Silicosis: a review.

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Silicosis: A Review
Michael I. Greenberg, MD, MPH,
Javier Waksman, MD, and John Curtis, MD
Introduction
Silicosis is a potentially fatal, irreversible, fibrotic pulmonary disease that
may develop subsequent to the inhalation of large amounts of silica dust
over time. In most circumstances, silicosis only develops subsequent to
substantial occupational exposures. The disease has a long latency period
and may clinically present as an acute, accelerated, or chronic disease.
The pathophysiology of chronic silicosis involves chronic inflammation
arising as a result of the accumulation of various inflammatory mediators
and fibrogenic factors. Under the influence of these factors, pulmonary
silicoproteinosis develops as eosinophilic proteinaceous material accu-
mulates in the pulmonary alveolar spaces. The rate of disease progression
appears to depend upon the rate of silica deposition in the lungs, as well
as the total amount of crystalline silica that is actually retained in the lung.
In some cases, silicosis may be associated with the concomitant develop-
ment of other diseases, including tuberculosis, cancer, or autoimmune disease.
Currently, no cure or effective treatment is available for silicosis.
Due to the association between occupational exposure to silica and the
subsequent development of silicosis, a variety of federal and state agencies
have initiated strict regulations aimed at preventing the development of
silicosis in certain workers. These regulations generally emphasize ade-
quate ventilation on job sites and limiting the amount of time workers
may spend in potentially exposing environments.1-3
Historical Perspective
Respiratory disease associated with occupational exposure to crystalline
silica has been described throughout history. Hippocrates described a
condition of “breathlessness” in miners, and in 1690, Lohneiss noted that
when “the dust and stones fall upon the lungs, the men have lung disease,
breathe with difficulty.”1Bernardo Ramazzini studied so-called “miners’
phthisis,” and other trades of the day wherein workers inhaled substantial
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amounts of dusts. These dust-related afflictions have been known by various
names, including “miners’ phthisis,” “dust consumption,” “mason’s disease,”
“grinders’ asthma,” “potters’ rot,” and “stonecutters’ disease.”1These prob-
lems are now collectively referred to as silicosis. Peacock and Greenhow
reported finding silica dust in the lungs of miners in the 1860s, and 10
years later, Visconti used the term “silicosis” to describe the disease
caused by inhalational exposure to silex.1,4
Statistical and epidemiological analyses have contributed important
information towards understanding silicosis. At the turn of the 20th
century, the Metropolitan Life Insurance Company reported that workers
from foundries, quarries, and machine shops were absent from work
substantially more frequently than other workers. This constituted the first
modern suggestion of the clinical importance of silica exposure. How-
ever, it was not until the so-called Hawk’s Nest Tunnel disaster of the
early 1930s that silicosis was clearly defined as an important public health
concern.
The Hawk’s Nest Tunnel was a hydroelectric power project constructed
by blasting into massive natural rock formations in the area of Gauley
Bridge, West Virginia. The construction techniques used in this project
did not employ so-called wet-drilling techniques to reduce drilling and
blasting-related dust. Consequently, enormous amounts of construction-
related dusts were generated during the course of this project.5During the
project, it was noted that a number of workers became ill and many died
from what appeared to be a nondescript, yet severe, form of respiratory
disease. Pneumonia was noted as the cause of death in many of the
workers and company doctors for the project dubbed the respiratory
problems plaguing these workers “tunnelitis.”1,6
When the drilling and blasting phase of the Hawks Nest project was
complete, an epidemic of silicosis was identified among men who had
worked at the Gauley Bridge site. Four hundred drillers died, and
lung-related disabilities were reported in the majority of surviving
workers.6Investigations eventually revealed that blasting for the Hawks
Nest project actually involved the disruption of rock formations com-
posed largely of pure silica. Eventually it was shown that the exposures
at Gauley Bridge involved prolonged and unprotected exposure times in
conjunction with extremely high levels of pure silica liberated into the
immediate breathing space of workers.
Investigators working with the U.S. Public Health Service first classi-
fied silicosis by state, industry, and job description. Since then, a variety
of occupations have been identified as having the potential to be associated
with silica exposure. These occupations are listed in Table 1. The Occupa-
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tional Safety and Health Administration (OSHA) has reported that as many
as two million American workers may be chronically exposed to crystalline
silica.7Of these workers, roughly 100,000 are employed in what may be
considered potentially “high-risk” settings, such as sandblasting, rock
drilling, roof bolting, and foundry-related industries.7In 2002, the
National Institute for Occupational Safety and Health (NIOSH) published
an updated report entitled “Work-Related Lung Disease Surveillance Re-
port.”8This report indicated that approximately one-third of all decedents
who had silicosis from 1990 through 1999 had been employed in the
construction and mining industries (Table 2).
