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The following
conclusions about the health effects caused by exposure to respirable
crystalline silica are derived from
studies in humans and animals published since the 1974 criteria document
[NIOSH 1974]. These studies support the risk of silicosis, lung cancer,
and several other debilitating and fatal diseases from occupational
exposure to crystalline silica. The onset of silicosis and lung cancer
is thought to be related to the biological activity and the lack of
solubility of crystalline silica particles in body fluids and tissues.
5.1
Lung Cancer
In 1988
testimony to the U.S. Department of Labor, NIOSH recommended that respirable
crystalline silica be considered a potential occupational carcinogen
[54 Fed. Reg. 2521 (1989)]. Since then, additional studies have supported
a lung cancer risk from exposure to crystalline silica:
- Lung
cancer is associated with occupational exposures to crystalline silica
[ATS 1997], specifically quartz
and cristobalite [IARC 1997].
- An
exposure-response relationship has been reported in studies of miners,
diatomaceous earth workers, granite workers, pottery workers, refractory
brick workers, and other workers (see Section
3.4.2).
- Meta-analyses
of the epidemiologic studies of silica exposure and lung cancer reported
a moderate summary relative risk of 1.3 for silica exposed workers
[Steenland and Stayner 1997] and higher summary relative risks of
2.2 to 2.8 for silicotic workers [Steenland and Stayner 1997; Tsuda
et al. 1997; Smith et al. 1995]. Some of the studies of silica-exposed
workers controlled for the effects of smoking and others did not.
The available data also support the conclusion that silicosis produces
an increased risk for bronchogenic carcinoma, but the data are less
clear as to whether silica exposure is associated with lung cancer
in the absence of silicosis [ATS 1997].
5.2
Noncarcinogenic Health Effects
In 1974,
NIOSH established an REL for respirable
crystalline silica of 0.05 mg/m3 as a 10-hr TWA to prevent
the risk of silicosis from occupational exposure [NIOSH 1974]. Since
then, additional studies have indicated that a risk for silicosis exists
at the NIOSH REL. Three recent epidemiologic studies have shown that
the estimated risk of silicosis for a 45-year working lifetime is 47%
to 90% for cumulative silica exposures at concentrations
equal to the current OSHA and MSHA PELs, and approximately 10% to 30%
at concentrations equal to the NIOSH
REL (see appendix) [Kreiss and Zhen 1996;
Steenland and Brown 1995a; Hnizdo and Sluis-Cremer 1993]. The results
from these studies support the need for continued medical and epidemiologic
surveillance of workers after they leave employment and for revision
of OSHA and MSHA standards for respirable crystalline silica.
Additional
studies have reported the risk for several other debilitating and fatal
diseases:
- Several
epidemiologic studies have reported statistically significant numbers
of excess deaths or cases of immunologic disorders and autoimmune
diseases in silica-exposed workers. These diseases and disorders include
scleroderma [Steenland and Brown
1995b; Cowie 1987], rheumatoid arthritis [Sluis-Cremer et al. 1986;
Klockars et al. 1987; Rosenman and Zhu 1995], systemic lupus erythematosus
[Steenland and Brown 1995b], and sarcoidosis
[Rafnsson et al. 1998].
- Recent
epidemiologic studies have reported statistically significant associations
of occupational exposure to crystalline
silica with renal diseases and subclinical renal changes [Steenland
et al. 1990, 1992; Steenland and Brown 1995b; Calvert et al. 1997;
Nuyts et al. 1995; Hotz et al. 1995; Boujemaa et al. 1994; Ng et al.
1993].
- Crystalline
silica may affect the immune system, leading to mycobacterial infections
(tuberculous and nontuberculous) or fungal infections [ATS 1997; NIOSH
1992a,b, 1996b; Ziskind et al. 1976; Parkes 1982; Parker 1994], especially
in workers with silicosis [Corbett et al. 1999; Kleinschmidt and Churchyard
1997; Althouse et al. 1995; Goldsmith et al. 1995; Hnizdo and Murray
1998; ATS 1997].
- Occupational
exposure to respirable crystalline silica is associated with bronchitis,
COPD, and emphysema (see Section
3.5). Some epidemiologic studies suggest that these health effects
may be less frequent or absent in nonsmokers.
5.3
Exposures, Monitoring, and Controls
Published
studies on workers exposed to crystalline silica indicate that exposures
still occur at concentrations exceeding
the OSHA and MSHA PELs and the NIOSH REL. Engineering control methods
used to control silica exposures in some industrial environments may
not be feasible for reducing airborne exposures in other workplaces
where their implementation is hindered by the type of work being performed.
In addition, sampling and analytical techniques used to measure airborne
crystalline silica exposures are limited in their ability to accurately
quantify exposures below the NIOSH REL. The following issues must be
resolved to prevent silicosis and other debilitating and fatal diseases:
- Many
occupational exposures to crystalline silica still exceed applicable
Federal standards. Of the 255 industries targeted for OSHA inspection
between 1980 and 1992, 48% had overall average exposures for respirable
quartz that exceeded the PEL [Freeman
and Grossman 1995]. Analysis of OSHA compliance data for five of the
three-digit SICs (masonry and plastering, heavy construction, painting
and paper hanging, iron and steel foundries, and metal services) for
the period 1979-1995 indicated that an estimated number of workers
were exposed to concentrations of
respirable crystalline silica that were at least 10 times the NIOSH
REL of 0.05 mg/m3 (10-hr TWA) [Linch et al. 1998] (see
Section 2.3).
- Workers
are exposed to crystalline silica in a variety of industries and occupations
in which engineering controls may not be feasible for reducing exposures
and may necessitate the use of other worker protection measures such
as substitution (use of a less hazardous material) or respirator use.
- Current
sampling and analytical methods used to evaluate occupational exposure
to crystalline silica do not meet the appropriate accuracy criterion
needed to quantify exposures at concentrations
below the NIOSH REL of 0.05 mg/m3 (see Section
2.4). However, the recent introduction of a new sampler that can
operate at a higher flow rate and the ongoing improvements in the
analysis of crystalline silica should soon make it possible to measure
crystalline silica exposure accurately
when it is below 0.05 mg/m3.
- Until
these improved sampling and analytical methods are developed for respirable
crystalline silica, NIOSH will continue to recommend an exposure
limit of 0.05 mg/m3 to reduce the risk of developing silicosis,
lung cancer, and other adverse health effects. NIOSH also recommends
minimizing the risk of illness that remains for workers exposed at
the REL by substituting less hazardous materials for crystalline silica
when feasible, by using appropriate respiratory protection when source
controls cannot keep exposures below the NIOSH REL, and by making
medical examinations available to exposed workers.
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