OSHA Silica PEL explained…sort of!

[tweetmeme]I often get a lot of questions regarding the OSHA Silica PEL.  What is it?  Why is so difficult to obtain?  Why isn’t it a simple number like other PELs?  Good questions to be sure, and OSHA says they are working to revamp the PEL, so that it is easier to work.  How long that will take is anyone’s guess.  But I thought I’d take a stab at trying to explain the PEL.  Of course all of this information can be found on the OSHA website: www.osha.gov.  They also have a great Silica e-tool to help you with determining your silica exposure.

Okay!  ready?  Here we go…Firts you need to know that there is not one crystalline silica exposure limit for all cases.  Rather, the limit is derived from a calculation that takes into account the percentage of quartz, cristobalite, and tridymite silica found in the respirable dust specific to your particular work-site. 

The current OSHA permissible exposure limit (PEL) for crystalline silica respirable dust is as follows:

Where 10 mg/m3 equals the current PEL for respirable Quartz Silica and the percentage % SiO2 equals the percentage of cristobalite and/or tridymite in the sample.  Keeping up?  Good!  continuing on…For total dust you use 30 mg/m3(PEL for total Quartz Silica), the formula is as follows: 


In other words, the PEL is dependent on the amount of crystalline silica that is present in the dust.   This is critical to the determination for exposure.  Also 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.

Remember you are calculating a PEL for a mixture (silica and dust) so you must compare it to the monitoring results for a mixture to make a valid exposure determination.  As expressed in Table Z-3 (OSHA Mineral Dusts PELs), the PELs for crystalline quartz silica and for inert (or nuisance) dust are interrelated.   The PEL formula operates to regulate silica exposure by determining its concentration in respirable dust and setting variable limits on exposure to the respirable dust depending on its silica concentration.

As a side note, how many of you have seen the 1985 movie Better Off Dead with John Cusack wherein, his high school science teach played by Vicent Schiavelli is explaining a complicated formula on the blackboard and everyone in the class is on the edge of their seats throughout, and even bursts into applause upon finding out the answer, and poor Lane Meyer (Cusack’s character) is the only one who hasn’t a clue as to what is being said.  Do you feel that way now?  No, keeping up?  Good to hear…let’s continue then. 

The numerator in the silica PEL formula, “10mg/m3,” represents the weight of the total respirable dust sample, and its denominator, “% SiO2 + 2,” represents the percentage of silica therein.  Where “% SiO2 ” equals “0,” representing dust with no quartz silica in it, the PEL formula yields a PEL of 5 mg/m3: ( ).    A 5 mg/m3 PEL is the fixed PEL listed in Table Z-3 for “inert or nuisance dust,” which demonstrates that the PEL for silica works in harmony with the PEL for inert dust.

Where there is no silica detected in the air, the PEL yielded should logically be and is equal to that allowed for 100% inert dust. Where “% SiO2” equals “100,” representing dust that is pure quartz silica, the PEL formula yields a PEL of .098 mg/m3: ( )

The only variable in the PEL formula is “% SiO2,” the percentage of the total respirable dust sample that is silica.   Thus, the PEL formula operate[s] in such a way as to create a sliding scale of PEL’s depending on the percentage of the total respirable fraction of the dust that is crystalline quartz silica.   In this way the PEL’s are lowered as the percentage of silica in the dust becomes higher.   The upper limit of the sliding scale is the PEL for inert or nuisance dust (containing no quartz); the lower limit is the PEL for pure quartz dust.

One may therefore conclude (can you feel the energy and tempo building into a cresendo?) that if you sample for only a limited time during heavy exposure and then stopped sampling, that the overall % of quartz silica in the sample may be relatively high compared to the total amount of dust collected.   However, if one continued to sample for the remaining portion of the work shift (without exposure to silica related dusts, but rather only nuisance dusts), that the amount of silica in the sample may be reported as a lower overall percentage (%) of total amount of dust collected. Therefore the PEL is inversely related to the denominator (% SiO2) in the formula.

See it’s as easy as that…



  1. Thanks for writing that article. I now fully understand how it all works to determine exposure, and it was also great to find out that there was that Silica E-Tool. I owe you lunch if you get to California.

  2. Mr.Badarudin says:

    I am involved in OSH in Malaysia and would like to exchange knowledge with u tq

  3. Abel Hackman says:

    this article has done me good in understanding the next step after calculating my TWA. Yea that really a good piece.


  1. […] OSHA Silica PEL explained…sort of! « The Safety Director's Cut Share and […]

  2. […] variables and conditions that prevent it from being expressed as a simple number. You can read this blog post from The Safety Director’s Cut for a detailed […]

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