Chemical Properties of Silica
Silica refers to the chemical compound SiO2(silicon dioxide) that
occurs in two specific and distinct forms: amorphous and crystalline
(Tables 3 and 4). The word “crystalline” implies that the silicon and
oxygen atoms are oriented and related to each other in a fixed pattern as
opposed to the random fashion that predominates in the amorphous form
of silica. Crystalline silica naturally exists in a polymerized tetrahedral
framework producing several polymorphs. These polymorphs are a
function of the temperature and pressure: alpha quartz (or quartz), the
most common polymorph found on the earth’s surface, is stable over most
temperatures and pressures found in the earth’s crust. In contrast, beta
quartz is stable at high temperatures, whereas tridymite and cristobalite
TABLE 1. Industrial processes and activities associated with silica exposure7
Type of
Exposure
Exposure
Crystalline SilicaAmorphous Silica
IndustrialAbrasive blasting
Cleaning fossil fuel furnaces,
flues
Metal preparation, pouring
Mining
Diatomaceous earth removal
Refractory brick production
Electrometallurgical processes
Agricultural work (burning/incineration
of rice, sugar cane harvesting, etc)
Ferro-alloy production
Silicon, ferrosilicon, and chromium
manufacture
Occupational exposure to pumice dust
Animal feed industry
Molding, core making
Petroleum refining
Smelting copper or lead
Steel production
GlassblowingOther (hobbyists,
artisans)
Sculptoring stone containing
granite and other sources
of crystalline silica
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are stable only at high temperatures and low pressures. Other silica
polymorphs include coesite and stishovite. These may be encountered at
a wide range of temperatures but only in a high-pressure environment and
therefore they may be created during a variety of industrial processes
including ceramic manufacturing, foundry processes, and any other
industrial operation wherein quartz may become heated to high
temperatures at elevated pressures. The most common crystalline
forms of silica involved in workplace exposures include quartz,
tridymite, and cristobalite. Silica may also occur naturally and at
varying concentrations in rocks such as sandstone (67% silica) and
granite (25 to 40% silica).
TABLE 2. Most frequently recorded industries on death certificates, US residents aged 15 years
and over: selected states and years (1990-1999)8
IndustryDeaths (n)
Percent of All
Industries
Construction
Metal mining
Coal mining
Blast furnaces, steelworkers, rolling/finishing mills
Nonmetallic mining/quarrying (except fuel)
Iron and steel foundries
Nonmetallic mineral and stone products, miscellaneous
Manufacturing industries (not specified)
Machinery (except electrical; not elsewhere classified)
Structural clay products
All other industries
Industry not reported
Total
118
86
69
51
48
48
44
33
23
20
317
23
880
13.4
9.8
7.8
5.8
5.5
5.5
5.0
3.8
2.6
2.3
36.0
2.6
100.0
TABLE 3. Synonyms for crystalline silica7
Name/FormulationAlternative or Trade Names
? Quartz CSQZ
DQ 12
Min-U-Sil
Sil-Co-Sil
Snowit
Sykron F300
Sykron F600
Keatite
Silica W
Porosils Zeosils
Clathrasils
Tridymite
Cristabolite
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Silicates are structures composed of silicon dioxide bound to cations
such as magnesium, aluminum, or iron. Examples of silicates include
mica, soapstone, talc tremolite, Portland Cement, and others.
Opal, diatomaceous earth (tripolite), silica-rich fiberglass, fume silica,
mineral wool, and silica glass (vitreous silica) are common amorphous
forms of silica. Other forms of amorphous silica are listed in Table 3.
Dusts composed of amorphous silica, with the exception of fiberglass, are
not generally considered to be harmful to humans.9
Quartz, cristobalite, and some forms of tridymite are inherently
piezoelectric. Piezoelectricity is a property that produces opposite
electric charges on opposite sides of the physical structure when
pressure is applied directly to the crystal. This phenomenon occurs in
crystalline silica because the chemical structure does not have a center,
reflecting an inversion symmetry. In addition, the opposite sides of these
crystals have dissimilar surfaces and carry opposite electrical charges. It
is theorized that these piezoelectric characteristics may play a role in the
pathophysiology of silica-related illness by the generation of oxygen free
radicals produced on the cleaved surfaces of silica molecules and as a
result of silica-damaged alveolar macrophages.10,11Silanol (SiOH)
groups present on the surface of silica particles are capable of forming
hydrogen bonds with oxygen and nitrogen groups found in biologic cell
TABLE 4. Synonyms for amorphous silica7
Aerosil (130, 200, 255, 300, 380)
B-6C
Biogenic silica
Cab-O-Sil (EH-5, LM-130, MS55, M-5)
CI 7811
Diatomaceous earth
Pyrogenic silica
Ronasphere
Silica, anhydrous 31
Silica gel
Sipernat
Sorbosil (AC33, AC 35, AC 37, AC 39, AC
77, BFG50, TC15)
Sorbso BF G10
Spherica
Speriglass
Spheron (L-1500, N-2000, P-1000, P-1500,
PL-700)
Tripolite
Vitreous silica
Wacker HDK (P 100H, N 20P, N 25P, T 30,
V 15, V15P)
Wessalon
Zelec sil
Diatomite
EP 10TP
Fossil flour MBK
Fumed silica
Fused silica
Greensil K
Kieselguhr
LUDOX HS 40
Neosil (CBT50, CBT60, CBT70, CBT60S,
CL2000, CT11, PC10, PC50S)
Opal
Pigment white 27
Precipitated silica
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