Asbestos

Classification and properties

Occurrence and mining

Uses

Health considerations

Resources

Asbestos is the general name for a wide variety of silicate minerals, mostly silicates of calcium, magnesium, and iron. Their common characteristics are a fibrous structure and resistance to fire. The two most common families of asbestos minerals are called amphibole and serpentine. The mineral has been known and used by humans for centuries. The ancient Romans, for example, wove asbestos wicks for the lamps used by vestal virgins. A story is also told about Charlemagne’s effort to impress barbarian visitors by throwing a tablecloth woven of asbestos into the fire.

One of the first complete scientific descriptions of asbestos was provided by J. P. Tournefort in the early 1700s. He explained that the substance “softens in oil and thereby acquires suppleness enough to be spun into Threads; it makes Purses and Handkerchiefs, which not only resist the Fire, but are whiten’d and cleansed by it.” Travelers to North America in the 1750s also told of widespread use of asbestos among both colonists and Native Americans.

Classification and properties

The minerals that make up the asbestos group are very diverse. The form known for the longest time and most widely used is chrysotile, or white asbestos, a member of the serpentine asbestos family. Its fibers are long, hollow cylinders with a diameter of about 25 nanometers (10^–9 m). The fibers are strong and relatively inflexible. The chemical formula assigned to chrysotile is Mg₃ Si₂O₅ (OH). Like other forms of asbestos, chrysotile is noncombustible. The whole class of minerals was, in fact, named after the Greek word asbeston, for noncombustible. The amphibole asbestos minerals are:

• riebeckite (Na₂ Fe²⁺3Fe³⁺2Si₈ O₂₂ (OH)₂)
• anthophyllite (Mg₇ Si₈ O₂₂ (OH))
• actinolite (Ca₂ (Mg, Fe²⁺)₅ Si₈ O₂₂ (OH)₂)
• tremolite (Ca₂ Mg₅ Si₈ O(OH)₂).

Riebeckite is also called crocidolite, or blue asbestos. The asbestos amosite is sometimes included among the amphibole asbestos minerals and sometimes placed in its own group. Amosite (Fe ₇Si₈ O₂₂ (OH)) is also called grenerite. It is typically ash-gray in color.

In general, amphibole minerals and amosite tend to have longer, more rigid fibers with a lower melting point than that of chrysotile. This fact makes them less desirable as fireproofing materials.

Occurrence and mining

The primary sources of the asbestos minerals are Quebec and the Yukon in Canada, the Ural Mountain region of Russia (chrysotile), and southern Africa (the amphiboles and amosite). Some asbestos is also found in Mexico, Italy, and in the United States mainly in Arizona, California, North Carolina, and Vermont.

By far the greatest amount (95%) of asbestos produced today is chrysotile. An additional 3.5% consists of crocidolite, and the final 1.5% is amosite.

The largest supplier of asbestos minerals has traditionally been Russia, which accounts for about half of all the asbestos mined in the world. The second largest source has been Canada (about 30% of the world’s output), followed by the European nations, Zimbabwe, China, South Africa, and the United States.

Asbestos occurs either in seams that run at or just beneath Earth’s surface or in veins that may go as deep as 327 yards (300 m). One method of quarrying the seams is known as block caving. In this process, trenches are dug underneath an asbestos seam and the whole section is then allowed to fall and break apart. In another technique, open seams of the mineral are plowed up and allowed to dry in air.

Underground veins are mined in much the same way as coal. The distinctive fibrous character of asbestos makes it relatively easy to separate from other rocky material with which it is found.

Processing

After asbestos is removed from the earth, it is sorted by fiber length. Longer fibers are woven into a cloth-like material. Shorter fibers, known as shingles, are combined with each other and often with other materials to make composite products. Perhaps the best known of these composites is asbestos cement, which was invented in the late 1800s. Asbestos cement contains about 12.5% asbestos with the remainder consisting of portland cement; this mixture is used for a variety of construction purposes.

The first step in making asbestos cement is to form a thick, pasty mixture of cement and asbestos in water. That mixture is then passed along a conveyor belt, where water is removed. At the end of the belt, the damp mixture of cement and asbestos is laid down on some type of base. Layers are allowed to build up until a material of the desired thickness if obtained, then dried.

Uses

About two-thirds of the world’s production of chrysotile is used to make asbestos cement, which can be fabricated into corrugated or flat sheets for use as a building material in industrial and agricultural structures. Altering the process by which the asbestos cement is made changes its thermal, acoustical, and other properties to make it more suitable for interior structures. Asbestos cement can also be fabricated as cylinders, making a material that is suitable for ducts and pressure pipes.

Long-fiber asbestos is used in other kinds of applications. It can be woven alone or with other fibers (such as glass fibers) to make protective clothing for fire fighters, brake and clutch linings, electrical insulation, moldings for automobile components, and linings for chemical containers.

