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Are the results on my report a Time Weighted Average (TWA)?

This is one of the most frequently asked questions by customers once they receive their report.

The answer is your result is most definitely a TWA.  Which means the result is the average concentration for the period of time that you performed the sampling.  However, what almost everyone is really asking is “Is my result an 8 Hour TWA?”  My answer to that is, if you monitored for 8 hours, it’s an 8 hour TWA.  If you monitored for 12 hours, it’s a 12 hour TWA;  6 hours, then a 6 hour TWA, etc.

The follow-up question is usually, how do I convert my result to an 8 hour TWA?  Now the answer to that is tricky and it depends on what you know about the facility.  Here’s what I mean:

If the exposure lasted 6 hours and you monitored for 6 hours, the result you have is the TWA for the 6 hours.  To determine the eight hour TWA, you have to ask yourself if you believe:

  • the exposure was zero after I stopped the monitoring?
  • the exposure was the same after I stopped the monitoring?

Obviously, I used the word “believe.”  I could also say, “in your professional judgment.”  This is due to the fact that you didn’t monitor during this time and you have to use your judgment based on what you know was going on at the facility.  Let’s presume you know the person being monitored left the facility or continued to do the same job during hours 7 and 8.

If the person was performing the same task for the rest of the day and you would expect the same concentration all day, then you do not have to change your result.  A concentration is like the speed of a car.  If the car was going at a constant speed for 8 hours and you watched the car go 55 MPH for 6 hours, then you know the 8 hour MPH will be 55.  No adjustment necessary.

However, if you observed the car going 55 MPH for 6 hours, then the car stopped for 2 hours, then to determine the average speed of the car for 8 hours, you have to factor in 6 hours at 55 MPH and 2 hours at 0 MPH.

The shorthand equation for doing this is simple:  Your result X is the time on the report / 8 hours or 480 minutes = the 8 hour TWA.


Monitored 5 hours and the lab report says 100 ppm.  The last three hours there was zero exposure.  So

100 ppm X 5 hours / 8 hours = 62.5 ppm

If you do not want to make a judgment about the exposure when not sampling, then monitor for 8 hours whether or not you believe there is any exposure.

In addition, if you monitored for 12 hours and want to know the 8 hour time waited average, there is another choice.  Obviously, if the exposure was the same all day, then the 8 hour TWA will equal the 12 hour TWA.  So no adjustment necessary.

But if you want to assume all the exposure happened during 8 hours, then the equation is the same.  (This is the most conservative approach.)  Your result X is the time on the report / 8 hours or 480 minutes = the 8 hour TWA.  So:

100 ppm X 12 hours / 8 hours = 150 ppm

The lab frequently is asked why all results are not reported as an 8 hour TWA.  I hope after reading this, the answer is clear.  We cannot make a judgment about the exposure at the facility after monitoring.  Only the people actually at the site can do that.

Questions?  Give us a call.