Permissible Exposure Limits for Silica |
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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.
PEL Equations: PEL (respirable fraction) = 10 ÷ [(% 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: |
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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% Step 2. Calculate the PEL for the mixture. PEL(mixture) = 10 mg/m3 ÷ (% quartz + (% cristobalite × 2) + (% tridymite × 2) + 2) |
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= 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 |
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= ((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 |