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BACKGROUND INFORMATION ON ASBESTOS
Asbestos is a general term used to describe a
group of naturally occurring fibrous hydrated mineral silicates. These have been
divided into two mineral groups: serpentines and amphiboles. Commercially, the
most important form of asbestos is chrysotile, a serpentine which accounts for
more than 95% of domestic use. Commercially important amphiboles are crocidolite,
amosite, and anthophyllite. Of lesser commercial importance are two other
amphiboles: tremolite and actinolite.
SOURCES AND PROPERTIES
Chrysotile, also known as white asbestos, is
mined in Canada, Africa, the U.S.S.R., and at scattered locations in the U.S.
Chrysotile is the only fibrous (asbestiform) member of the serpentine mineral
group. The most diagnostic physical property of a chrysotile fiber is its silky
feel and appearance. A chrysotile fiber is usually very fine, elongated,
flexible (hardness 3-4), and slightly curled. Since chrysotile demonstrates
variegated color, the use of color as a diagnostic physical property is invalid.
Chrysotile is very heat-resistant, but easily degraded by acids and alkalis.
Partly because it is so available, historically, about 96% of the asbestos used
in this country was chrysotile from Canadian mines.
Both antigorite and lizardite are non-fibrous (non-asbestiform) in occurrence
and are therefore rarely encountered in commercial use as is their co-member,
chrysotile.
Crocidolite, or blue asbestos, is mined mainly in South Africa and Australia.
The fibers are less silky, coarser, and less workable than chrysotile, but still
flexible. Chemical resistance is good, but heat resistance is poor.
Amosite, or light-brown asbestos (it may be yellow or gray) is a trade name
(derived from the initials for the Asbestos Mines of South Africa) given to
fibrous cummingtonite and grunerite, mined in South Africa. These fibers also
appear silky, but are coarse to the touch, and not very flexible (hardness 5
1/2-6). Amosite has good chemical and thermal resistance, because of the
insulation provided by natural air pockets.
The other amphiboles (anthophyllite, tremolite, and actinolite) are mined in
various locations (Finland, Italy, South America) in small quantities. The
fibers of all three are brittle, with fair to good heat and chemical resistance.
These materials are of minor commercial importance, but do have specialty uses
such as laboratory filters (tremolite) and reinforcing certain plastics (anthophyllite).
Asbestos has been known and used throughout history, but did not become
important commercially until about 100 years ago, following the discovery of
large chrysotile deposits in Quebec and Russia.
Asbestos has been widely used. One study has estimated that asbestos is
contained in over 3,000 products. This widespread use of asbestos was due to its
relatively low cost and its unique combination of strength, flexibility, thermal
and electrical resistance, frictional properties, and resistance to corrosion.
The strength, insulating, and fire- resistant properties of asbestos result in
many construction industry applications, where approximately 75% of all asbestos
was conventionally used.
Spray application of asbestos fibers began in the 1930s and allowed the rapid
covering of irregular surfaces without the use of mechanical support or
extensive preparation leading to the use of sprayed-on asbestos for decoration
and acoustic insulation. In 1950, the Underwriters Laboratories approved the use
of sprayed asbestos where previously concrete had been required for prevention
of deformation of steel from fire in multistory buildings.
As a result, during the 1960s and 1970s in the United States, asbestos was
sprayed onto structural members of a large number of buildings for the purpose
of fireproofing. The sprayed asbestos was mixed with a binder, usually a
cementitious material.
However, as evidence of the health hazards of occupational asbestos exposure
began to accumulate in the late 1960s and early 1970s the sprayed-on application
of asbestos began to be restricted. In 1972, the New York City Council banned
asbestos spray application because of the health hazard to spray operators,
other construction workers, and the general public. In 1973, the Environmental
Protection Agency (EPA) banned sprayed asbestos application for structural
insulation or fireproofing. In 1978, the EPA banned all sprayed-on application
of asbestos materials, except those in which the asbestos fibers are
encapsulated with a resinous binder during spraying, and that are not friable
after drying.
