from open pits. The annual consumption of asbestos in the U.S. peaked at
about 800,000 t in 1973, but by 1984 it had declined to slightly above 200,000 t,
mainly because of concerns about its toxicity.
18
9.7.1 CHEMICAL AND PHYSICAL PROPERTIES
Asbestos minerals have a number of desirable properties that made it useful in
commercial applications. These include tensile strength, heat stability, thermal
and electrical insulation, wear and friction characteristics, the ability to be
woven, and resistance to chemical and biological degradation.
16
Chrysotile, the most abundant form of asbestos, occurs naturally in lengths
from 1 to 20 mm. Its chemical composition is Mg 3 (Si 2 O 5 )(OH) 4 and it exists as
a curled sheet that forms a spiral around a central hollow tube silicate. Small
amounts of Fe, Al, Ni, calcium (Ca), chromium (Cr), Mn, sodium (Na), and
potassium (K) may be present as imprities. Chrysotile may be white, gray,
green, or yellowish, and has a silky luster.
16
9.7.2 USE
Although asbestos use dates back at least 2000 years, modern industrial use
began around 1880. Asbestos demand peaked in the late 1960s and early 1970s,
when more than 3000 industrial applications or products were listed. It has
been estimated that in the U.S. more than 320,000 km of asbestos-cement
pipes, carrying drinking water and other materials, had been laid. Asbestos has
been used in brake linings, roofing, clutch facings, thermal and electrical
insulation, cement pipes and sheets, filters, gaskets, friction materials, textiles,
steam and fire hoses, plastics, gas-mask filters, paper, and other products.
16
Consumption of asbestos in the U.S. has been declining for two decades.
Reported consumption in 1980 was 360 million kg. By 1998 and 1999, U.S.
consumption had declined to about 16 million kg/year. Only chrysotile is
presently used for manufacturing in the U.S., most of it is used in plastics.
16
9.7.3 EXPOSURE
Asbestos is released to the environment from both natural and anthropogenic
sources, and has been detected in indoor and outdoor air, soil, drinking water,
food, and medicines. Asbestos has also been detected within the Greenland ice
sheet. The primary routes of potential human exposure to asbestos are
inhalation and ingestion. Worker exposure is a concern in the mining and
milling of asbestos, during manufacture of all asbestos products, and in the
construction and shipbuilding industries. It has been reported that in the U.S.
an estimated 37,000 persons were employed in manufacture of primary
asbestos products, while 300,000 persons were in secondary asbestos industries.
In addition, workers may be exposed to asbestos in consumer industries, such
as brake repair, asbestos insulation, and asbestos abatement. According to a
1990 Occupational Safety and Health Administration (OSHA) report,
144
Environmental Toxicology
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Ref: 4365 MING-HO YU Chap-009 Page: 144 135-148
about 800,000 t in 1973, but by 1984 it had declined to slightly above 200,000 t,
mainly because of concerns about its toxicity.
18
9.7.1 CHEMICAL AND PHYSICAL PROPERTIES
Asbestos minerals have a number of desirable properties that made it useful in
commercial applications. These include tensile strength, heat stability, thermal
and electrical insulation, wear and friction characteristics, the ability to be
woven, and resistance to chemical and biological degradation.
16
Chrysotile, the most abundant form of asbestos, occurs naturally in lengths
from 1 to 20 mm. Its chemical composition is Mg 3 (Si 2 O 5 )(OH) 4 and it exists as
a curled sheet that forms a spiral around a central hollow tube silicate. Small
amounts of Fe, Al, Ni, calcium (Ca), chromium (Cr), Mn, sodium (Na), and
potassium (K) may be present as imprities. Chrysotile may be white, gray,
green, or yellowish, and has a silky luster.
16
9.7.2 USE
Although asbestos use dates back at least 2000 years, modern industrial use
began around 1880. Asbestos demand peaked in the late 1960s and early 1970s,
when more than 3000 industrial applications or products were listed. It has
been estimated that in the U.S. more than 320,000 km of asbestos-cement
pipes, carrying drinking water and other materials, had been laid. Asbestos has
been used in brake linings, roofing, clutch facings, thermal and electrical
insulation, cement pipes and sheets, filters, gaskets, friction materials, textiles,
steam and fire hoses, plastics, gas-mask filters, paper, and other products.
16
Consumption of asbestos in the U.S. has been declining for two decades.
Reported consumption in 1980 was 360 million kg. By 1998 and 1999, U.S.
consumption had declined to about 16 million kg/year. Only chrysotile is
presently used for manufacturing in the U.S., most of it is used in plastics.
16
9.7.3 EXPOSURE
Asbestos is released to the environment from both natural and anthropogenic
sources, and has been detected in indoor and outdoor air, soil, drinking water,
food, and medicines. Asbestos has also been detected within the Greenland ice
sheet. The primary routes of potential human exposure to asbestos are
inhalation and ingestion. Worker exposure is a concern in the mining and
milling of asbestos, during manufacture of all asbestos products, and in the
construction and shipbuilding industries. It has been reported that in the U.S.
an estimated 37,000 persons were employed in manufacture of primary
asbestos products, while 300,000 persons were in secondary asbestos industries.
In addition, workers may be exposed to asbestos in consumer industries, such
as brake repair, asbestos insulation, and asbestos abatement. According to a
1990 Occupational Safety and Health Administration (OSHA) report,
144
Environmental Toxicology
[16:52 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-009.3d]
Ref: 4365 MING-HO YU Chap-009 Page: 144 135-148