NIOSH Case Study in Occupational
Epidemiology
SILICOSIS IN
SANDBLASTERS
A
Case Study Adapted for Use in U.S. High Schools
DHHS (NIOSH) Publication Number
2002–105
June 2002
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PART I. CASE STUDY
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Question 1. How did epidemiologists
find out what caused a silicosis outbreak in Texas?
In
November 1988, a doctor in Texas reported three cases of silicosis to
his local health department. These workers went to the doctor because
they were becoming very short of breath. All three men worked at a
facility where they sandblasted oil-field drilling pipes. One of the
workers (aged 34) died of acute silicosis. The doctor reported the
cases to the health department because silicosis is a very serious
occupational disease. The doctor knew that the facility where these
people worked might be hazardous. The doctor wanted the facility to be
checked by the local health department to protect other people from
developing silicosis. Recognition of silicosis usually signals a
serious problem in the workplace.
The
county health department and the Texas Department of Health contacted
other local doctors and identified seven more workers who had suffered
from silicosis since 1985. Thus the health departments identified a
total of 10 workers who had been diagnosed with silicosis.
Epidemiologists from the Texas Department of Health conducted an
investigation of the workers with silicosis. The investigators reviewed
personal and occupational histories from each worker who had been
diagnosed with silicosis. They reviewed lung biopsy reports and
inspected the facility for hazardous agents. The workers' chest X-rays
were examined for lung diseases.
Each
of the 10 workers had been exposed to crystalline silica at the
workplace. Their chest X-rays were all abnormal and showed
pneumoconiosis. Nine of the workers had abnormal lung biopsy reports of
silicotic nodules or acute silicosis. The workers' ages ranged from 24
to 50 at the time of diagnosis. Seven workers were under age 30.
All
10 workers had used sandblasting machines for periods of 18 months to 8
years (the average was 4.5 years). Nine workers reported no silica
exposure before working at the current site. The other worker had
sandblasted oil-field drilling equipment for 3 years before working at
the current site for 5 years. The epidemiologists from the Texas
Department of Health consulted with occupational safety and health
experts (called industrial hygienists) to learn more about the
industrial processes that could have caused silicosis at this facility.
The
sandblasting process at the facility used a blasting rod with a
sandblasting agent containing 20.5% crystalline silica sand. The rod,
which was connected to a high-pressure air supply hose, was held by the
worker and directed through the drilling pipe. The abrasive silica sand
from the rod was air-blasted against the inner walls of the pipe. This
process removed contaminants to create a cleaner, smoother interior
surface for application of a protective plastic coating. Although the
sandblasting operation was enclosed by blasting cabinets connected to
exhaust systems, the cabinets were in poor condition and permitted
clouds of dust to be released throughout the work area. Workers stood
inside booths intended to protect them from inhaling crystalline
silica. However, the booths actually increased the workers' inhalation
of crystalline silica by drawing their air from surrounding areas with
high silica contamination. Workers then shoveled the used sandblasting
mixture into the machinery for reuse.
In
March 1989, air samples collected in the workers' breathing zones
showed high concentrations of crystalline silica in the sandblasting
area.
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PART II. UNDERSTANDING
SILICOSIS
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Question 2.
What is silica and what are the possible dangers of working with silica?
Silica
may be of several distinct types. For example, quartz is a crystalline
form of silica and is the most common mineral in the earth's crust.
When rock containing quartz is chipped, hammered, drilled, crushed,
loaded, hauled, or dumped, small particles of silica are released into
the air and can be inhaled by workers.
Silica
sand containing quartz is used in sandblasting equipment to clean
surfaces. The outsides of buildings and bridges and the insides of
storage tanks and pipes are some of the surfaces that are cleaned by
sandblasting.
The
silica sand used in sandblasting breaks into fine particles that stay
in the air (Figure 1). If these particles are small enough to be
inhaled deeply into the lungs, they are known as respirable crystalline
silica. Inhaling these fine silica particles causes more lung damage
than inhaling larger particles. This process causes rapid and severe
forms of silicosis in sandblasters.
Figure 1. Detailed
microscopic view of silica particles on a filter. (Scanning electron
micrograph by William Jones, Ph.D.)
Question
3. What is silicosis?
When workers
inhale crystalline silica, the lung develops scar tissue around the
silica particles. This process results in a lung disease known as
silicosis. As more lung tissue is damaged by silica dust, breathing
becomes more difficult, chest pain occurs, and death may result.
Silicosis patients suffer shortness of breath, fever, and cyanosis.
Some patients are diagnosed incorrectly as having pulmonary edema,
pneumonia, or other lung diseases.
