Silica Part III: How Much Dust is Too Much?Joel Guth
UPDATED FOR THE NEW OSHA PEL 2017
In our introduction to this series on understanding the OSHA PEL, we described what a PEL is and outlined the three factors which determine the PEL: Air, Dust and Time. Here in part III we’re going to explain the second factor: Dust.
There are a few questions we need to answer about dust:
- How much dust is produced from cutting materials?
- How much silica is in that dust?
- How much dust is too much?
Let’s start by figuring out how much dust is released from a common cut. For this example we’ll use a familiar material, a standard 60mm paver.
The typical weight for masonry materials is 145 lbs. per cubic foot, or 65,770 grams. This works out to 38 grams per cubic inch. Remember that, we’re going to use it later.
Now let’s look at the dimensions of the cut we’re making. Our depth of cut is 60mm or 2.36 inches, and our length of cut is 4 inches. If we’re using a standard table saw our blade width plus overcut is 0.125 inches. Multiplying these will give us the volume of material removed in a single cut:
2.36 inches * 4 inches * 0.125 inches = 1.18 cubic inches
If we’re removing 1.18 cubic inches of material, and we know that a cubic inch weighs 38 grams, we can calculate the weight of material removed:
1.18 cubic inches * 38 grams = 45 grams
So in one standard 60mm paver cut using a table saw we are removing 45 grams of material.
Converting to micrograms we can start to understand how much dust we’re talking about in OSHA terms:
= 45,000 milligrams
= 45,000,000 micrograms
We’ve determined that one paver cut releases 45 million micrograms of dust into the air. But not all of that dust is silica.
Our tests have shown an average silica content in masonry materials of +/- 20%. If 20% of that dust is silica, we can calculate silica by weight:
45,000,000 * 20% = 9,000,000 micrograms
Through these calculations we now know that 9 million micrograms of respirable crystalline silica is released into the environment in one paver cut.
Great, so what does that mean?
The OSHA PEL
Looking again at the current OSHA PEL:
50 micrograms of respirable crystalline silica per cubic meter of air, or:
In other words, for every cubic meter of air a guy breathes, he’s allowed to inhale 50 micrograms of silica.
Let’s go back to part II of this series, where we learned that an average male working at a moderate pace breathes 16.8 cubic meters of air in an eight-hour day. At 50 micrograms for every cubic meter, that means a guy is allowed to breath 840 micrograms in a day within the current OSHA PEL.
And one standard paver cut releases 9 million micrograms.
That’s enough silica to exceed 10,714 days worth of dust exposure.
Using our example, if you were to breathe 100% of the dust from one single paver cut, you’d be breathing over 29 years worth of silica in the OSHA PEL.
We don’t recommend doing that.
Considering these numbers, smart and responsible contractors should understand how dust is measured, how much is too much, and how much their workers are being exposed.
In this example we’ve learned how much silica is produced from a single paver cut relative to the current OSHA PEL. While we know how to calculate the amount of dust being released, we still need to figure out how much of that dust your workers are being exposed to.
The last factor we need to consider is time. For how much time can workers cut, at what level of exposure, and still be within the OSHA PEL?
You’ll have to read Part IV in our series on Understanding the OSHA PEL.
Previous articles in this series:
Silica Part I: What Contractors Should Know About the OSHA PEL
Silica Part II: The OSHA PEL in Simple Terms