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6.1
Health-Related Research
The relationship
of occupational crystalline silica
exposure with silicosis and other silica-related diseases is well documented
in the literature. However, the mechanisms and particle characteristics
that cause silicosis and other silica-related diseases have not been
precisely defined. Prevention of silicosis, lung cancer, and other silica-related
diseases can be facilitated by the following:
- Development
of methods for earlier detection or more definitive noninvasive evaluation
of silica-related pulmonary disease, such as methods to improve the
sensitivity of radiography for detecting silicosis (these methods
were reviewed by Wilt et al. [1998] and Talini et al. [1995])
- Further
in vitro and in vivo studies of mechanisms for development of
- silicotic
nodules [Craighead 1996]
- autoimmune
diseases
- DNA
damage by silica particles [Saffiotti et al. 1994]
- Further
in vitro and in vivo studies of the toxicity and pathogenicity of
- alpha
quartz compared with its polymorphs
[Craighead 1996]
- crystalline
silica compared with crystalline glass, amorphous silicone, and
silicates [Craighead 1996]
- crystalline
silica compared with substitute materials for abrasive blasting
and other tasks that use crystalline silica
- dust
mixtures that contain crystalline silica [Craighead 1996; Donaldson
and Borm 1998; Dufresne et al. 1998]
- quartz
contaminated with trace elements [Castranova et al. 1997]
- The
association of surface properties of silica particles with specific
work processes and health effects
- Cellular,
molecular, and animal models of silica carcinogenesis to explore whether
silica dust is an initiator or a promoter of lung cancer [Craighead
1996] and to evaluate a dose-response relationship
- Animal
models of individual susceptibility and the development of fibrosis
[Craighead 1996], including the translocation of silica particles
from the lungs [Adamson and Prieditis 1998]
- Animal
models of the adverse effects of crystalline silica on the kidneys
and liver
- Routes
and kinetics of lymphatic transport and deposition of silica particles
[Craighead 1996]
Further
epidemiologic studies and surveillance of silica-exposed workers are
needed to do the following:
- Determine
the exposure-response relationship between occupational silica dust
exposure and lung cancer in nonsmokers
- Determine
why lung cancer risks appear to be higher in silicotic workers (e.g.,
determine the histologic type and anatomic location of lung cancers
in workers with and without silicosis [Ducatman et al. 1997])
- Evaluate
exposure-response relationships between occupational silica dust exposure
and
-
TB [ATS 1997] and
-
changes in cellular components (lymphocytes, Clara cell protein)
or immunoglobulin concentrations
- Determine
the relationship between occupational exposure to silica dust and
- TB
in silica-exposed workers without diagnosed silicosis
- clinically
significant changes in the lung function of nonsmokers
- emphysema
in nonsmokers
- gastric
cancer and other nonpulmonary cancers
- Gather
uniform national and international prevalence
and incidence data about silicosis
cases to identify industries, occupations, and work areas where preventive
measures could be implemented [CSTE 1996; Wagner 1997]
- Gather
prevalence, incidence, and mortality data about silica-related diseases
such as cancer, scleroderma and other
autoimmune diseases, nonmalignant renal disease, and other adverse
health effects to assess morbidity and mortality risk factors and
to identify areas where preventive measures could be implemented
- Determine
whether silicosis or silica-related lung cancers are related to a
specific gene, gene pattern, or other individual susceptibility factors
- Improve
the methods for estimating historical exposures for retrospective
cohort studies
- Improve
the assessment of potential confounding and synergistic effects of
smoking in silica-exposed workers [Checkoway 1995]
- Improve
the assessment of potential confounding and synergistic effects of
other carcinogens present in the work environment of silica-exposed
workers [Dufresne et al. 1998]
- Determine
whether adverse health effects are associated with occupational exposure
to materials that could be substitutes for crystalline
silica [NIOSH 1992a]
6.2
Research Related to Exposure Measurement
Reducing
the OSHA and MSHA PELs for crystalline silica to concentrations below
the NIOSH REL (0.05 mg/m3 for up to a 10-hr workday during
a 40-hr workweek) would require new methods that can accurately measure
low airborne concentrations at the NIOSH accuracy criterion. (Limitations
of current NIOSH methods for measuring worker exposure to airborne crystalline
silica are discussed in Chapter 2). Such
new methods will depend on the following types of research and development:
- Reevaluation
of the 10-mm nylon cyclone, the GK2.69 cyclone, or other proposed
devices at exposure concentrations below 0.05 mg/m3
- Ascertainment
of the sampling efficiency of proposed samplers versus particle aerodynamic
diameter
- Side-by-side
comparison of proposed samplers under field conditions
- Development
of samplers that can operate at higher flow rates than those currently
available
- Development
of working standards that use different types of filter media (e.g.,
PVC) to reduce errors in calibration
- Further
improvement of the system used to produce replicate crystalline silica
samples for the PAT Program* to
- improve
the reproducibility of inter-laboratory results for silica analysis,
- eliminate
problems with sample over-loading, and
- determine
how to account for bias between results from different analytical
methods
*This
system has undergone improvements from its original form to reduce the
intersample variability. Currently, intersample CV is on the order of
0.08 to 0.12. Only cursory testing of these improvements has been carried
out, and further improvements may be necessary.
