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Silica, crystalline (as respirable dust)

May 1994
Immediately Dangerous to Life or Health Concentrations (IDLH)

CAS number: 14808–60–7

NIOSH REL: 0.05 mg/m3 TWA; NIOSH considers crystalline silica to be a potential occupational carcinogen as defined by the OSHA carcinogen policy [29 CFR 1990].

Current OSHA PEL: Crystalline quartz (respirable): 250 mppcf/(%SiO2 +5) TWA

10 mg/m3/(%SiO2 + 2) TWA

Crystalline quartz (total dust): 30 mg/m3/(%SiO2 + 2) TWA

Cristobalite: Use ½ the value calculated from the count or

mass formulae for quartz;

Tridymite: Use ½ the value calculated from the formulae for


1989 OSHA PEL: Cristobalite: 0.05 mg/m3 TWA

Tridymite: 0.05 mg/m3 TWA

Quartz: 0.1 mg/m3 TWA

Tripoli: 0.1 mg/m3 TWA

1993-1994 ACGIH TLV: Quartz (respirable dust): 0.1 mg/m3 TWA

Cristobalite (respirable dust): 0.05 mg/m3 TWA

Tridymite (respirable dust): 0.05 mg/m3 TWA

Description of substance: Varies

Original (SCP) IDLH*: No Evidence [*Note: "Effective" IDLH = 500 times the OSHA PEL -- see discussion below.]

Basis for original (SCP) IDLH: The available toxicological data show no evidence that indicates an acute exposure to a high concentration of crystalline silica would impede escape or cause any irreversible health effects within 30 minutes. For this draft technical standard, therefore, respirators have been selected on the basis of the assigned protection factor afforded by each device. However, for some particulate substances for which no evidence of an IDLH exists, the determination of allowable respiratory protection based on protection factors may result in the assignment of respirators for concentrations that are not likely to be encountered in the occupational environment. Therefore, for all such particulate substances it has been arbitrarily determined that only the "most protective" respirators are permitted for use in concentrations exceeding 500 ´ the OSHA PEL.

Short-term exposure guidelines: None developed


Animal or human data: None relevant for use in determining the revised IDLH.

Revised IDLH: 25 mg/m3 (Cristobalite & Tridymite); 50 mg/m3 (Quartz & Tripoli)

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.]

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