Health considerations

The deleterious health effects of asbestos have become apparent only since the end of World War II. Prior to that time, very few measurements had been made of the concentration of asbestos in the air around workplaces and other settings in which it was used. In addition, the connection between the mineral and its health effects was difficult to recognize, since they typically do not manifest themselves for 20 years or more after exposure.

Today scientists know that a rather narrow range of asbestos fiber lengths (less than 2 microns and 5-100 microns in length) can cause a range of respiratory

KEY TERMS

Asbestos cement

—A composite material made by mixing together cement and asbestos.

Asbestosis

—A disorder that affects the respiratory tract as a result of inhaling asbestos fibers, leading eventually to a variety of serious and generally fatal respiratory illnesses.

Fabricate

—To shape a material into a form that has some commercial value, as in shaping asbestos fibers into a flat or corrugated board.

Fiber

—A complex morphological unit with an extremely high ratio of length to diameter (typically several hundred to one) and a relatively high tenacity.

Mesothelioma

—Tumors that occur in structures found in the lining of the lungs.

Quarrying

—A method by which some commercially valuable substance is extracted, usually from Earth’s surface.

Shingles

—A name given to shorter fibers of asbestos.

problems, especially asbestosis, lung cancer, and mesothelioma. These problems begin when asbestos fibers enter the respiratory system and become lodged between the alveoli in the lungs. As the fibers continue to accumulate, they can cause fibrous scar tissue to develop, reducing air flow through the respiratory system.

Symptoms that gradually develop include coughing and shortness of breath, weight loss, and anorexia. Other respiratory conditions, such as pneumonia and bronchitis, become more common and more difficult to cure. Eventually the fibers may initiate other anatomical and physiological changes, such as the development of tumors and carcinomas. Individuals most at risk for asbestos-related problems are those continually exposed to the mineral fibers. This includes those who work in asbestos mining and processing as well as those who use the product in some other manufacturing line, as in the production of brake linings.

Over the past two decades, intense efforts have been made to remove asbestos-based materials from buildings where they are especially likely to pose health risks—particularly schools and public auditoriums. Recent critics of asbestos removal maintain that if not done properly, asbestos removal actually spreads more fibers into the air than it removes. Further, no satisfactory asbestos substitute materials has been found.

See also Poisons and toxins; Respiratory diseases.

Resources

BOOKS

Brodeur, Paul. Outrageous Misconduct: The Asbestos Industry on Trial. New York: Pantheon Books, 1985.

Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements. Oxford: Butterworth-Heinneman Press, 1997.

PERIODICALS

Yuspa, S. H. “Overview of Carcinogenesis: Past, Present and Future.” Carcinogenesis 21 (2000): 341–344.

OTHER

Weisstein, Eric. “Asbestos” Wolfram Research: Eric Weisstein’s World of Science. <http://scienceworld.wolfram.com/chemistry/Asbestos.html> (accessed October 17, 2006).

David E. Newton

Asbestos

Background

Asbestos is a general name that applies to several types of fibrous silicate minerals. Historically, asbestos is best known for its resistance to flame and its ability to be woven into cloth. Because of these properties, it was used to make fireproof stage curtains for theaters, as well as heat-resistant clothing for metal workers and firefighters. More modern applications of asbestos take advantage of its chemical resistance and the reinforcing properties of its fibers to produce asbestos-reinforced cement products including pipes, sheets, and shingles used in building construction. Asbestos is also used as insulation for rocket engines on the space shuttle and as a component in the electrolytic cells that make oxygen on submerged nuclear submarines. Much of the chlorine for bleach, cleansers, and disinfectants is produced using asbestos products.

The earliest known use of asbestos was in about 2500 b.c. in what is now Finland, where asbestos fibers were mixed with clay to form stronger ceramic utensils and pots. The first written reference to asbestos came from Greece in about 300 b.c. when Theophrastus, one of Aristotle's students, wrote a book entitled On Stones. In his book, he mentioned an unnamed mineral substance, which looked like rotten wood, yet was not consumed when doused with oil and ignited. The Greeks used it to make lamp wicks and other fireproof items. When the Roman naturalist and statesman Pliny the Elder wrote his comprehensive Natural History in about 60 a.d., he described this fire-proof mineral and gave it the name asbestinon, meaning unquenchable, from which we get the English word asbestos.

Although the fireproof qualities of asbestos continued to fascinate the scientific community for hundreds of years, it wasn't until the 1800s that asbestos found many commercial uses. The first United States patent for an asbestos product was issued in 1828 for a lining material used in steam engines. In 1868 Henry Ward Johns of the United States patented a fireproof roofing material made of burlap and paper laminated together with a mixture of tar and asbestos fibers. It became an immediate success. Large-scale mining of asbestos deposits near Quebec, Canada, began in 1878 and spurred the development of other commercial uses. By 1900 asbestos was being used to make gaskets, fireproof safes, bearings, electrical wiring insulation, building materials, and even filters to strain fruit juices.

