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

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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.