- Further
research to validate the feasibility of on-filter analysis
under field conditions (preliminary investigation of particle transition
between the cyclone and the sample collection cassette indicates that
it is possible to improve the uniformity of particles deposited on
the filter to permit an accurate on-filter analysis)
- Collaborative
testing of any improved or new sampling and analytical methods to
demonstrate equivalence
6.3
Research Related to the Control of Exposure
Protecting
workers from crystalline silica exposures can be accomplished through
a number of means. Respiratory protection and administrative controls
are important means of protecting workers, but they should not be used
as the primary method of preventing worker exposure. Other exposure
control methods (including process modifications to eliminate hazards,
substitution, and engineering controls) should be the primary focus
of any safety and health program in preventing occupational exposures.
For some industries, research is needed to develop cost effective controls;
whereas in other industries, work is needed to increase the availability
and use of control measures and to explore barriers that prevent the
introduction of control technology. Specific types of research are needed
in the following industries:
- Construction.
The construction industry presents a major challenge for protecting
workers. In this industry, crystalline silica is present in many of
the building materials and construction substrates (i.e., rock and
soil). Silica sand is a major component of concrete and mortar and
is used in the production of brick and concrete masonry units. In
addition to the ubiquitous presence of silica in construction, this
industry also faces a challenge from the ever changing nature of the
worksite. These changes create two problems in the control of silica
exposures. First, permanent control measures are not feasible for
many worksites because of the short duration of the task (e.g., concrete
cutting or coring operations). Second, the manner in which the work
is performed at a worksite can create a silica exposure for workers
at adjacent worksites. Control methods such as wet cutting of bricks
and concrete masonry units and use of high-velocity/low-volume (HVLV)
ventilation systems during cutting and grinding of concrete have been
effective in reducing exposures to silica at some worksites. However,
the following research is needed to improve these techniques and the
feasibility of their use:
- The
use of water is not a feasible control method for reducing exposures
on many interior jobs or in cold temperatures. Research is needed
to find methods for increasing
-
the applicability of water to more operations and
-
the use of water in applications where it is considered feasible.
- The
use of HVLV ventilation involves problems such as insufficient
hood capture velocity, obstruction of the work area by the control,
and poor dust collector performance. Research is needed to improve
the performance of HVLV systems and the feasibility of their use
in other operations.
- Alternative
materials and work methods can be used to reduce crystalline silica
exposures. For example, concrete forms can be used to impart smoother
surface finishes and reduce the need for additional grinding or
rework. Additional research is needed to investigate alternative
methods for blowing and sweeping on construction sites (e.g.,
the use of vacuums instead of compressed-air lances to remove
debris from cracks in road construction).
- Foundries.
Foundries use large volumes of sand in the molds and the cores to
produce castings. In general, foundries that cast higher temperature
metals (steel, gray iron, and stainless steel) have the potential
for creating higher silica exposures than foundries that cast lower-temperature
metals (aluminum, brass, and bronze). The molding sand used in most
foundries contains a small percentage of water and other binders.
High temperatures dry the sand, making it more likely to become airborne.
Various types of controls are being used in foundries, but additional
research is needed:
- Alternative
processes such as the lost foam casting process have been used
for some metal castings, but they require additional investigation
to determine whether they can effectively reduce exposures by
minimizing the amount of casting cleaning and sand handling required
to produce high- quality castings.
- Industrial
ventilation is widely used to capture and contain silica-containing
aerosols. However, its effectiveness is only as good as its design,
installation, and maintenance. Research is needed on methods for
effectively communicating the need for routine and proper maintenance
of ventilation systems.
- Automated
processes in foundries need to be explored so that workers can
be removed from operations that generate high silica exposures.
- The
use of HVLV ventilation systems during casting cleaning needs
to be evaluated.
- Alternative
methods should be investigated for blowing and sweeping in foundries.
Vacuums may be feasible as an alternative to compressed-air lances
and dry sweeping.
- Abrasive
blasting operations. Abrasive blasting operations have been documented
to generate some of the highest crystalline
silica exposures. Other blasting materials such as steel shot,
steel grit, and boiler slag have been used as substitutes for silica
sand. However, additional research is needed to determine the safety
of substitute blasting materials. In addition, replacing silica sand
with a substitute blasting material will not eliminate silica exposures
when blasting on silica substrates such as concrete or granite. Many
of these operations may be modifiable to reduce the amount of blasting
required. Additional research is needed on alternative blasting methods
such as high-pressure water jetting, slurry blasting, and vacuum blasting.
All of these may reduce exposures associated with silica-containing
substrates.
- Surface
and other mining. Technology exists in the surface mining industry
to control exposure to crystalline silica. However, silicosis persists
because controls are often not implemented or properly maintained
[NIOSH 1996b]. Effective methods are needed for informing drillers
and drill owners about the need for continued maintenance and proper
use of dust controls on drills. Mine workers at other than surface
sites have silica exposures that have not been well characterized.
For example, little or no information is available about dust control
measures for hard-rock tunneling operations. Research is needed to
determine which control measures provide the best protection and are
feasible to implement.
-
Paints,
coatings, glass, cosmetics, plastics, and cleaning products. Crystalline
silica is used in a diverse number of products, including paints,
coatings, glass, cosmetics, plastics, and cleaning products. However,
the hazards associated with silica exposure are often not recognized
in these industries. Research is needed to develop methods for communicating
hazards and controls to workers and employers. The need is for innovative
technologies that can be transferred across industries. Additional
research is needed to investigate the feasibility of using HVLV
ventilation systems and water to reduce exposures in these industries.
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