In a scene from the 1978 movie "Superman," the suave superhero tries to demonstrate his X-ray vision on Lois Lane, who is standing behind a planter. When unsuccessful, he explains that the planter must be made of lead and that he has trouble seeing through the substance. Later in the movie, bad guy Lex Luthor captures Superman by hiding the mineral kryptonite inside a lead box. When Superman opens the box, he's exposed to kryptonite, which is toxic to him. What is this substance that can trick superheroes, and why is lead such a problem for mere mortals, too?
Lead is an element, specifically one of the heavy metals. It has the following properties:
- Bluish-white color
- Solid at room temperature
- Dense (11.4 times denser than water)
- Malleable, or able to be molded
- Ductile, or easily drawn into wire
- Doesn't conduct electricity well
Chemically, the most common form of lead atom has 82 protons, 82 electrons and 125 neutrons (for a refresher, see How Atoms Work). It has four electrons in its outer energy level or shell, which means it can form up to four chemical bonds with other elements. In the Earth's crust, lead is rarely found as a pure metal, but rather as lead compounds such as lead sulfide, lead sulfate or lead oxide. These lead compounds often occur with silver.
Lead has no role in the human body and, as you're undoubtedly aware, is quite toxic. In the 1970s, many American children suffered from lead poisoning through exposure to lead paints. Such paints are no longer produced in the United States, and the incidence of lead exposure through this source has gone down, except in older houses (built before 1978). However, lead paints are still used in other countries. For example, in 2007, lead hit the news when many toys made in China were recalled because they contained lead-based paints (see What's with China and lead poisoning?). Lead remains a leading environmental hazard worldwide. Here, we'll discuss the extraction and refining of lead, its industrial uses, and its adverse health and environmental effects.
Leaded Gas and Other Uses for the Heavy Metal
Since ancient times, humans have known about lead. Lead was originally a novelty with little use or value. Its first use was for artwork. But entrepreneurial Romans used lead extensively and took advantage of its malleability and resistance to corrosion. They made lead pipes to carry water and to drain sewage water. They also used lead to line containers that stored water. The English word "plumbing" and the chemical symbol for lead (Pb) come from the Latin word plumbum, which just means "lead." Lead plumbing from Roman times survives today in Roman baths and structures.
Aside from its role in plumbing, lead has been added to cosmetics, colorful paints and pigments, glass, pewter jewelry and tableware, munitions and pottery. In the 20th century, lead could also be found in household paints, plumbing, cable sheathing and additives to gasoline (tetraethyl lead). However, as health officials began to recognize the metal's toxic effects on human and environmental health, the use of lead in these areas has been greatly reduced, if not eliminated.
Today, you predominantly find lead in lead acid batteries for electrical power, like your car battery. According to the International Lead Association, 70 percent of lead-acid batteries are recycled and used for secondary lead production.
Besides batteries, you can find lead in roofing products and radiation shielding -- the element's high density makes it ideal for absorbing gamma radiation and X-rays. Most glass cathode ray tubes (like those in computer monitors) have leaded glass in them to shield the viewer from radiation made inside. You'll also find lead added to glass for decorative crystal. Lastly, lead solder is good for making electrical connections, and the element is also handy for ceramic devices used in the electronics industry.
Where does all that lead come from? It's hidden in the Earth's crust, primarily as a sulfide (PbS) in the mineral galena. Currently, 75 percent of the world's lead comes from China, the United States, Australia, Canada, Mexico and Peru [source: International Lead Association]. More lead is produced secondarily by recycling lead scrap.
Before lead can exist in its finished form, you have to process and refine the lead ore. The Romans used a process called cupellation to separate silver from lead. Today, the lead industry extracts the element using the processes of roasting and smelting, which are similar to the methods the Romans used.
1. Roasting: Galena is heated in air to convert lead sulfide (PbS) to lead oxide (PbO), plus sulfur dioxide.
2PbS + 3O2 --> 2PbO + 2SO2
2. Smelting: Coke (carbon -- C) is added to lead oxide (PbO) and mixed with air in a high-temperature blast furnace to obtain lead metal. In the shaft of the furnace, the carbon displaces the lead from lead oxide to form carbon dioxide gas (CO2) and molten lead (Pb).
2PbO + C --> 2Pb + CO2
The molten lead sinks to the bottom of the furnace, is siphoned off and cools to form lead bricks or pigs (big, oblong masses). Slag, a nonmetallic by-product of metal smelting, separates from molten lead, gets siphoned off, cooled and dumped as waste. Molten lead extracted through smelting often contains other metal impurities, such as zinc, arsenic, copper, silver, gold and bismuth. Electrowinning removes these impurities with the help of an electrical current.
Besides extracting lead from ore, the heavy metal can also be produced from secondary sources such as used batteries and scrap lead. Secondary extraction also involves processing the raw materials, smelting (or turning reclaimed lead back into alloys or its elemental form), cooling and casting. For a virtual tour of a secondary lead smelter and its environmental controls, see OSHA's Secondary Lead Smelter eTool.
Next, learn how lead gets into our bodies.
How Are We Exposed to Lead?
