TABLE 2. U.S. AND U.K. GUIDELINES AND LIMITS FOR OCCUPATIONAL EXPOSURE TO CRYSTALLINE SILICA.
| Reference | Substance | Guideline or limit (mg/m3) |
|---|---|---|
| NIOSH [9] | Crystalline silica:* quartz, cristobalite andtridymite as respirable dust | REL = 0.05 (for up to a 10-h workday during a 40-h workweek) |
| OSHA [29 CFR 1910.1000-Table Z-3]† CFR [20] |
dust (respirable) containing quartz | 8-h TWA PEL = 10 ÷ (% quartz + 2) |
| dust (respirable) containing cristobalite | PEL = half of the value calculated from the formula for dust-containing quartz | |
| dust (respirable) containing tridymite | PEL = half of the value calculated from the formula for dust-containing quartz | |
| MSHA [30 CFR 56, 57, 70, 71] CFR [20] |
dust (respirable) containing > 1% quartz in surface and underground metal and nonmetal mines | 8-h TWA TLV = 10 ÷ (% quartz + 2) |
| dust (respirable) containing > 1% cristobalite or tridymite in surface and underground metal and nonmetal mines | 8-h TWA TLV = 5 ÷ (% SiO2 + 2) | |
| dust (total) containing < 1% crystalline silica in surface and underground metal and nonmetal mines | 8-h TWA TLV = 10 (for particulates listed in Appendix E, 1973 ACGIH TLVs only) | |
| dust (respirable) containing >5% crystalline silica in surface and underground coal mines | TWA RDS = 10 ÷ % SiO2 (MRE equivalent) | |
| dust (respirable) containing # 5% crystalline silica in surface and underground coal mines | TWA RDS = 2 (MRE equivalent) | |
| ACGIH [17] | Respirable crystalline silica, quartz | 8-h TWA TLV = 0.05 |
| Respirable crystalline silica, cristobalite | 8-h TWA TLV = 0.05 | |
| Respirable crystalline silica, tridymite | 8-h TWA TLV = 0.05 | |
| UK-HSE [21, 22, 23] | Respirable crystalline silica, total (quartz + cristobalite) | 8-h TWA MEL = 0.300 |
Adapted from Hearl [24].
* Identified by NIOSH as a potential occupational carcinogen [16].
† See this website for sample calculations of the OSHA PEL: http://www.osha-slc.gov/SLTC/silica_advisor/ mainpage.html. Other useful information is available on this website: http://www.osha-slc.gov/SLTC/ silicacrystalline/rosem/index.html. On this web page, click on "Frequently Asked Questions."
CFR = Code of Federal Regulations; REL = recommended exposure limit; PEL = permissible exposure limit; TWA = timeweighted average; RDS = respirable dust standard; MRE = Mining Research Establishment (United Kingdom); TLV = threshold limit value; MEL = maximum exposure limit.
Under the OSHA Hazard Communication Standard (HCS) materials containing 0.1% or more crystalline silica by weight must follow federal guidelines concerning hazard communication and worker training [6, 8, 16]. Although HCS does not require the analysis of materials for crystalline silica content, material suppliers or employers may want to have materials analyzed for crystalline silica to be exempt from HCS requirements. Unfortunately, laboratories may need to deal with clients that assume a low concentration of a commonly occurring mineral like crystalline silica can be measured reliably and accurately. Sample inhomogeneity, measurement sensitivity and specificity limitations when other minerals are present make it difficult if not impossible for a laboratory to provide the conclusive evidence that laboratory clients seek to avoid HCS requirements.