Technological developments in the early 1900s resulted in even more uses for asbestos. Many of the early plastic materials relied on asbestos fibers for reinforcement and heat resistance. Vinyl-asbestos tile became one of the most commonly used floor coverings and remained in use well into the 1960s. Automobile brake linings and clutch facings also used large amounts of asbestos, as did a multitude of building materials. After World War II, the use of asbestos in products continued to expand. Heart surgeons used asbestos thread to close incisions, Christmas trees were decorated with asbestos artificial snow, and a brand of toothpaste was marketed using asbestos fibers as an abrasive.

The widespread use of asbestos was not without a dark side, however. Health problems associated with exposure to airborne asbestos particles had been noted since the early 1900s, and resulted in the passage of the Asbestos Industry Regulations of 1931 in England. By the mid-1960s, health problems began to surface among shipyard workers who handled asbestos insulation during World War II. In the United States, the problem reached the crisis stage by the 1970s, forcing the Environmental Protection Agency (EPA) to place severe restrictions on the use of asbestos. Although the EPA lifted the ban for certain kinds of asbestos in 1991, the public's faith had been severely shaken, and most manufacturers had voluntarily removed asbestos from their products. As a result, asbestos usage in the United States fell from about 880,000 tons/yr (800,000 metric tons/yr) in 1973 to less than 44,000 tons/yr (40,000 metric tons/yr) in 1997.

In other countries, asbestos products are still widely used, especially in the construction industry. Worldwide usage of asbestos in 1997 was estimated at about 2.0 million tons/yr (1.8 million metric tons/yr). Most of this asbestos is used to make asbestos-reinforced concrete products, where the asbestos fibers are locked within the concrete.

Asbestos mining operations are found in 21 countries. The leading producers of asbestos are Russia (formerly the USSR), Canada, Brazil, Zimbabwe, China, and South Africa. Smaller deposits are found in the United States and several other countries.

Raw Materials

There are six types of asbestos: actinolite, amosite, anthophyllite, crocidolite, tremolite, and chrysolite. The first five types are known as amphiboles. They are characterized by having very strong and stiff fibers, which makes them a serious health hazard. Amphibolic asbestos fibers can penetrate body tissue, especially in the lungs, and eventually cause tumors to develop. The sixth type of asbestos, chrysotile, is known as a serpentine. Its fibers are much softer and more flexible than amphibolic asbestos, and they do less damage to body tissue. All six types of asbestos are composed of long chains of silicon and oxygen atoms, locked together with various metals, such as magnesium and iron, to form the whisker-like crystalline fibers that characterize this mineral.

Chrysotile is the most commonly used type of asbestos and accounted for about 98% of the worldwide asbestos production in 1988. It is usually white, and is sometimes known as white asbestos, although it can also be amber, gray, or greenish in color. Most chrysotile fibers are about 0.25-0.50 in (6.4-12.7 mm) long and are usually added to concrete mixes to provide reinforcement. Only about 8% of chrysotile fibers are long enough to be spun into fabric or rope.

Amosite, sometimes called brown asbestos, accounted for about 1% of worldwide production in 1988. It often has a light brown tinge, but is also found in dark colors, as well as white. Amosite has coarse fibers that are about 0.12-6.0 in (3.0-152.0 mm) long. The fibers are difficult to spin into fabric or rope and are mostly used as an insulating material, although that use is banned in many countries.

Crocidolite, sometimes called blue asbestos, accounted for the remaining 1% of world-wide production. It has a bluish tinge, and its fibers are about 0.12-3.0 in (3.0-76.0 mm) long. Crocidolite has very high tensile strength and excellent resistance to chemicals. One of its uses is as a reinforcement in plastics.

The other three types of asbestos—anthophyllite, actinolite, and tremolite—have no significant commercial applications and are rarely mined.

The Manufacturing
Process

Asbestos deposits are found underground, and the ore is brought to the surface for processing using conventional mining practices. Chrysotile asbestos is usually found near the surface and can be accessed with an open-pit mine. Other asbestos deposits are found at varying depths and may require tunnels as deep as 900 ft (300 m) to gain access.

Asbestos fibers are formed by the gradual growth of mineral crystals in cracks, or veins, found in soft rock formations. The crystals grow across the vein, and the width of the vein determines the resulting asbestos fiber length. Because the minerals come from the surrounding rock, the chemical composition of the fibers is similar to the rock. As a result the asbestos must be separated from the rocky ore using physical methods, rather than the chemical methods sometimes used to process other ores.

Here are the steps used to process the chrysotile asbestos ore commonly found in Canada:

Mining

• 1 Chrysoltile asbestos deposits are usually located using a magnetic sensor called a magnometer. This method relies on the fact that the magnetic mineral magnetite is often found near asbestos formations. Core drillings are used to pinpoint the location of the deposits and to determine the size and purity of the asbestos.

