Health Effects of Occupational Exposure
to Respirable Crystalline Silica

This section provides an abbreviated review of various experimental research studies. The reader is encouraged to consult the cited materials for complete information.

4.1 Biomarkers

A biomarker can indicate

1. the occurrence of exposure,
   
   
2. the effects of exposure,
   
   
3. the presence of early or frank disease, or
   
   
4. the susceptibility to disease or early effects of exposure [Committee on Biological Markers of the National Research Council 1987; Schulte 1995].

Useful biomarkers require

1. a definitive, validated link with the exposure or the risk of disease and
   
   
2. evidence of a dose-response relationship between the marker and the exposure [Schulte 1995].

The relationship between respirable silica dust exposure and silicosis is well established. However, the complex chain of cellular responses that leads to fibrosis and silicosis has not been fully discovered. The usefulness of biomarkers as a screening tool for silicosis risk will be realized when biomarkers in the chain of complex cellular responses are validated for their relationship to disease. In addition, the studies of blood, serum, sputum, bronchoalveolar lavage samples, and gene patterns of silica-exposed workers or silicotics (Table 20) are inconclusive for the following reasons:

1. The numbers of subjects are small, and few studies of similar markers exist for comparison.
   
   
2. The studies lack control for factors other than silica exposure that could change immunoglobulin concentrations.
   
   
3. The studies lack information about control groups, diagnostic criteria for silicosis, and baseline levels of markers.
   
   
4. Study results are inconsistent.

Further research on biomarkers in silica-exposed workers is needed to do the following:

1. Quantify the exact amount of soluble products in bronchoalveolar lavage in individual patients to provide more information about the mechanisms of fibrogenesis [Sweeney and Brain 1996]
   
   
2. Determine whether silicosis or silica-related lung cancers are associated with a specific gene or gene pattern
   
   
3. Determine whether a relationship exists between changes in immunoglobulin concentrations and silica exposure
   
   
4. Determine whether a dose-response relationship exists between changes in certain cellular components (lymphocytes and Clara cell protein) and silica exposure

Detailed reviews of the immunologic response to silica and other mineral dusts are available elsewhere (i.e., Heppleston [1994]; Haslam [1994]; Weill et al. [1994]; Davis [1991,1996]; Kane [1996]; Driscoll [1996]; Sweeney and Brain [1996]; Hook and Viviano [1996]; Gu and Ong [1996]; Iyer and Holian [1996]; Weissman et al. [1996]; Mossman and Churg [1998]).

4.2 Cytotoxicity

Respirable crystalline silica is known to cause silicosis; however, the molecular mechanism responsible for the cellular injury that precedes the lung disease is unknown. Extensive in vitro and in vivo research has been conducted to evaluate the effects of crystalline silica on mammalian cells. Several mechanisms have been proposed to explain the cause of the cellular damage [Lapp and Castranova 1993]:

1. Direct cytotoxicity of crystalline silica
   
   
2. Stimulation of the alveolar macrophages by silica and subsequent release of cytotoxic enzymes or oxidants
   
   
3. Stimulation of the alveolar macrophages to release inflammatory factors (e.g., interleukin-8, leukotriene B₄, platelet-activating factor, tumor necrosis factor, platelet-derived growth factor) that recruit polymorphonuclear leukocytes, which in turn may release cytotoxins
   
   
4. Stimulation of the alveolar macrophages to release factors that initiate fibroblast production and collagen synthesis (e.g., interleukin-1, tumor necrosis factor, platelet-derived growth factor, fibronectin, and alveolar macrophage-derived growth factor)

4.3 Genotoxicity and Related Effects

Some studies have demonstrated the ability of quartz to induce micronuclei in mammalian cells in culture [Hesterberg et al. 1986; Nagalakshmi et al. 1995; Oshimura et al. 1984] (Table 21). However, other in vitro studies did not observe chromosomal aberration [Nagalakshmi et al. 1995; Oshimura et al. 1984], hprt (hypoxanthine guanine phosphoribosyl transferase) gene mutation [Driscoll et al. 1997], or aneuploid or tetraploid cells [Price-Jones et al. 1980; Oshimura et al. 1984; Hesterberg et al. 1986]. An in vivo treatment of rats with quartz induced mutation in rat alveolar epithelial cells (Table 21) [Driscoll 1995; 1997].

