Safety Tidbit #29 – Noise Thresholds

Safety Tidbit #29 – Noise Thresholds

This week I was conducting some noise dosimetry.  I like dosimetry over just taking noise readings as the dosimeter goes wherever the worker goes and evaluates the sound pressure reading every minute.  Much more efficient than me lingering around using a handheld meter and trying to get a reading in the “hearing zone” of the worker.  So I find using the dosimeter a much more elegant solution.  Now if I am trying to pinpoint a noise source or even better getting a profile of the source so I can reduce the noise levels, then my handy-dandy meter is my best friend. Especially when the meter can split out the octaves.  But I digress. What I wanted to discuss today, although it will be a brief discussion, is the time-weighted averages we hear (no pun intended) so much about. 

Most of us know of OSHA’s 8-hour Permissible Exposure Limit (PEL) for noise which equals 90 decibels (on the A-weighted scale, again let’s not delve into what is dBA just now).  But OSHA also has the medical surveillance action level (AL) of 85 dBA.  Unfortunately, these two values are not quite apples and apples because OSHA dictates the parameters we must use to evaluate the workplace for each.  We use a dosimeter to help us evaluate however we have to program the equipment to look at the right noise and perform the correct calculations.  That’s where the parameters come in.  Since the Walsh-Healy Act, for the PEL, OSHA says to use a threshold of 90 dBA, capture everything between 90 and 140 dBA and use a five dBA excursion limit.  Then in 1982, Hearing Conservation Programs became all the rage and OSHA said to be more protective we want you also to evaluate noise between 80 and 130 dBA with a threshold of 80 dBA, and we will keep the excursion limit at five dBA.  We will not talk about the excursion limit today.

The threshold, 90 dBA for the PEL and 80 for the AL, what this parameter does, tells the dosimeter to ignore any sound levels that do not meet the threshold.  For instance, if I use my trusty handheld meter I find the noise level on the floor to be 88 dBA.  The dosimeter calculating for the AL will record this reading and add to the noise dose for the worker.  However, the dosimeter calculating for the PEL will ignore the level and not add to the noise dose for the PEL.  As you might guess a fair amount of noise may be overlooked when only evaluating for the PEL.  As a consultant, I wish to advise my client as well as I am able.  I usually don’t worry about evaluating the PEL since if they are protecting the worker starting at the AL, then the client is much more protective.  Saying that, if my dosimeter's reading comes back and the 8-hour time-weighted average exceeds 90 dBA the client must look to reduce the occupational exposure to noise, and I get out my trusty handheld meter or better yet my octave band analyzer and have some fun! Oh, and by the way, this is just OSHA’s take on noise and is quite old.  NIOSH and really everyone else sets the threshold for whatever is the limit of the equipment. 

I hope this was helpful. As usual, if you have comments or if you have a particular topic you wish me to discuss, please let me know.  Thanks for reading!