OSHA standards are a little more forgiving than audiology guidelines i have seen. Here are the OSHA noise guidelines:
The standards that I posted in another thread to a web page article on safe exposure limits come from a 1998 NIOSH Study of Safe Occupational Limits (see chart posted below). So did the good ol’ government decide that, hey, we can’t have a lot of jobs if we have these noise limits, hence the more generous job-tolerant OSHA standards you cite (you don’t give a link to the original source - did you check to make sure it’s not, say, 1974 limits?, @Don?).
Actually, NIOSH and OSHA have agreed to disagree. The following page discusses the differences, and includes a third standard for mining. The 1998 NIOSH standard is still in effect as far as the CDC and NIOSH are concerned, apparently. If you scroll down to the bottom of the CDC page (first link) there is a comparative table of the difference. So it’s like Clint Eastwood says in Dirty Harry, “Are you feeling luck today, punk?! Do ya like your chances?! …” or something like that!
If I had to guess I would say the difference is politics, where some industry needs higher guidelines and will pay to get it (political contributions).
That would kinda be my bet, too. NIOSH seems overly conservative, OSHA overly generous. Maybe we should split the difference but those logarithmic calculations get me down!
Edit_Update: Actually, the basic math is not that complicated. For each halving of allowed exposure time, the NIOSH standard only allows the REL (Recommended Exposure Level) to go up by 3 dB. OSHA more generous. For each halving of the allowed exposure time, OSHA allows the PEL (permitted exposure level) to go up 5 dB, at least up to the 115 dBA level.
The following graph, G-9 from the OSHA PEL Sound Standards, is interesting. It shows the equivalent sound levels in terms of exposure damage risk - basic a frequency “contour map” - kinda like a topo map. The contours dip around 4K, meaning frequencies around there are considered most damaging.
Perhaps two things are going on. High frequency sound is more energetic than low frequency sound because you are packing more amplitude peaks into the same time frame. But the human ear canal is most resonant aroud 3.5 KHz, according to ReSound literature on feedback. So perhaps the dip in the contours around 4 K is like feedback in reverse. It’s easiest for sound to efficiently get into the ear around 4 K and after that as the sound rises to higher frequencies, there is less efficient delivery of the sound to the ear (is my guess) so the contours rise again.
Equivalent sound level contours. Octave band sound pressure levels may be converted to the equivalent A-weighted sound level by plotting them on this graph and noting the A- weighted sound level corresponding to the point of highest penetration into the sound level contours. This equivalent A-weighted sound level, which may differ from the actual A-weighted sound level of the noise, is used to determine exposure limits from Table 1. G-16.
It is expensive to make a factory quieter. I would guess that both of these guidelines are generous compromises. I recall seeing some evidence that even 8 hours at 75 dB could induce some negative changes in the auditory system (I feel like it was something Jos Eggermont was looking at).
And there are other factors. Some people are genetically at a higher risk for NIHL. Smokers are at a higher risk. Workers in factories that use ototoxic solvents are at a higher risk (noise damage + ototoxic solvent damage = greater than what one would expect if it was just summative). Higher levels of stress seem to also increase your risk of damage.
On the other hand, there is some sort of inverse relationship with hearing loss. The gain targets for, for example, children with severe-profound hearing loss are way over these safety standards and yet their thresholds do not drop slowly over time with appropriate amplification. I am not aware of any work decribing this relationship at this time, but I may just not have found it yet. Outer hair cells are more vulnerable to noise damage than inner hair cells, and IIRC past a threshold of about 65 you can assume that you have few outer hair cells left at that frequency.
Find out on further reading that I’m obviously in error on the first point. dB SPL is dB SPL no matter what the sound frequency and a measure of the sound pressure outside your ear. It turns out that all the contours are related to the acoustics of the human ear and the bio-mechanics of sound transfer to the inner ear - and high frequency sound is better conducted to the inner ear and more damaging once there than lower frequency sound or much higher frequency sound (the rising part of the curves on the right that I’m referencing in my post above with the contour “equal loudness” curves).
The dBA variable that’s depicted changing in the curve is not a direct measure of damage potential. It’s just measure of dB after the sound is passed through a particular type of filter (A filter ) (once) thought to mimic human hearing sensitivity closely. The A-filter was designed based on the sensitivity of human hearing over a frequency range to pure tones compared to the loudness perceived relative to a 1 KHz standard tone. Sound measured through a A-filter, since it mimics the human range of hearing sensitivity, has been found in extensive hearing damage studies to reasonably reflect the potential for sound damage as a function of frequency. But the problem is is that noise is usually not a pure tone but more like white noise so the A-filter-based standard, an American standard, is considered inadequate by some and in Europe, a different standard based on more general types of noise is used as the basis of filtering to determine damaging noise levels. Another variable is the tests were done with headphones on, I believe, or at least the sound coming from the side, and the more usual assault of noise is from the front so modern day standards have been modified to take that into account. The dBA standard is popular in part because of its historical acceptance and history and dBA-based sound meters are cheap and plentiful.
Sorry for errors in my explanation. I think that if I were a Ph.D. in audiology, I would understand a bit better than I do currently!
I found a great in-depth article by an audi at etymotic’s on the difference between the OSHA and NIOSH standards. She says straight up that the OSHA standard is based on politics and the NIOSH standard is actually based on real scientific studies. She also points out that Europe, China, and most of the rest of the world follow the NIOSH standard with an 85 dBA limit to exposure for 8 hours and a 3 dBA exchange rate, not 5 dBA exchange rate as for OSHA, for 2-fold reduction in allowed exposure time every time the exposure level rises by the exchange amount. (etymotic’s interest in this is that they want to sell you noise dosimeters)
She points out that even the NIOSH standard does not guarantee complete hearing protection. If followed, the standard creators still expect there will be additional hearing loss from the amount of noise exposure allowed and 8% of the exposed population would suffer a hearing sensitivity decrement of > 25 dB (or whatever the cutoff is for defining loss) over a 40-year occupational exposure (or whatever the lifetime work period is - too lazy to look up the “whatever’s”). I think it is her article, too, that points out that these are just limits for the 8-hour work day and in no way take into account what happens during the other 16 hours of the day. And also that the EPA’s recommended limit (Yes! the EPA joins the crowd, too!) for a TWENTY FOUR HOUR day is an average exposure of 70 dBA. So, yep!, if you’ve been working a loud shift at 85 dBA for 8 hours, when you come home, you’d better live in mouse-like quiet and not talk too much to not exceed a 24 hour average of 70 dBA!