The
three types of silicosis are as follows:
- Chronic silicosis, which
occurs after 10 or more years of exposure to low concentrations of
crystalline silica.
- Accelerated silicosis, which
occurs 5 to 10 years after exposure to high concentrations of
crystalline silica.
- Acute silicosis, which
occurs a few weeks to 5 years after exposure to very high
concentrations of crystalline silica.
Hawk's Nest
Incident
Descriptions of chronic silicosis have been available since
the late 1800s. But acute silicosis first received widespread public
attention in 1936, when Congress investigated reports of 1,500 cases of
workers with acute silicosis near the town of Gauley Bridge, West
Virginia. The incident has been called “America's worst industrial
disaster.” The workers were hired to tunnel through a mountain with a
very high concentration of silica. As a result, the workers were
exposed to high concentrations of silica dust in the air while they
were working. More than 1,000 of these workers are believed to have
died from acute silicosis caused by their silica exposure on this
project. Almost no precautions were taken against inhalation of silica.
These events are described in the book The Hawk's Nest Incident:
America's Worst Industrial Disaster, by Martin Cherniak.
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Question 4. How many workers are
exposed to dusts containing crystalline silica?
More
than 2 million U.S. workers are potentially exposed to dusts containing
crystalline silica. More than 100,000 of them are in high-risk
occupations, including sandblasting. Most sandblasters work in
construction and shipbuilding. Workers may be exposed to crystalline
silica in many other industries, including surface and underground
mining, pottery, drywall hanging, glassmaking, foundry work, quarry
work, work with sandblasting materials, agriculture, and automotive
repair.
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PART III. PREVENTION OF
SILICOSIS
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Question 5. What fields of occupational health protect
workers from occupational hazards?
Industrial
hygiene is the science of recognizing, evaluating, and controlling
hazards in the workplace and environment. Industrial hygiene practices
protect workers from hazards. Other professionals important for
protecting workers' health include occupational health physicians and
nurses as well as engineers.
Question 6. What practices are used
to protect workers from silicosis?
Industrial
hygiene practices are used to protect workers from silicosis and other
workplace hazards. These practices include the use of substitution,
engineering controls, work practices, respiratory protection, and air
monitoring. Employers are responsible for implementing these practices
when they are needed to provide safe and healthful workplaces.
Substitution
Crystalline Silica as a Blasting Material
The use of crystalline silica was banned for most
blast-cleaning operations in Great Britain in 1950 (Factories Act of
1949) and in other European countries in 1966. In 1974, NIOSH
recommended that silica sand be prohibited for use as an abrasive
blasting material and that less hazardous materials be substituted for
silica sand during abrasive blasting.
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NIOSH
has studied several abrasive agents that might be used as substitutes
for silica sand during sandblasting. Some of the abrasives studied are
steel grit, specular hematite, nickel slag, copper slag, crushed glass,
garnet, staurolite, olivine, and coal slag. Most of these abrasives
work as well as silica sand and cost about the same or even less.
However, the use of a substitute may have other adverse effects.
Regardless of the type of abrasive, NIOSH recommends the use of
engineering controls, work practices, respiratory protection, and air
monitoring (all described below) to reduce hazards to workers during
sandblasting. Figure 2 illustrates good work practices and the proper
use of personal protective equipment for abrasive blasting.
Engineering Controls
Engineering
controls are hazard controls designed into equipment and workplaces.
Examples of engineering controls are
—automated
equipment operated from an enclosed booth,
—enclosed blast-cleaning machines and cabinets,
—tools with dust collection systems, and
—local exhaust ventilation.
Engineering
controls are sometimes called source controls when they are designed to
eliminate or reduce exposures at the source.
Figure 2. Sandblaster
using proper respiratory protection inside a ventilated booth. Note
that the worker is supplied with two separate air lines—one to supply
fresh air for the worker to breathe and the other to supply air for
sandblasting.
Work
Practices
Work
practices are procedures followed by both employers and workers to
control hazards in the workplace. Examples of work practices are
—wet
drilling or sawing of silica-containing materials,
—showering and changing into clean clothing before leaving work, and
—avoiding smoking, eating, and drinking in dusty areas.
Respiratory Protection
Respiratory
protection prevents workers from inhaling pollutants. Appropriate
respiratory protection equipment and programs should be provided
whenever engineering controls and work practices cannot keep
concentrations of airborne silica at or below safe limits.
Respirators—Workers
can wear special protective equipment called respirators to protect
them from inhaling respirable crystalline silica. Respirators are a
good option as protective equipment, but they should be used only when
controls (such as substitution, automation, enclosed systems, and local
exhaust ventilation) cannot keep exposures at or below safe limits.