  Most chrysotile asbestos mining operations are conducted in an open-pit mine. A spiraling series of flat terraces, or benches, are cut into the sloping interior sides of the pit. These are used both as a work platform and as a roadway for hauling the ore up and out of the pit. The asbestos ore deposits are loosened from the surrounding rock by careful drilling and blasting with explosives. The resulting rocky debris is loaded into large rubber-tired haul trucks and brought out of the mine. Some operations use an excavation technique called block caving, in which a section of the ore deposit is under-cut until it crumbles under its own weight and slides down a chute into the waiting haul trucks.

Separating

The ore contains only about 10% asbestos, which must be carefully separated from the rock to avoid fracturing the very thin fibers. The most common method of separation is called dry milling. In this method, the primary separation is done in a series of crushing and vacuum aspirating operations in which the asbestos fibers are literally sucked out of the ore. This is followed by a series of secondary separation operations to remove rock dust and other small debris.

• 2 The ore is fed into a jaw crusher, which squeezes the ore to break it up into pieces that are 0.75 in (20.0 mm) in diameter or less. The crushed ore is then dried to remove any moisture that may be present.
• 3 The ore falls on the surface of a vibrating 30-mesh screen, which has openings that are 0.002 in (0.06 mm) in diameter. As the screen vibrates, the loosened asbestos fibers rise to the top of the crushed ore and are vacuumed off. Because the crushed ore is much denser than the fibers, only the very smallest rock particles get vacuumed off with the asbestos.
• 4 The very fine silt and rock particles that fall through the vibrating screen are called throughs or tailings and are discarded. The crushed ore pieces that remain on the screen are called overs and are moved to the next stage of processing.

  The crushed ore from the first screen is fed through a second crusher, which reduces the ore pieces to about 0.25 in (6.0 mm) in diameter or less. The ore then falls on another vibrating 30-mesh screen and repeats the process described in steps 3 and 4.

• 5 The process of crushing and vacuum aspiration of the asbestos fibers is repeated twice more. Each time the pieces of ore get smaller until the last asbestos fibers are captured and the remaining ore is so small that it falls through the screen and is discarded. This four-step process also separates the asbestos fibers by length. The longest fibers are broken free from the surrounding rock in the first crusher and are vacuumed off the first screen. Shorter length fibers are broken free and captured on each successive set of crushers and screens, until the shortest fibers are captured on the last screen.
• 6 The asbestos fibers and other material captured from each screen are carried suspended in a stream of air and run through four separate cyclone separators. The heavier debris and rock dust particles fall to the center of the whirling air stream and drop out the bottom of the separators.
• 7 The air then passes through four separate sets of filters, which capture the different length asbestos fibers for packaging.

Quality Control

Asbestos fibers are graded according to several factors. One of the most important factors is their length, since this determines the applications where they may be used and, therefore, their commercial value.

The most common grading system for chrysotile asbestos fibers is called the Quebec Standard dry classification method. This standard defines nine grades of fibers from Grade 1, which is the longest, to Grade 9, which is the shortest. At the upper end of the scale, Grades 1 through 3 are called long fibers and range from 0.74 in (19.0 mm) and longer down to 0.25 in (6.0 mm) in length. Grades 4 through 6 are called medium fibers, while Grades 7 through 9 are called short fibers. Grade 8 and 9 fibers are under 0.12 in (3.0 mm) long and are classified by their loose density rather than their length.

Other factors for establishing the quality of asbestos fibers include tests to determine the degree of fiber separation or openness, the reinforcing capacity of the fibers in concrete, and the dust and granule content. Specific applications may require other quality control standards and tests.

Health and
Environmental Effects

It is now generally accepted that inhalation of asbestos fibers can be associated with three serious, and often fatal, diseases. Two of these, lung cancer and asbestosis, affect the lungs, while the third, mesothelioma, is a rare form of cancer that affects the lining of the thoracic and abdominal cavities.

It is also now generally accepted that different types of asbestos, particularly the amphiboles, pose a greater health hazard than chrysotile asbestos.

Finally, it is recognized that other factors, such as the length of the fibers and the duration and degree of exposure, can determine the health hazard posed by asbestos. In fact some studies have shown that some asbestos-induced lung cancers only occur when the exposure is above a certain level of concentration. Below that threshold, there is no statistical increase in lung cancer over that found in the general population.

Although not everyone agrees with these findings, overall concerns about the potential adverse health effects of inhaling asbestos fibers have led to stricter regulations on the amount of airborne asbestos allow-able in the workplace. These regulations vary from one country to another, but they all mandate significantly lower levels than previously found. In the United States, the Occupational Health and Safety Administration (OSHA) set the maximum permissible exposure to fibers longer than 0.005 mm at 0.2 fibers/cubic centimeter during an eighthour workday or 40-hour work week.

Airborne asbestos levels in the general environment outside the workplace are many times lower and are not considered a hazard.

The Future

Asbestos is still an important component in many products and processes, although its usage is expected to remain low in the United States. The stricter exposure regulations and improved manufacturing and handling procedures now in place are expected to eliminate health problems associated with asbestos.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th Edition. McGraw-Hill, 1997.

