Asbestos << as BEHS tuhs >> is any of a group of soft, threadlike mineral fibers. Asbestos has many properties that make it commercially valuable. It does not burn or easily conduct heat or electric current. It is also flexible and strong and is not affected by most chemicals.
Asbestos can cause serious health problems if it is inhaled. It is thought to present no health hazard so long as it remains intact. It is believed to become dangerous only after it crumbles and releases its tiny fibers into the air.
Asbestos that is dry and capable of crumbling with the slightest pressure is said to be friable. Inhaling asbestos is associated with a disease called asbestosis. It is also associated with certain types of cancer.
Manufacturers once used asbestos to reinforce thousands of products, many of which could release high concentrations of breathable asbestos fibers. Today, manufacturers mainly use asbestos in high-density building materials that do not release large amounts of breathable fibers. These include cement and asphalt-based roofing materials.
Types of asbestos.
Geologists use the word asbestos to refer to the fibrous varieties of certain hydrated silicates. Hydrated silicates are minerals that are composed of silica and a metallic element, which are chemically combined with water. The fibrous hydrated silicates that make up commercially used asbestos belong to either the serpentine group of minerals or to the amphibole group of minerals.
The serpentine mineral group includes chrysotile, which is the best known, most abundant, and most widely used type of asbestos. The crystal structure of chrysotile consists of alternate sheets of magnesia and silica. These sheets are rolled into tubes called fibrils that resemble a rolled newspaper.
The amphibole mineral group consists of several types of asbestos. These types are the actinolite-tremolite series, anthophyllite, crocidolite, and the cummingtonite-grunerite series. The most abundant types of amphibole asbestos are crocidolite and amosite, which belongs to the cummingtonite-grunerite series. The word amosite, which is a common industrial term, comes from Asbestos Mine of South Africa. Amphibole asbestos has coarser fibers than does chrysotile. These types of asbestos also consist of hydrated silicates plus other elements. The crystal structure of amphibole asbestos is made up of a double chain of silicon and oxygen atoms that is coordinated with atoms of other elements. The strands of amphibole fibers lie parallel to the double chain of atoms.
The actinolite-tremolite series is rich in aluminum, calcium, iron, and magnesium. Anthophyllite contains aluminum, iron, and magnesium. Crocidolite has a large amount of aluminum, iron, and sodium. The cummingtonite-grunerite series contains iron and magnesium.
The crystal structure of amphibole asbestos is made up of a chain of silicon and oxygen atoms. Atoms of various other elements are coordinated with this chain. The strands of amphibole fibers are parallel to the chain of atoms.
Uses of asbestos.
Each type of asbestos has certain qualities, and so each kind is used for different purposes. For example, most chrysotile has fine, curly fibers that are strong and resist heat. Manufacturers use chrysotile for such products as roof coatings and roof cements, asbestos cement sheets, and siding and shingles for houses. Chrysotile is also used for asbestos cement water pipes and for packings and gaskets used in the automotive and petrochemical industries. Short chrysotile fibers are used to make materials that can withstand heat generated by friction, such as brake linings for motor vehicles.
Amphibole asbestos is noted for its high resistance to heat and acids. Crocidolite is used in asbestos cement pipes and in diaphragms involved in the production of chlorine.
Sources of asbestos.
Most asbestos is found in metamorphic rocks (see Metamorphic rock). Chrysotile develops mainly as deposits in the cracks and seams of such rocks. Amosite and crocidolite, the most abundant types of amphibole asbestos, occur chiefly in highly folded metamorphic rocks.
Russia and China produce most of the world’s asbestos. Other leading producers include Brazil, Canada, Kazakhstan, and Zimbabwe.
How asbestos is mined and processed.
Most asbestos is obtained from enormous pits dug in the ground. This process is called open-pit mining. But some asbestos deposits are mined underground.
After the rocks have been mined, trucks take them to a plant called a mill. There, a series of machines crush the rocks into progressively smaller pieces with little damage to the fibers. In one common method, the loose fibers are aspirated (drawn by suction) through screens that allow only the asbestos to pass through. Special equipment divides the fibers according to length. The asbestos is then shipped to manufacturers.
Hazards of asbestos.
People who mine asbestos, manufacture asbestos products, install asbestos insulation, or remove asbestos may inhale the fibers. People who hold such jobs in countries that do not limit workers’ exposure to asbestos are at high risk of contracting a disease related to asbestos. Also at high risk are workers who held such jobs for many years before exposure was limited by law. In addition, asbestos can be a hazard to the families of asbestos workers, who may carry asbestos dust home on their clothing. Asbestos dust may also affect people who live near asbestos mines or processing plants.
Asbestosis is a disease in which scar tissue blocks the exchange of gases in the lungs. This condition causes shortness of breath. Another disease associated with asbestos is lung cancer. Asbestos workers who smoke are at an even higher risk for lung cancer. Asbestos also causes mesothelioma, a rare and fatal cancer of the lining of the chest or abdomen.
Small amounts of airborne asbestos fibers may be present anywhere because of the widespread use of asbestos products. These fibers are present in most people’s lungs, but not in harmful quantities.
Scientists do not know exactly how asbestos causes disease. Many researchers say that inhaling fibers longer than 5 to 10 micrometers (0.0002 to 0.0004 inch) and less than 2 micrometers (0.00008 inch) wide increases the risk of illness. Inhaling fibers for even a short amount of time may result in mesothelioma.
In the United States, government regulations established by the Occupational Safety and Health Administration (OSHA) limit the hazard of asbestos among workers. These regulations provide that workers must wear respirators and protective clothing if they are exposed to more than 3 1/3 fibers per cubic inch (0.2 fiber per cubic centimeter) during an eight-hour day. The fibers must be longer than 5 micrometers (0.0002 inch). Such employees also must have special rooms where they can change clothes after work. This precaution prevents people from carrying asbestos dust home on their clothing. In addition, companies must provide an annual medical examination for employees exposed to more than the stipulated amount of asbestos fibers.
Government regulations also protect consumers from certain products containing asbestos. For example, the Consumer Product Safety Commission bans any product that could release asbestos fibers into the air.
History.
The properties of asbestos have been known since ancient times. The Egyptians used asbestos cloth to prepare bodies for burial. The Romans collected the ashes of the dead by wrapping the bodies in asbestos cloth before cremation.
In 1774, Abraham G. Werner, a German mineralogist, wrote the first comprehensive scientific description of asbestos. People did not become aware of the health hazards of asbestos until the early 1900’s. Britain, in 1931, was probably the first country to establish health laws regulating exposure to asbestos. In 1935, the Canadian province of British Columbia passed a law requiring asbestos workers to wear protective equipment when working with asbestos. The United States enacted similar standards into law in 1972.
In 1973, the U.S. Environmental Protection Agency (EPA) banned the use of sprayed asbestos for insulation or fireproofing. In 1975, this ban was expanded to include wet applications of asbestos, such as application with a trowel, and the use of molded asbestos that could become friable.
In 1986, the EPA began a program to reduce the danger of asbestos in all public and private primary and secondary school buildings. These structures must be inspected to determine the amount of asbestos in them, and its location and condition. If a building contains asbestos, the school must develop and implement an asbestos management plan. The EPA also ordered that any removal, encapsulation, or enclosure of asbestos be followed by tests to ensure that the amount of asbestos has been reduced to a safe level. Synthetic fibers of fiberglass and plastics are being used as a substitute for asbestos fibers.
