a retained bullet or missile. Only one of these previously reported cases resulted in death. We report a second death due to lead poisoning from a retained bullet

with elevated blood lead levels documented by toxicologic analysis.

There is, however, concern about the environmental effects of large quantities of bullets and shotgun pellets in nature, and there have been efforts to remove lead from military firing ranges, where they may leach into groundwater. A number of programs, generically called "green bullets", have tried to replace lead with a less toxic metal, usually tungsten with nonmetalic material. Questions have been raised, however, about the safety of tungsten.

Health effects

Lead can be an acute and chronic poison. Lead compounds are more toxic than the metal, but the metal needs to be handled with care; respiratory protection is needed when aerosols of lead may be produced.

With the reduction in lead tetraethyl, most lead poisoning is from inorganic compounds. In children, oral ingestion from contaminated hands or chewing lead-painted objects is most common, but it can be inhaled or absorbed through the skin.

Factors influencing absorption are coupled to the absorption of essential metal nutrients such as iron and calcium. The amount absorbed is affected by factors including:

• Amount of absorbable solution; acid media have greater absorption
• Particle size. Smaller particles are more likely to be absorbed.
• In the presence of deficiencies of iron, calcium, zinc, copper, and protein, lead absorption will increase.
• High intake of fats and oils increase absorption, but leafy green vegetables and other nutrients decrease absorption.

Transcutaneous absorption of inorganic lead is minimal. However, organic lead, such as tetraethyl lead found in leaded gasoline, may enter through the skin. Tetraethyl lead, the main organic compound in leaded gasoline, is converted in the body to triethyl lead and inorganic lead.

Diagnosis and treatment of childhood lead poisoning

From a public health standpoint, the most serious concern is chronic lead poisoning in children. There are a number of suggestive physical symptoms, most notably a "lead line" along the gums. A thorough history is extremely important, to identify factors such as exposure to lead paint. Blood lead levels are the definitive test.

With low to moderate levels, prevention, decontamination, and supportive measures usually suffice. At significant levels, after confirmatory retesting, chelation therapy is warranted, starting with oral succimer (an analog of dimercaprol), with parenteral edetate calcium disodium, then a combination of this agent and dimercaprol. These are not benign drugs and careful risk-benefit analysis is needed.

Behavioural effects

Antisocial behaviours and serious, violent criminal activities were shown to be correlated to male sex, poverty, tobacco smoke exposure in utero, and being reared by antisocial parents. A 2008 prospective study suggested that lead exposure in the womb and in early life was another important determinant of later life violence and antisocial behaviour.

In the United States of America, low-level lead exposure in infancy is a major factor of attention deficit hyperactivity disorder (ADHD), coming slightly before prenatal tobacco exposure. Lead exposure, as opposed to tobacco exposure through maternal smoking, is not a clear-cut issue. Subclinical hyperactivity and consecutive intellectual deficits may well be attributed to background levels of lead to which the whole population is exposed. In a sample of 97 children and teens diagnosed with ADHD, blood lead levels were correlated to hyperactivity-impulsiveness (but not inattention-disorganization), and this behavioural derangement was demonstrated to be causative of IQ deficits. It was concluded that lead impaired cognitive control and, neurophysiologically, the circuits connecting the striatum to the frontal cortex.

References and notes

This article outline was originally sourced from http://periodic.lanl.gov/elements/82.html (Los Alamos National Labs) and the history section from http://www.epa.gov/history/topics/perspect/lead.htm (Jack Lewis EPA Journal - May 1985) accessed on 3/22/08.