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Sampling and Analysis of Crystalline Silica - X-ray Diffraction (XRD) >
Crystalline Silica Mixture Calculation |
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Crystalline Silica Mixture Calculation |
Sample Calculation for a mixture of crystalline silica:
Figure I:1-14 from TED 1.15 Chapter 1 Appendix I:1-5.
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 (Liters) |
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
| 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% |
| Percentage |
= |
[(wt. of cristobalite in sample A)+(wt.
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
|
|
|
| |
|
|
|
|
| |
= |
[0.020 mg + 0.011 mg] x (100)
1.474 mg |
|
|
| |
|
|
|
|
0.031 mg x (100)
1.474 mg |
= |
0.021(100) |
= 2.1% |
= 2% |
c. Tridymite: None Detected = 0%
Step No. 2 : Calculate the PEL for the mixture (use the formula
in the OSHA Technical manual Appendix I-1.5)
| PEL(mixture) |
= |
10 mg/m3
[% quartz + 2(% cristobalite) + 2(%tridymite)
+ 2] |
|
|
| |
|
|
|
|
| |
= |
10 mg/m3
[5.0 + 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/m(3) |
|
|
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
PEL (mixture) |
|
|
| |
|
|
|
|
| |
= |
(1.8 mg/m3)
(0.91 mg/m3) |
= |
2.0 |
Subject to SAE considerations, if the result from Step 5 is greater than
1.0 then an overexposure to the mixture of crystalline silica exists.
End of this page.
Last Updated on March 4, 1997 by Mike
C. Rose
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