Hornbostel, Caleb. Construction Materials, 2nd Edition. John Wiley and Sons, Inc., 1991.

Kroschwitz, Jacqueline I. and Mary Howe-Grant, ed. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.

Periodicals

Alleman, James E., and Brooke T. Mossman.

"Asbestos Revisited." Scientific American (July 1997): 70-75.

Other

http://www.asbestos-institute.ca.

http://www.epa.gov/ttnuatwl/hlthef/asbestos.html.

—ChrisCavette

Asbestos

Asbestos is a mineral rock with a chemical composition of mostly silicon, water, and magnesium. Most asbestos fibers are long, thin, strong, flexible, fireproof, and resistant to chemical attack. Of the six varieties of asbestos fibers found in nature, only three are commonly found in construction materials: chrysotile, amosite, and crocidolite. Chrysotile, the variety most often found in building materials, absorbs water readily, which allows for easier removal. Chrysotile was commonly used as a binding and strengthening agent in plastics, cement, and insulation. Extremely long chrysotile fibers were woven into fire- and heat-resistant cloth.

Asbestos is a carcinogen , and medical reports indicate a single fiber can cause lung cancer. There is little health risk if the material is fully intact and is properly maintained; but it can quickly turn dangerous if any of the fibers become friable and airborne, and are inhaled.

Asbestos has been used in a wide variety of products and materials. Its positive properties of heat and chemical resistance were discovered early in history: Egyptians wove asbestos fibers into cremation shrouds and the Greeks made lamps with "inextinguishable" wicks of asbestos. Asbestos fibers have been used in approximately 3,000 different applications. At asbestos's commercial peak, the United States used nearly one million tons of asbestos per year. Common asbestos-containing materials (ACM) include thermal and acoustic insulation, fireproofing, concrete, flooring, roofing felts, building papers, shingles, electrical insulation, decorative sprays, gaskets, packing, and textiles.

The principal sources of airborne asbestos fibers are the quarrying, mining, milling, manufacturing, and application of asbestos products. Medical reports have documented laboratory and clinical evidence that inhalation of asbestos fibers can lead to an increased risk of developing asbestosis , lung cancer, and mesothelioma . Epidemiological studies also show that the risk of lung cancer increases tenfold for smokers compared to nonsmokers exposed to asbestos. In the past, the individuals at greatest risk of developing these diseases were asbestos workers who were exposed to high concentrations of asbestos fibers each working day with virtually no respiratory protection. The combination of cautionary medical reports and a better-informed public spurred the U.S. Environmental Protection Agency (EPA) to begin banning the manufacture of asbestos-containing products in the early 1970s.

The mineral vermiculite, mined in the United States and elsewhere, is also used as insulation and can be, though is not always, contaminated with asbestos. Asbestos-contaminated vermiculite mined in Libby, Montana, from 1963 to 1990 has caused hundreds of mine workers and family members in Libby to become sick or die from asbestos-related disease. According to the

EPA, about 70 percent of the vermiculite mined worldwide came from the Libby mine and most was sold as zonolite attic insulation between 1963 and 1984. The EPA recommends that vermiculite insulation in homes be tested for asbestos.

The EPA regulates environmental exposure to asbestos while the Occupational Safety and Health Administration (OSHA) regulates occupational exposure to asbestos. The most recent EPA regulation is the Asbestos Hazard Emergency Response Act (AHERA) for schools. This regulation became effective in 1987 and specifically outlines inspection, reinspection, periodic surveillance, and management plans for all schools to minimize exposure to asbestos. This regulation was considered state-of-the-art when it first came out and it soon became applicable to all public and private buildings.

The asbestos workers of today wear high-efficiency respirators and protective clothing to minimize the risk of developing one of the asbestos diseases. OSHA limits a worker's exposure (over an eight-hour, time-weighted average) to no more than 0.2 fibers per cubic centimeter. In the last fifteen years, asbestos regulations have been put into effect, prompting the need for asbestos abatement policies.

The current policies on asbestos are centered on the protection of the building occupants and maintenance and repair personnel. Many building occupants believe that any ACM must be removed immediately. In some cases, this is a reasonable choice, but in most situations, immediate removal is not required. To deal with an asbestos material in any building requires planning and continuous management. Improper removal can increase asbestos-related health risks significantly.

The first step in developing an effective long-term asbestos management program involves defining the nature and scope of the problem. This requires a complete building survey, including a walk-through of the entire building to include basements, crawl spaces, and attics. Bulk samples of suspected ACM material should be taken, including wallboard, insulation, roofing, floor tile, mastic, fireproofing, plaster, concrete, mortar, sprayed-on ceiling and ceiling panels, exterior siding, and fire doors. The bulk samples of each suspect material should then be analyzed by a certified/accredited laboratory, and a management plan should be developed.