Workers
who perform sandblasting must use the type of respirator recommended by
NIOSH for their particular job. Regardless of the abrasive used, NIOSH
recommends a respirator that provides a constant supply of clean air
through a line that connects to a hood or helmet worn by the worker.
The respirator air line must be separate from the line that supplies
air for blasting. This device is known as a type CE abrasive-blasting
respirator operated in the continuous-flow mode.
Respiratory
Protection Program—To make sure that respirators are used
properly, the employer must establish a comprehensive respiratory
protection program as required in the OSHA respiratory protection
standard of 1998 (see the NIOSH Guide to Industrial Respiratory
Protection for guidance). The following are important elements of this
standard:
- Periodic air
monitoring
- Regular
training of workers in respirator use
- Selection of
proper NIOSH-approved respirators
- A medical
determination of the worker's ability to perform the work while using a
respirator
- Respirator fit
testing
- Maintenance,
inspection, cleaning, and storage of respirators
The respiratory
protection program should be evaluated regularly by the employer.
Air
Monitoring
Air-monitoring
equipment measures air pollutants such as silica to assess worker
exposure. Industrial hygienists and air-sampling technicians perform
air monitoring. The samples from air monitoring are analyzed to
identify concentrations and types of pollutants in the air. The results
of air monitoring for crystalline silica are used to select and
evaluate engineering controls and respiratory protection. These air
monitoring results are also compared with exposure limits that have
been recommended on the basis of health studies of animals and humans.
NIOSH, OSHA, and MSHA have exposure limits for respirable crystalline
silica. For more information about these exposure limits, see Suggested
Reading List and Other Useful Resources in Question 8.
Question
7. What else is done to detect and control silicosis in a workplace?
Medical
exams should be offered to all workers who may be exposed to respirable
particles of silica. Each exam includes a job history, a medical
history, and a physical exam. The job history questionnaire asks about
all the jobs the worker has had, all the materials worked with, and all
protective equipment used. These exams should occur before job
placement or entry into a trade. Workers should continue to have
followup medical evaluations as well as careful monitoring of their
work and exposures.
Question
8. What steps can employers take to prevent silicosis in workers who
work with crystalline silica?
- Use
abrasive blasting materials that are less hazardous than those
containing more than 1% crystalline silica. Because substitute
abrasives may also have toxic effects, NIOSH recommends the use of
engineering controls, work practices, and respiratory protection for
all abrasive blasting, regardless of the type of abrasive used.
- Recognize
situations in which silica dust may be generated; plan to eliminate or
control dust at its source.
- Inform
workers about their exposures to crystalline silica dust, the possible
health effects of this dust, and the risk of silicosis.
- Establish
effective programs for monitoring the exposure of workers to
crystalline silica dust. Conduct air monitoring to measure worker
exposure and check that controls are working properly.
- Train
workers about safe work practices to reduce exposure to crystalline
silica. For example, workers should remove dust from equipment with a
water hose instead of compressed air. Use vacuums with high-efficiency
particulate air filters (HEPA filters). Use wet sweeping instead of dry
sweeping.
- Post
warning signs to mark the boundaries of work areas that may be
contaminated with respirable crystalline silica.
- Provide
workers with material safety data sheets (MSDSs) for silica, masonry
products, alternative abrasives, and other hazardous materials to which
they may be exposed.
- Provide
workers with information about safe handling, labeling, and storage of
toxic materials.
- Write
all signs, labels, instructions, and other material for workers in
easy-to-read English and in any other languages needed for workers'
understanding of the material.
- Provide
engineering controls such as containment methods (enclosed
blast-cleaning machines and cabinets). Provide equipment for wet
drilling or wet sawing of silica-containing materials. Make sure
workers use tools with dust collection systems. Use local exhaust
ventilation to prevent release of dust into the air.
- Provide
appropriate respiratory protection for workers, including
NIOSH-certified respirators and a respiratory protection program to
ensure proper use and maintenance of respirators.
- Provide
regular medical exams for workers who may be exposed to respirable
crystalline silica.
- Provide
either disposable protective clothes or protective clothes that are
laundered at the workplace.
- Provide
areas for workers to shower and change into clean clothes before
leaving the worksite. This practice prevents contamination of other
work areas, cars, and homes.
- Report
all cases of silicosis to State health departments and to OSHA
(1–800–321–6742) or MSHA (1–800–746–1553).
Question
9. What steps can workers take to reduce exposure to silica and prevent
silicosis?
