Masonry Magazine November 2002 Page. 13
Permissible Exposure Limits
Substance
Silica, Crystalline (respirable)
Quartz: mg/m³ (10mg/m³) / (%SIO2+2)
Cristobalite: Use 1/2 the value calculated from the above mass formula for quartz.
Tridymite: Use 1/2 the value calculated from the above mass formula for quartz.
The chart accompanying this article shows a sample calculation for a mixture of crystalline silica.
One can easily see why so many in the construction industry believe this formula is complicated and should be revised and simplified. Yet a federal appeals court ruled recently that OSHA's method of calculating actual worker exposure to respirable silica is reasonable. The precedent setting decision stems from a case in Ohio where OSHA cited a casting company for violating Section 1910.100 of the General Industry Standard for failing to protect its workers against respirable dust containing crystalline silica. At the heart of the company's contest of the citation was its belief that OSHA incorrectly calculated employee exposure. The company claimed that its method, as opposed to OSHA's, was the correct one to use to calculate exposure. OSHA calculates actual silica exposure by dividing the total weight of the entire respirable dust sample (silica and non-silica) by the volume of air flowing through the sampling pump.
The company argued that by using the total weight of the dust sample instead of just the weight of the respirable silica, OSHA was penalizing the company for exposing its workers to ALL respirable dust, regardless of silica content. The company's sample showed that workers were not exposed above the PEL while OSHA's sample said they were. The cited company used its calculating method to justify not providing its workers with respiratory protection. In supporting OSHA's calculation method, the court ruled that respirable silica and nuisance dust are interrelated. Workers exposed to silica dust, the court stated, were usually exposed to a mixture of silica and nuisance dust.
What sort of impact will this court decision have on OSHA's revisions to the existing silica exposure standard? It is difficult to say but it should give us all at MCAA a greater sense of urgency to be an active and prudent participant in the development of a new silica exposure standard to ensure that it is not overly broad and correspondingly difficult to administer and costly to enforce.
Marian Marshall was named director of the MCAA Government Affairs office July 1, 2002. She has nearly 25 years experience working with the Congress and Executive branches of the U.S. government and oversees all MCAA legislative, regulatory and political action activities.
Sample Calculation for a mixture of crystalline silica: ¹
Two consecutive samples from the same employee taken from a combined exposure to crystalline silica dusts have the following results:
Sample | Sampling Period (Min.) | Total Volume (L) | Respirable Weight (Mg) | Respirable Concentration Mg/m³ | Laboratory Results
------- | -------- | -------- | -------- | -------- | --------
A | 238 | 405 | 0.855 | 2.1 | 5.2 quartz 2.3 cristobalite ND tridymite
B | 192 | 326 | 0.619 | 1.9 | 4.8 quartz 1.7 cristobalite ND tridymite
TOTAL | 430 | 731 | 1.474 | |
ND Non Detected
Calculation of the TWA from the sampling and analytical data:
Step No. 1: Calculate the percentage of quartz, cristobalite, and tridymite in the respirable particulate collected
Quartz: Percentage (weight of quartz in Sample A) + (weight of quartz in sample B) x (100) Total weight of respirable particulate collected
-0.052(0.855 mg) 0.048(0.619 mg) x (100)/(0.855 mg 0.619 mg)
-0.044 mg + 0.03 mg x (100)/1.474 mg 0.074 mg x (100)/1.474 mg 0.05(100) -5%
Cristobalite: Percentage (weight of cristobalite in sample A) (weight of Cristobalite in sample b) x (100)/Total weight of respirable particulate collected
0.023(0.855 mg)+0.017(0.619 mg) x (100)/1.474 mg
=0.02 mg +0.011 mg x (100)/1.474 mg
0.031 mg x (100)/1.474 mg 0.021(100) 2.1% 2%
Tridymite: None Detected=0%
Step No. 2: Calculate the PEL for the mixture (use the formula in the OSHA Technical manual Appendix 1-1.5)
PELmixture 10 mg/m3/[% quartz + 20% cristobalite)+ 20% tridymite) + 2]
10 mg/m3/15.0+2(2.0)+2(0)+2]
10/11-0.91 mg/m3
Step No. 3: Calculate the employee's exposure to respirable dust
Exposure (sample weight A+ Sample weight B)/Total volume of air sampled
(0.855 mg +0.619 mg)/731 liters (1 m3/1000 liters)
-2.0 mg/m3
Step No. 4: Adjust (where necessary) for sampling period less than 8-hours. Assume a zero exposure time for the sampling period remaining.
Adjusted Exposure (2.0 mg/m3)(430 minutes) + 0(50 minutes)/480 minutes
2.0 mg/m3 (430 minutes)/ 480 minutes = 1.8 mg/m3
Step No. 5: Calculate the Severity of the exposure:
Severity Adjusted Exposure/PELmixture
(1.8 mg/m3)/(0.91 mg/m3) 2.0
the result from Step 5 is greater than 1.0 than an overexposure to the mixture of crystalline silica exists.
References
1. Occupational Safety and Health Administration Technical Manual OSHA Instruction TED 1.15.
Refferences Related to Appendix A