The EPA endorses ACM management and recently released a regulation that endorses management versus blanket removal. An operations and maintenance (O&M) program describes the steps to maintain ACM in a building to minimize exposure to airborne asbestos fibers, and to prevent uncontrolled disturbance of ACM. It describes what must be removed, what ACM is repairable, how repairs are performed, and how remaining ACM is maintained and/or repaired. Any ACM that must be removed from a building for offsite disposal will be subject to waste transportation and disposal regulations. Most states require haulers to have waste-transportation permits. Friable asbestos is considered a hazardous substance under the Federal Superfund Law, and therefore requires special handling.

Building surveys should be an automatic requirement for any building erected before 1985. Recently, a city in northern California leased an existing movie theater with the intention of renovating it for use as a performing arts theater. Shortly before work was to begin, ACMs were identified in many portions of the facility. The city council was extremely surprised by the presence of asbestos in the building, even though the building was built in 1950. There was no asbestos survey conducted prior to the commencement of the renovation, and the building owner had verbally assured the council that the presence of ACMs was highly unlikely. The project continued, but at substantial, and unexpected, additional expense and delay. Building identification surveys are being conducted by most building owners who want to prevent any costly confrontations with ACMs. The Army Corps of Engineers in Sacramento requires an identification survey for any building that is scheduled for demolition or remodel. These surveys have indicated that mastic, boiler refractory, flexible duct connectors, silver (heat-resistant) paint, and wall taping compound can also contain asbestos and must be tested. Each asbestos identification survey has taught the Corps of Engineers new precautions to take when conducting the next one.

All building owners need to have asbestos identified and located, and its condition recorded. An operation and maintenance plan needs to be developed for each building, and maintenance workers need to be educated before they come in contact with asbestos. Legislative and public attention has led to the requirement of a long-term approach aimed at ensuring the safety of those who come into contact with asbestos.

see also Cancer; Health, Human.

Internet Resource

Agency for Toxic Substances and Disease Registry Asbestos pages. Available from http://www.atsdr.cdc.gov/ToxProfiles/phs9004.html and http://www.atsdr.cdc.gov/HEC/CSEM/asbestos/who's_at_risk.html.

EPA's Asbestos and Vemiculite homepage. Available from http://www.epa.gov/asbestos.

National Institute for Occupational Safety and Health Asbestos topic page. Available from http://www.cdc.gov/niosh/topics/asbestos.

OSHA Web site. Asbestos information pages. Available from http://www.osha-slc.gov/SLTC.

Linda N. Finley-Miller

Asbestos

Asbestos is the general name for a wide variety of silicate minerals , mostly silicates of calcium , magnesium , and iron . Their common characteristics are a fibrous structure and resistance to fire. The two most common families of asbestos minerals are called amphibole and serpentine. The mineral has been known and used by humans for centuries. The ancient Romans, for example, wove asbestos wicks for the lamps used by vestal virgins. The story is also told about Charlemagne's effort to impress barbarian visitors by throwing a table cloth woven of asbestos into the fire.

One of the first complete scientific descriptions of asbestos was provided by J. P. Tournefort in the early 1700s. He explained that the substance "softens in oil and thereby acquires suppleness enough to be spun into Threads; it makes Purses and Handkerchiefs, which not only resist the Fire, but are whiten'd and cleansed by it." Travelers to North America in the 1750s also told of widespread use of asbestos among both colonists and Native Americans.

Classification and properties

The various minerals that make up the asbestos group are so diverse that they share only one major property, their fibrous character. The form known for the longest time and most widely used is chrysotile, or white asbestos, a member of the serpentine asbestos family. Its fibers are long, hollow cylinders with a diameter of about 25 nanometers (10-9 meter). The fibers are strong and relatively inflexible. The chemical formula assigned to chrysotile is Mg3Si2O5(OH). Like other forms of asbestos, chrysotile is noncombustible. The whole class of minerals was, in fact, named after the Greek word asbeston, for noncombustible. The amphibole asbestos minerals are:

• riebeckite (Na Fe2+3Fe3+2Si8O22(OH)2)
• anthophyllite (Mg7Si8O22(OH))
• actinolite (Ca2(Mg,Fe2+)5Si8O22(OH)2)
• tremolite (Ca2Mg5Si8O(OH)2).

Riebeckite is also called crocidolite, or blue asbestos.

The asbestos amosite is sometimes included among the amphibole asbestos minerals and sometimes placed in its own group. Amosite (Fe 7Si8O22(OH)) is also called grenerite. It is typically ash-gray in color .

In general, amphibole minerals and amosite tend to have longer, more rigid fibers with a lower melting point than that of chrysotile. This fact makes them less desirable as fireproofing materials.

Occurrence and mining

The primary sources of the asbestos minerals are Quebec and the Yukon in Canada and the Ural Mountain region of Russia (chrysotile) and southern Africa (the amphiboles and amosite). Some asbestos is also found in Mexico and Italy, and in the United States, in Arizona, California, North Carolina, and Vermont.

By far the greatest amount (95%) of asbestos produced today is chrysotile. An additional 3.5% consists of crocidolite, and the final 1.5% is amosite.

