Silica Part II: The OSHA PEL in Simple TermsJoel 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 II of this series we explain the first factor: Air.
The root question we need to answer is, how much air does a person breathe? And how do we measure that? As we described in part I, the answer depends on a number of factors. How big of a person? What is their fitness level? At what activity level are they breathing? As contractors we were always pretty good at math, so we’ve broken these questions down into a relatively simple math problem.
First of all let’s consider the new OSHA PEL: 50 micrograms of respirable crystalline silica per cubic meter of air, or:
How much is 50 μg/m3? Since the PEL is expressed in micrograms per cubic meter, we’ll have to convert our measurements into those terms. Starting with air, the generally accepted scientific measurement is liters per minute. We can also use scientific studies to find an average range of air consumption. See the listed ranges for each activity level in the chart below:
For the construction industry we’re going to make an assumption that the average adult male employee works at a moderate activity level (hopefully). Looking at this chart, we’ll use 35 liters per minute, an activity level right between a brisk walk and mowing the lawn.
Most of us are familiar with a liter bottle of soda. So, the average adult male working at a moderate level breathes 35 of these every minute.
With what we know, let’s start to multiply. If one minute equals 35 liters, then one hour (60 minutes) is equal to 2100 liters.
35 liters/min. x 60 min. = 2,100 liters/hour
Now, using an 8-hour work day, we can calculate the average worker breathes 16,800 liters of air each day.
2,100 liters/hour x 8 hours = 16,800 liters/day
This is helpful to know, but still doesn’t get us to the OSHA terms of micrograms per cubic meter. We have a little more math to do, but fortunately our problem is in metric terms which makes it easier.
One cubic meter is equal to 1000 liters. If we divide our 8-hour day average of 16,800 liters by 1000, we get 16.8 cubic meters.
16,800 liters/day ÷ 1000 liters in a cubic meter = 16.8
In other words, the average adult male working at a moderate activity level breathes 16.8 cubic meters of air over the course of an 8-hour work day. For our visual readers, here is what 16.8 cubic meters looks like:
We’ve made it to cubic meters! So what the heck is a microgram, and how many of those are in 16.8 cubic meters?
You’ll have to read part III in our series on Understanding the OSHA PEL. Subscribe below to get the next installment.