Time – How long can I work in the dust cloud?


How long can I work
in the dust cloud?

16.8m3 of air x 50μg of Silica =

840µg in 8 hours TWA

The Time Factor

Time is the third and final factor used to calculate the PEL. Once we know how much air is being breathed, how much dust is contained within that air, and how much of that dust is respirable silica, we still need to factor the amount of time that air and dust is being inhaled.
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).

Time Weighted Average

Using this chart, it shows that dependent upon the silica content in the breathing zone, an operator can perform a task that could be 200 micrograms or even 400 micrograms per cubic meter and still stay under an 8 hour TWA as long as the exposure in the other 6-7 hours per day is in clean air.

So you see, this allows us to go in for a limited time, 1 hours, 2 hours, 3 hours, into a dusty environment and still keep our 8 hour TWA below safe limits.

To know what your exposure levels are, you will still need to do air monitoring for tasks that might be above 50mc per cm but this still gives you many options for tasks that are required for a short duration of the day.

Know our exposure level, and use the time weighted average to work to your benefit.

Air monitoring graph iqcolors iq power tools

TWA Example

If you’re cutting at an exposure level of 200ug/m3 , you can cut for two hours and still be within the OSHA PEL. In this case your total exposure is 840ug.

Air monitoring graph example 1 iq power tools