The largest supplier of asbestos minerals has traditionally been Russia, or the former Soviet Union, which accounted for about half of all the asbestos mined in the world. The second largest source has been Canada (about 30% of the world's output), followed by the European nations, Zimbabwe, China, South Africa, and the United States.

Asbestos occurs either in seams that run at or just beneath the earth's surface or in veins that may go as deep as 327 yd (300 m). One method of quarrying the seams is known as block caving. In this process, trenches are dug underneath an asbestos seam and the whole section is then allowed to fall and break apart. In another technique, open seams of the mineral are plowed up and allowed to dry in air.

Underground veins are mined in much the same way as is coal . The distinctive fibrous character of asbestos makes it relatively easy to separate from other rocky material with which it is found.

Processing

After asbestos is removed from the earth, it is processed in order to divide it into groups according to fiber length. Longer fibers are separated out for weaving into a cloth-like material. Shorter fibers, known as shingles, are combined with each other and often with other materials to make composite products. Perhaps the best known of these composites is asbestos cement which was invented in the late 1800s. Asbestos cement contains about 12.5% asbestos with the remainder consisting of portland cement; this mixture is used for a variety of construction purposes.

The first step in making asbestos cement is to form a thick, pasty mixture of cement and asbestos in water . That mixture is then passed along a conveyor belt, where water is removed. At the end of the belt, the damp mixture of cement and asbestos is laid down on some type of base. Layers are allowed to build up until a material of the desired thickness if obtained. It is then dried.

Uses

About two-thirds of the world production of chrysotile is used to make asbestos cement. That material can be fabricated into corrugated or flat sheets for use as a building material in industrial and agricultural structures. Altering the process by which the asbestos cement is made can improve thermal, acoustical, and other properties to make it more suitable for interior structures also. Asbestos cement can also be fabricated as cylinders, making a material that is suitable for ducts and pressure pipes.

Long-fiber asbestos is used in other kinds of applications. It can be woven alone or with other fibers (such as glass fibers) to make protective clothing for fire fighters, brake and clutch linings, electrical insulation, moldings for automobile components, and linings for chemical containers.

Health considerations

The deleterious health effects of asbestos have become apparent only since the end of World War II. Prior to that time, very few measurements had been made of the concentration of asbestos in the air around workplaces and other settings in which asbestos was used. In addition, the connection between the mineral and its health effects was difficult to recognize since those effects typically do not manifest themselves for 20 years or more after exposure.

Today scientists know that a rather narrow range of asbestos fiber lengths (less than two microns and five to 100 microns in length) can cause a range of respiratory problems, especially asbestosis, lung cancer , and mesothelioma. These problems begin when asbestos fibers enter the respiratory system and become lodged in the interstitial areas—the areas between the alveoli—in the lungs. As the fibers continue to accumulate in the lungs, they can cause the development of fibrous scar tissue that reduces the flow of air through the respiratory system.

Symptoms that gradually develop include coughing and shortness of breath, weight loss, and anorexia. Other respiratory conditions, such as pneumonia and bronchitis , become more common and more difficult to cure. Eventually the fibers may initiate other anatomical and physiological changes, such as the development of tumors and carcinomas. Individuals most at risk for asbestos-related problems are those continually exposed to the mineral fibers. This includes those who work in asbestosmining and processing as well as those who use the product in some other manufacturing line, as in the production of brake linings.

Over the past two decades, intense efforts have been made to remove asbestos-based materials from buildings where they are especially likely to pose health risks, as in school buildings and public auditoriums. Recent critics of asbestos removal maintain that if not done properly, asbestos removal spreads more asbestos fibers into the air than it actually removes. Also, there has not been a satisfactory substitute found for the asbestos materials being removed.

See also Poisons and toxins; Respiratory diseases.

Resources

books

brodeur, paul. outrageous misconduct: the asbestos industry on trial. new york: pantheon books, 1985.

greenwood, n. n., and a. earnshaw. chemistry of the elements. oxford: butterworth-heinneman press, 1997.

periodicals

yuspa, s. h. "overview of carcinogenesis: past, present andfuture." carcinogenesis 21 (2000): 341–344.

David E. Newton

KEY TERMS

Asbestos cement

—A composite material made by mixing together cement and asbestos.

Asbestosis

—A disorder that affects the respiratory tract as a result of inhaling asbestos fibers, leading eventually to a variety of serious and generally fatal respiratory illnesses.

Fabricate

—To shape a material into a form that has some commercial value, as in shaping asbestos fibers into a flat or corrugated board.

Fiber

—A complex morphological unit with an extremely high ratio of length to diameter (typically several hundred to one) and a relatively high tenacity.

Mesothelioma

—Tumors that occur in structures found in the lining of the lungs.

Quarrying

—A method by which some commercially valuable substance is extracted, usually from the earth's surface.

Shingles

—A name given to shorter fibers of asbestos.