- Use
proper respiratory protection when engineering controls cannot keep
silica exposures at or below safe limits.
- Remove
dust from equipment with a water hose instead of compressed air. Use
vacuums with HEPA filters. Use wet sweeping instead of dry sweeping.
- Change
into disposable protective clothes at the worksite.
- Shower
and change into clean clothes before leaving the worksite to prevent
contamination of other work areas, cars, and homes.
- Do
not eat, drink, use tobacco products, or apply cosmetics in dusty areas.
- Wash
hands and face before eating, drinking, or smoking outside dusty areas.
- Do
not park cars where they can be contaminated with silica or other
pollutants.
The personal hygiene habits mentioned above are important
because workers may take home silica and other harmful substances on
their clothes, skin, hair, and automobiles. Exposure to these
substances may make other household members sick.
Question
10. What additional information is available about preventing silicosis?
Additional information about preventing silicosis is
available in the publications on the following Suggested Reading
List and from the Other Useful Resources listed at the end
of this document. Note that the NIOSH documents on this list are
available at the Web site printed at the end of each reference. Single
copies of NIOSH documents are also available free from NIOSH at
Telephone: 1–800–35–NIOSH (1–800–356–4674)
Fax: 513–533–8573
E-mail: pubstaft@cdc.gov
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SUGGESTED
READING
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Cherniak M
[1986]. The Hawk's Nest incident: America's worst industrial disaster.
New Haven, CT: Yale University Press.
CPWR
[1999]. Silica in sandblasting and rock drilling: hazard alert.
Washington, DC: The Center to Protect Workers' Rights.
DiNardi
SR, ed. [1997]. The occupational environment—its evaluation and
control. Fairfax, VA: American Industrial Hygiene Association.
NIOSH
[1987]. NIOSH guide to industrial respiratory protection. Cincinnati,
OH: U.S. Department of Health and Human Services, Public Health
Service, Centers for Disease Control, National Institute for
Occupational Safety and Health, DHHS (NIOSH) Publication No. 87–116
(full text of document is available at www.cdc.gov/niosh/87–116.html).
NIOSH
[1992]. NIOSH Alert: request for assistance in preventing silicosis and
deaths from sandblasting. Cincinnati, OH: U.S. Department of Health and
Human Services, Public Health Service, Centers for Disease Control,
National Institute for Occupational Safety and Health, DHHS (NIOSH)
Publication No. 92–102 (full text of document is available at www.cdc.gov/niosh/92–102.html).
NIOSH
[1996]. NIOSH Alert: request for assistance in preventing silicosis and
deaths in construction workers. Cincinnati, OH: U.S. Department of
Health and Human Services, Public Health Service, Centers for Disease
Control and Prevention, National Institute for Occupational Safety and
Health, DHHS (NIOSH) Publication No. 96–112 (full text of document is
available at www.cdc.gov/niosh/consilic.html).
NIOSH
[1997]. Construction workers: it's not just dust! Cincinnati, OH: U.S.
Department of Health and Human Services, Public Health Service, Centers
for Disease Control and Prevention, National Institute for Occupational
Safety and Health, DHHS (NIOSH) Publication No. 97–101 (full text of
document is available at www.cdc.gov/niosh/1997–101.html).
NIOSH
[1997]. NIOSH pocket guide to chemical hazards. Cincinnati, OH: U.S.
Department of Health and Human Services, Public Health Service, Centers
for Disease Control and Prevention, National Institute for Occupational
Safety and Health, DHHS (NIOSH) Publication No. 97–140 (full text of
document is available at www.cdc.gov/niosh/npg/npg.html).
NSC
[1996]. Fundamentals of industrial hygiene. 4th ed. Chicago, IL:
National Safety Council.
Parkes
WR [1994]. Occupational lung disorders. 3rd ed. Oxford, England:
Butterworth-Heinemann, Ltd.
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OTHER USEFUL RESOURCES
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The
Center to Protect Workers' Rights (CPWR)
Telephone: 1-301-578-8500
Web site: www.cpwr.com
Mine
Safety and Health Administration (MSHA)
Toll-free telephone: 1–800–746–1553
Other telephone: 1–703–235–1452
Web site: www.msha.gov
National
Institute for Occupational Safety and Health (NIOSH)
Toll-free telephone: 1–800–35 NIOSH (1–800–356–4674)
Web site: www.cdc.gov/niosh/homepage.html
Occupational
Safety and Health Administration (OSHA)
Toll-free telephone: 1–800–321–OSHA (1–800–321–6742)
Web site: www.osha.gov
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