The current American Conference of Governmental Industrial Hygienist TLVs(tm) (threshold limit value) for Silica - are:
Note (j): These TLVs are for the respirable fraction of particulate matter for the substance listed . The concentration of respirable dust for the application of this limit is to be determined from the fraction passing a size selector with the characteristics defined in the "C" paragraph of Appendix D.
That is: a cyclone device with a median cut off point for particles from 3.5 to 4.0 um operating at a rate of 1.7 lpm (liters per minute) for an Mine Safety Appliances cyclone or 2.6 lpm for an aluminum SKC cyclone.
The magnitude of the personal exposure of silica can be determined by the quite straight forward procedure of comparing the laboratory results for silica to the TLV.
Currently both OSHA and MSHA use the ACGIH 1973 TLVs for their legal exposure limits or PELs (permissible exposure limits). The agencies believe this limit is more valid for mixtures containing lower percentages of silica.
The OSHA / MSHA PEL calculation serves effectively to lower the PELs for Respirable Dust (5 mg/m³) and Total Dust (10 mg/m³) down to levels which take into account the adverse health effects of breathing silica dust. Thus workers in ceramics plant would be more at risk for lung disease breathing 10% silica dust than non-silica dust.
Quartz (Respirable) PEL = 10 mg/m³ ÷ %Silica + 2
Quartz (Total Dust) PEL = 30 mg/m³ ÷ %Silica + 2
For Cristobalite and Tridymite use 1/2 the value calculated from the formula for quartz.
The % of silica or %SiO(2) [silicon dioxide] is found on your lab report.
Remember you must compare the respirable dust or total dust exposure monitoring results to the calculated PELs. Comparing the raw numbers of the silica results to the calculated PELs is like comparing apples to oranges. Remember you are calculating a PEL for a mixture so you must compare it to the monitoring results for a mixture to make a valid exposure determination.
Steven R. Pressman
With thanks to ACGIH, Sharon Dannan, Cleveland OSHA Office and Fred Usbek CIH, Phillp Analytical of Reading, PA.
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