Asbestos

Asbestos is the general name for a wide variety of silicate minerals, primarily silicates of calcium, magnesium, and iron. Silicates are commonly occurring minerals that all contain a characteristic combination of silicon and oxygen similar to that found in silicon dioxide (sand). The two most common families of asbestos minerals are called the amphiboles and serpentines. Most people know of asbestos today because of health problems attributed to it. Prolonged exposure results in a pneumonia-like condition known as asbestosis, which is often fatal.

More than a dozen minerals are classified under the general name of asbestos. These minerals differ from each other in many ways, but all have one important property in common: their fibrous character. The most common form of asbestos, chrysotile, consists of long, hollow, cylindrical fibers that are about 25 nanometers in diameter. (A nanometer is one-billionth of a meter.)

All asbestos fibers are more or less rigid and more or less noncombustible. Their resistance to burning is, in fact, responsible for their name. The Greek word asbeston means "noncombustible."

History

Because of its unusual properties, asbestos has an interesting history. The ancient Romans were said to have woven asbestos wicks for the lamps used by the vestal virgins. (The vestal virgins were women who watched over the temple to the goddess Vesta.) And a story is also told about the Frankish king and emperor of the West named Charlemagne (742–814), who sought to impress barbarian visitors by throwing a tablecloth woven of asbestos into a fire.

One of the first complete scientific descriptions of asbestos was provided by J. P. Tournefort in the early 1700s. He explained that the substance "softens in Oil and thereby acquires suppleness enough to be spun into Threads; it makes Purses and Handkerchiefs, which not only resist the Fire, but are whiten'd and cleansed by it." Travelers to North America in the 1750s also told of widespread use of asbestos among both colonists and Native Americans.

Obtaining asbestos

The largest supplier of asbestos minerals has traditionally been Russia (or the former Soviet Union), which accounted for about half of all the asbestos mined in the world. The second largest source has been Canada (about 30 percent of the world's output), followed by the European nations, Zimbabwe, China, South Africa, and the United States.

Asbestos occurs either in seams that run at or just beneath Earth's surface or in veins that may go as deep as 300 meters. One method of quarrying the seams is known as block caving. In this process, trenches are dug underneath an asbestos seam and the whole section is then allowed to fall and break apart. In another technique, open seams of the mineral are plowed up and allowed to dry in air.

Underground veins are mined in much the same way as coal. The distinctive fibrous character of asbestos makes it relatively easy to separate from other rocky material with which it is found.

Processing

After asbestos is removed from Earth, it is processed in order to divide it into groups according to fiber length. Longer fibers are separated out for weaving into a clothlike material. Shorter fibers, known as shingles, are combined with each other and often with other materials to make some type of composite (mixed) product. Perhaps the best known of these composites is asbestos cement, invented in the late 1800s. Asbestos cement

contains about 12.5 percent asbestos and the remainder, portland cement, which is used for a variety of construction purposes.

Uses

Most asbestos cement is fabricated (processed) into corrugated (wavy) or flat sheets to be used as a building material in industrial and agricultural structures. The cement can be altered by adding various materials to improve its thermal (heat), acoustical (sound), and other properties. Some asbestos cement is also used in the manufacture of heating ducts and piping.

Long-fiber asbestos is more commonly used in the production of fabrics. It can be woven alone or with other fibers (such as glass fibers) to make protective clothing for fire fighters, brake and clutch linings, electrical insulation, moldings for automobile components, and linings for chemical containers.

Words to Know

Asbestos cement: A composite material made by mixing together cement and asbestos.

Asbestosis: A disorder that affects the respiratory tract as a result of inhaling asbestos fibers, leading eventually to a variety of serious and generally fatal respiratory illnesses.

Fabricate: To shape a material into a form that has some commercial value, as in shaping asbestos fibers into a flat or corrugated board.

Fiber: A long, threadlike material, often used in the manufacture of cloth.

Quarrying: A method by which some commercially valuable substance is extracted, usually from Earth's surface.

Shingles: A name given to shorter fibers of asbestos.

Health considerations

During the first half of the twentieth century, asbestos cement was widely used for construction. In many respects, it was the ideal building material: strong, fireproof, and a good insulator. Untold numbers of homes, schools, office buildings, naval ships, and other structures were built with linings of asbestos.

After World War II (1939–45), scientists discovered that asbestos fibers can cause a variety of respiratory disorders in humans, such as lung cancer and asbestosis, a pneumonia-like condition. Discovering the relationship between asbestos and these conditions took a long time; the disorders commonly do not appear until 20 years or more after a person is exposed to asbestos.

Individuals most at risk for asbestos-related problems are those continually exposed to the mineral fibers. This includes those who work in asbestos mining and processing as well as those who use the product in some other manufacturing line, as in the production of brake linings. Over the past two decades, massive efforts have been made to remove asbestos-based materials from buildings where they are especially likely to pose health risks, as in school buildings and public auditoriums. Recent critics of asbestos removal maintain that—if not done properly—the removal process actually intensifies the problem by spreading more asbestos fibers into the air.

[See also Respiratory system ]