Silica Part IV: TimeJoel Guth
UPDATED FOR THE NEW OSHA PEL 2017
What We’ve Learned
In part I of this series on silica dust and the OSHA PEL, we described what a permissible exposure limit (PEL) is and the three factors that make up the OSHA PEL: air, dust and time. We also learned that exposure is calculated in terms of micrograms per cubic meter (μg/m3), and we interpreted the current OSHA PEL as 50μg/m3.
In part II we set out to learn how much air a person breathes. We found this depends on a number of factors, but that generally speaking an average adult male working at a moderate activity level breathes 16.8m3 of air over the course of an 8-hour work day.
And in part III we calculated that 45 million μg of dust is released into the environment in one standard paver cut. With an average silica content in masonry materials of +/– 20%, that means one paver cut releases approximately 9 million μg of respirable silica – enough to exceed 29 years worth of silica exposure under the OSHA PEL.
Total Maximum Exposure
We can use what we’ve learned so far to determine the total maximum silica exposure for a person over an eight-hour period. Since a worker breathes 16.8m3 of air in eight hours, and the OSHA PEL is 50μg/m3, the total maximum exposure for eight hours is 840μg (16.8m3 * 50μg).
In other words, 840μg is the total amount of silica a worker can breathe in an eight-hour work day using a time-weighted average (TWA) – regardless if that happens in eight hours or five minutes. We’ll come back to maximum exposure shortly.
What We Still Need to Know
With an understanding of air and dust, the question we still need to answer is time. For how much time can a worker cut, at what level of exposure, and still be within the OSHA PEL?
While we can calculate how much dust is released from cutting, and the content of silica in that dust, we can’t accurately calculate how much of that dust is being breathed without using other tools. For this we have to use air monitoring.
Air monitoring uses a battery-operated vacuum attached to an operator’s shirt collar, collecting air samples right where the operator breathes. The results will tell us the exposure level for specific work practices. Once we know exposure level, we can calculate time.
Let’s use an example. Using round numbers, if we learn from air monitoring that during a specific work practice a worker is exposed at exactly 50μg/m3, he’d be able to cut for eight hours. At an exposure level of, say, 100μg/m3 he’d be able to cut for only four hours. And at 200μg/m3, he’d be able to cut for only two hours.
What does this mean to you? It means that to know where you stand, you need to do air monitoring to learn what your exposure levels are. This is an inexpensive and relatively easy process for average contractors. Once you learn your exposure levels, you can decide what options are best-suited to control silica exposure on your job sites. Check out our dustless tile saws and the impact they can have on your health.
Know the hazard. Know your exposure. Know the standards. Take action.
Want more? Visit our Silica Dust Home Page.