Pairon et al. [1990] tested tridymite (i.e., Tridymite 118) and quartz (i.e., Min-U-Sil 5) particles for genotoxic effects. Tridymite induced a significant number of sister chromatid exchanges (SCEs) in co-cultures of human lymphocytes and monocytes (P<0.05 compared with control cells) at doses of 5 and 50 µg/cm² (87.9% of the tridymite particles had a diameter <1 µm). However, the number of SCEs in purified human lymphocytes that were treated with the same doses of tridymite particles did not differ significantly from control cells [Pairon et al. 1990]. Results of the same experiments with quartz did not yield a clear conclusion about the ability of quartz to induce a significant number of SCEs (Table 21) [Pairon et al. 1990].

In vitro cellular transformation systems model the in vivo process of carcinogenesis [Gao et al. 1997; Gu and Ong 1996]. The ability of quartz to induce dose-dependent morphological transformation of cells in vitro has been demonstrated in experiments with Syrian hamster embryo cells [Hesterberg and Barrett 1984] and mouse embryo BALB/c-3T3 cells [Saffiotti and Ahmed 1995]. Gu and Ong [1996] also reported a significant increase in the frequency of transformed foci of mouse embryo BALB/c-3T3 cells after treatment with Min-U-Sil-5 quartz. These studies indicate that crystalline silica can morphologically transform mammalian cells. However, further studies are needed to determine whether the transforming activity of silica is related to its carcinogenic potential.

Researchers at the National Cancer Institute have examined the ability of quartz, cristobalite, and tridymite particles to cause deoxyribonucleic acid (DNA) damage (i.e., strand breakage) [Saffiotti et al. 1993; Shi et al. 1994; Daniel et al. 1993; Daniel 1993, 1995]. Although the results of those studies demonstrated the ability of crystalline silica to cause damage to isolated DNA in acellular systems, reviewers at IARC [1997] recently stated that the relevance of these assays to assess quartz related genetic effects in vivo was questionable because

1. the nonphysiological experimental conditions did not apply to intracellular silica exposure and
   
   
2. very high doses of silica were used in the DNA breakage assays [IARC 1997].

Several studies conducted since the IARC review found that crystalline silica induced DNA damage (i.e., DNA migration). Zhong et al. [1997] found that by using the alkaline single cell gel/comet (SCG) assay, crystalline silica (Min-U-Sil 5) induced DNA damage in cultured Chinese hamster lung fibroblasts (V79 cells) and human embryonic lung fibroblasts (Hel 299 cells) [Zhong et al. 1997]. Amorphous silica (Spherisorb), but not carbon black, was also found to induce DNA damage in these mammalian cells. However, the DNA-damaging activity of amorphous silica was not as high as the damaging activity of crystalline silica [Zhong et al. 1997]. Liu et al. [1996, 1998] challenged Chinese hamster lung fibroblasts with dusts pretreated with a phospholipid surfactant to simulate the condition of particles immediately after deposition on the pulmonary alveolar surface. Results of the experiments showed that untreated Min-U-Sil 5, Min-U-Sil 10, and noncrystalline silica induced micronucleus formation in a dose-dependent manner, but surfactant pretreatment suppressed that activity [Liu et al. 1996]. A subsequent experiment found that surfactant pretreatment suppressed quartz induced DNA damage in lavaged rat pulmonary macrophages, but DNA damaging activity was restored with time as the phospholipid surfactant was removed by intracellular digestion [Liu et al. 1998].

Shi et al. [1998] recently reviewed published literature on

1. the generation of reactive oxygen species (ROS) directly from silica and from silica-stimulated cells,
   
   
2. the role of ROS in silica-induced DNA damage and silica-induced cell proliferation, and
   
   
3. other silica-mediated reactions.
   
   

A proposed mechanism for silica-induced generation of ROS species and carcinogenesis is described by Shi et al. [1998]. Experimental research is continuing to determine whether crystalline silica particles have a direct genotoxic effect that could cause lung tumor formation in humans.

4.4 Carcinogenicity

Experimental evidence of the carcinogenicity of quartz particles is based on the results of long-term inhalation and intratracheal instillation studies of rats, which are summarized in Tables 22 and 23 [Saffiotti et al. 1996]. Several issues are apparent from the results of the rat studies [Holland 1995]:

1. The appearance of tumors (usually adeno-carcinomas or epidermoid carcinomas) is a late phenomenon.
   
   
2. Lung fibrosis is usually present in the rats with tumors.
   
   
3. No adequate dose-response data exist because multiple-dose experiments have not been conducted in the rat except for the inhalation study by Spiethoff et al. [1992].
   
   
4. Comparability of the intratracheal instillation and inhalation studies is difficult because of notable differences in methods and materials.

Although new long-term carcinogenesis studies in animals may provide information about dose-response relationships and inhibition of quartz toxicity or reactivity in vivo, in vitro studies are needed to develop effective cellular and molecular models of carcinogenesis [Holland 1995; Saffiotti et al. 1996].