In 1989 the EPA announced a three-stage ban to phase out manufacturing,
importing and processing of most U.S. asbestos products over seven years.
Although this ban has been challenged in court, manufacturers and importers have
continued to phase out production and distribution of asbestos products for
liability reasons.
HEALTH HAZARDS OF ASBESTOS EXPOSURE
The inhalation of asbestos fibers has been shown
to increase the risks of lung disease including asbestosis, lung cancer, and
mesothelioma. Each of these lung diseases will be discussed in greater detail in
the following sections.
Generally, the risk of these diseases is directly related to the duration and
level of exposure. The majority of cases of asbestos disease now occurring are a
result of exposure to concentrations in excess of the current Occupational
Safety and Health Administration (OSHA) permissible exposure limit (PEL). These
exposures occurred during the 1940s and 1950s in industrial or mining
operations.
The presence of an asbestos-containing product in a building is neither
surprising, nor does it mean that building occupants are being exposed. In fact,
on a day-to-day basis, airborne asbestos fiber concentrations in a building
environment tend to be relatively low, on the order of 0.002 fibers (greater
than 5 micrometers) per cubic centimeter of air (fibers/cc) or less. This is
only slightly higher than the concentrations which may be measured in the
outdoor air in a typical urban environment (approximately 0.0005 fibers/cc).
ASBESTOSIS
It has been known since the early part of this
century that workers who inhaled large amounts of asbestos dust sometimes
developed a disabling or fatal fibrosis of the lungs. This condition has come to
be called asbestosis or asbestotic pneumoconiosis. Early studies of asbestosis
in Great Britain lead to the adoption by that country in 1932 of regulations
limiting occupational exposure to asbestos dust. In the United States,
asbestosis cases were first reported in 1930 and dust control guidelines were
first proposed in 1938.
Asbestosis, a diffuse interstitial pulmonary fibrosis, or fibrotic
pneumoconiosis, occurs almost exclusively among workers with chronic exposure to
high concentrations of airborne asbestos fibers, such as those in the textile
and insulation industries.
The disease, which on an average requires 15 or more years to develop, is
characterized by the formation of scars or collagen tissue in the lungs, and is
associated with progressive deterioration of pulmonary function and work
capacity.
Asbestosis can be diagnosed using x-ray and lung function techniques, but there
is currently no successful medical treatment for asbestosis.
Retrospectively, it is difficult to determine the average exposure
concentrations which lead to asbestosis, but they were certainly very much
higher than concentrations to which U.S. workers are currently exposed.
LUNG CANCER
Asbestos exposure was first linked to
bronchogenic carcinoma (lung cancer) in 1935, but it was not until 1947 that
epidemiological evidence of a causal connection was established.
Since then, numerous studies have confirmed an association between occupational
exposure to asbestos and significantly elevated rates of bronchogenic cancer.
This form of lung cancer generally occurs 20 to 30 years after exposure begins,
and a strong relationship between bronchogenic cancer and asbestosis has been
demonstrated.
MESOTHELIOMA
The first link between asbestos and pleural mesothelioma (a
cancer of the chest cavity lining) was reported in 1943. A strong association
was not established until 1960 when a study in a crocidolite mining area of
South Africa found that 31 of 33 persons with pleural mesothelioma had
experienced some asbestos exposure. Additional evidence of a causal relationship
between asbestos and pleural mesothelioma has since accumulated and asbestos has
similarly been implicated as a cause of peritoneal mesothelioma (a cancer of the
abdominal lining).
This type of carcinoma is virtually unknown in the general population; the
incidence rate among asbestos workers has been estimated at approximately 5 to
10%. Mesothelioma is invariably fatal, and as a rule, the pleural origin is much
more common than the peritoneal.
Epidemiological evidence has not clearly demonstrated mesothelioma to be
dose-related, although in general, exposure levels associated with this disease
tend to be less than those which produce asbestosis. Unlike bronchogenic
carcinoma, cigarette smoking does not appear to be related to the development of
mesothelioma.