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OSHA regulates silica exposure using the permissible exposure limit (PEL), which is the maximum amount of airborne dust an employee may be exposed to during a full work shift.

The PEL is dependent on the amount of crystalline silica that is present in the dust. The equation for this calculation is given below. For the most part, OSHA is more concerned with the respirable fraction of the sample because it is more hazardous; however, both respirable and total dust equations are shown. The following parameters are necessary to calculate the PEL and the exposure level.
  • % crystalline silica in the air samples (if the laboratory reports the silica concentrations in units of mass convert the mass to percent - e.g., [sample weight (mg) ÷ dust weight (mg) ] × 100% ):
    • % quartz
    • % cristobalite
    • % tridymite
  • Total weight of the dust collected in the air samples in milligrams
  • Total volume of air sampled for each sample in cubic meters (1000 liters = 1 cubic meter)
  • Total sampling time for each air sample in minutes
PEL Equations:
PEL (respirable fraction) = 10 ÷ [% quartz + (% cristobalite × 2) + (% tridymite × 2) + 2] 

PEL (total dust) = 30 ÷ [% quartz + (% cristobalite × 2) + (% tridymite × 2) + 2] 

The total dust sample is normally taken using the same sampling technique described previously, but without a cyclone.
Exposure Equation:
Exposure = [(mg/m3(1) × time(1)) + (mg/m3(2) × time(2)) + ... + (mg/m3(n) × time(n))] ÷ 480 minutes
Example:
The following example is based on collecting a respirable sample using a cyclone. If a total dust sample were collected (i.e., no cyclone used) the example would only differ in the value used to calculate the PEL.

Two consecutive samples (respirable fraction) from the same employee taken for a combined exposure to silica dusts have the following results:
Sample Sampling Period (minutes) Total Volume (m3) Weight of Dust (mg) Concentration (mg/m3) Laboratory Results (%)
A 238 0.405 0.855 2.1 5.2 quartz
2.3 cristobalite
ND tridymite
B 192 0.326 0.619 1.9 4.8 quartz
1.7 cristobalite
ND tridymite
Total 430 0.731 1.474    
        ND = not detectable

Step 1. Calculate the percentage of quartz, cristobalite, and tridymite in the particulate collected.
Quartz: [5.2% × (0.855/1.474)] + [4.8% × (0.619/1.474)] = 3.0 + 2.0 = 5.0%
Cristobalite: [2.3% × (0.855/1.474)] + [1.7% × (0.619/1.474)] = 1.3 + 0.7 = 2.0%
Tridymite: none detected = 0%
Step 2. Calculate the PEL for the mixture.
PEL(mixture) = 10 mg/m3 ÷ [% quartz + (% cristobalite × 2) + (% tridymite × 2) + 2]
 = 10 ÷ [5 + (2.0 × 2) + ( 0 × 2) + 2] = 10 ÷ 11.0 = 0.91 mg/m3
Step 3. Calculate the employee's exposure (8 hour TWA).
Exposure = [(mg/m3(1) × time(1)) + (mg/m3(2) × time(2)) + ... + (mg/m3(n) × time(n))] ÷ 480 minutes
= [(2.1 × 238) + (1.9 × 192)] ÷ 480 = 1.8 mg/m3
Step 4. Calculate the severity of the exposure.
Severity = 1.8 mg/m3 ÷ 0.91 mg/m3 = 2.0
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