Even though public health officials grew wise to the toxicity of lead decades ago, lead still lingers in the paint and pipes of older homes and other buildings (built before 1978). Lead-based paint chips and flakes that peel away from the interior and exterior walls are one source of exposure. Water that travels through and leaches from lead pipes is another source. Some home repairs, such as removing painted drywall, can send lead dust into the air. In addition, lead from soldered metal containers, lead-glazed pottery, lead glass crystal and lead-painted toys serve as other routes of exposure in the home.
In the environment, lead-based products can leach the element into the water and the soil. For example, water running from lead drainpipes or running off walls painted with lead-based paints will carry the metal into the soil. Waste dumps where lead batteries may have been improperly disposed also can leach lead into the ground.
What about the air? In 1970, the bulk of lead emissions (78 percent) came from on-road sources, such as cars fueled with leaded gasoline, according to the U.S. Environmental Protection Agency. Metal processing -- remember smelting? -- contributed another 11 percent,. When the EPA ran the numbers about a quarter-century later, the agency found that the emission sources had changed dramatically. Metal processing now claimed 52 percent of lead emissions, while waste disposal (16 percent), fuel combustion (13 percent), nonroad sources (13 percent) and other sources (6 percent) trailed far behind. The drastic change, of course, was due to the ban on using lead additives in motor fuels that took effect in February 1996. Many other countries have enacted similar bans.
Lead in the air, water or home can accumulate in your body in several ways. For example, you could drink contaminated water or swallow lead dust. You probably don't like to eat paint chips or lick toys with lead-based paints, but your kids might. Either way, once it's ingested, lead gets absorbed by the small intestine and enters the bloodstream.
Lead also can enter the body through the lungs when you inhale lead dust or lead-containing air pollution. Workers in the metal processing industry (smelting) and construction workers may be exposed to lead dust and fumes. Once inhaled, lead gets absorbed through capillaries in the lungs and enters the bloodstream. People who work with lead may also absorb it directly through the skin.
Doctors measure lead exposure by taking a sample of blood (finger or heel stick). They can also measure exposure by assessing the zinc protoporphyrin levels in your blood. This zinc compound is a by-product of the breakdown of red blood cells observed in high lead exposure; however, this test is not as sensitive as the blood lead test.
Keep reading to learn how to get the lead out.
Lead Effects and Treatment
Unlike zinc or iron, say, there's no biological role for lead in the body. It's just a toxin. Your body, however, treats lead like calcium, so it accumulates in places where calcium is stored, like your bones. Because calcium has essential roles in the communication between nerve cells, muscle contraction and hormones, lead can interfere with these processes.
- Cardiovascular system: high blood pressure, enlarged heart, electrical conduction problems
- Digestive system: pain, constipation or diarrhea, lack of appetite
- Nervous system: headaches, moodiness, irritability, fatigue, memory loss
- Renal system: kidney failure, increased urea levels in the blood, anemia
- Reproductive system: decreased sexual drive and sterility/infertility in both sexes
- Skeletal system: pain in bone and joints as lead accumulates in these areas
These symptoms will be readily noticed in cases of acute lead poisoning with relatively high blood lead levels. But they may not be noticeable in less acute cases, only surfacing during chronic exposure as the blood lead levels gradually rise.
The predominant effect of lead poisoning in children is on their brain. A child's brain isn't completely "hard-wired" like your home. Instead, as a child's brain develops, new neural circuits form in response to new experiences, learning and memories (neuroscientists call this ability to form new connections plasticity). These circuits involve one or more neurons communicating with others in various parts of the brain by sending and receiving chemical messages called neurotransmitters. The new circuits use the neurotransmitter, glutamate.
Now, because the body doesn't distinguish lead from calcium very well, the lead can interfere with glutamate release from the presynaptic cell and a special receptor in the postsynaptic cell called the NMDA receptor. This interference disrupts the plasticity that's so important in the developing brains of children.
The best treatment for lead poisoning is prevention. The U.S. Centers for Disease Control and Prevention recommends that all children ages 6 and under be tested for lead. If you have potential sources of lead in your house -- peeling lead-based paint or lead pipes -- then you or a professional should try to remove them (by professional cleaning, painting over old paint or removing hazardous material).
If blood lead levels are high, prescription medications like Succimer can reduce blood lead. In extreme cases of lead exposure, such as an industrial accident, then toxic levels of lead can be removed by chelation therapy. In this therapy, a chelating agent such as ethylenediaminetetraacetic acid (EDTA) is infused into the blood stream. The agent binds to the lead. The kidneys then excrete the lead-bound EDTA. Again, this treatment is usually reserved for extreme cases of lead poisoning.
We're not the only bodies negatively affected by lead. Lead enters the environment from contaminated water -- water coming from lead drainage pipes, lead-containing rainwater from air pollution or rainwater that filters through contaminated soil. The contaminated water runs into groundwater and aquatic ecosystems. Lead adversely affects the organisms in the lower parts of the food chain in these aquatic ecosystems, such as plankton (microscopic plants and animals) and mollusks. When these organisms die, they decompose and release their lead back into the environment. When they're eaten by organisms higher in the food chain such as fish, marine mammals, birds and humans, then the lead accumulates in the tissues of these higher organisms, thereby producing more ill effects.
Keep reading for more information on this heavy metal and related stories.
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More Great Links
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