Silica, crystalline

Agent Name
Silica, crystalline
CAS Number
14808-60-7; 14464-46-1; 15468-32-3; 1317-95-9
Major Category
Mineral Dusts
Cristobalite; Quartz; Tridymite; Tripoli; [NIOSH]
Fibrogenic Dusts
Colorless, odorless solid. [Note: A component of many mineral dusts.]; [NIOSH]
Quartz is present in most sedimentary and igneous rocks: granite has 25-40% quartz; shales have 22% quartz; sandstones have 67% quartz. Workers are exposed in hard-rock mining, foundries, ceramic manufacturing, and road construction. Cristobalite, mostly from calcined diatomaceous earth, is used to manufacture water glass, abrasives, ceramics and enamels. Cristobalite is also used to purify oils. [ACGIH] Crystalline silica not bound to other minerals is "free" silica. [Rom, p. 364] For conversion of units from mppcf (used prior to the 1970s) to mg/m3 and to see "dustiness classification time periods," see "Silica exposure assessment in a mortality study of Vermont granite workers." [PMID 21229455] "OSHA air sampling data revealed that in certain industries workers are exposed at or above 10 times the NIOSH REL, including construction (eg, poured concrete foundation and structure contractors, commercial and institutional building construction, masonry contractors) and manufacturing (eg, ready-mix concrete manufacturing, all other nonmetallic mineral product manufacturing, ferrous metal foundries)." [PMID 32270550] "Artificial Stone Silicosis: Rapid Progression Following Exposure Cessation" [PMID 32563682]
Silica occurs in both crystalline and amorphous forms. Quartz is the most common crystalline form, and it causes most cases of silicosis. [Harber, p. 373] Exposure to crystalline silica (alpha-quartz and cristobalite) is associated with lung fibrosis (silicosis) and increased risk of lung cancer. [ACGIH] Classic silicosis is caused by low to moderate silica exposure for 20 years or more. Disabling complications include tuberculosis and progressive massive fibrosis. Diagnosis is made by 1.) history of sufficient exposure; 2.) x-ray findings of silicosis; and 3.) absence of miliary tuberculosis and fungal infections. Open lung biopsy is usually not necessary. [Rom, p. 371] "The possible association of silica and glomerulonephritis is suggested by animal studies, case-control studies, and multiple case reports."[LaDou, p. 422] "Overall the evidence is still too sparse to be summarized as conclusive, but it seems very probable that silica causes kidney disease." [PMID 15940719] Silica-associated kidney disease was not detected in a study of 4 million death certificates of US workers for the period from 1982 to 1995. [PMID 12554850] While the studies of cohorts exposed to silica found elevated SMRs for renal disease, no clear evidence of a dose-response relationship emerged." [PMID 28409224] "Low grade silicosis cannot be excluded in workers with normal chest radiographs (ILO 0/0). In relatively highly exposed construction workers, a sevenfold increased risk of simple (nodular) silicosis was found. Emphysema on HRCT was associated with current or former smokers, but not with exposure, and contributed to reduced diffusion capacity. Airflow limitation was mainly determined by current smoking and was not associated with simple (nodular) silicosis." [Reference #2] See "Respirable crystalline silica exposure-response evaluation of silicosis morbidity and lung cancer mortality in the German porcelain industry cohort." [PMID 21346639] "The absence of an excess mortality from lung cancer does not support the hypothesis of an association between silica exposure and the development of lung cancer, in the absence of silicosis." [PMID 28691999] See "Silicosis, simple."
Reference Link #1
Biomedical References

Exposure Assessment

Skin Designation (ACGIH)
0.025 mg/m3, respir. fraction (cristobalite and alpha-quartz)
30 mg/m3/%SiO2+2, total dust, 10 mg/m3/%SiO2+2, respir. dust(quartz, tripoli), Value calculated from respir. dust formula for quartz(cristobalite, tridymite)
25 mg/m3(cristobalite, tridymite), 50 mg/m3(quartz, tripoli)
Excerpts from Documentation for IDLHs
Basis for revised IDLH: The available toxicological data contain no evidence that an acute exposure to a high concentration of crystalline silica would impede escape or cause any irreversible health effects within 30 minutes. However, the revised IDLHs for crystalline silica are 25 mg/m3 for Cristobalite and Tridymite and 50 mg/m3 for Quartz and Tripoli, based on being 500 times the 1989 OSHA PELs of 0.05 mg/m3 and 0.1 mg/m3, respectively (500 is an assigned protection factor for respirators and was used arbitrarily during the Standards Completion Program for deciding when the "most protective" respirators should be used for particulates). [Note: NIOSH recommends that the "most protective" respirators be worn for all varieties of crystalline silica at concentrations above 25 mg/m3.]

Adverse Effects

Chronic Bronchitis
IARC Carcinogen
NTP Carcinogen
Human carcinogen
ACGIH Carcinogen
Suspected Human

Diseases, Processes, and Activities Linked to This Agent