From time to time, certain articles come across my desk that make me sit up (typically spilling my coffee in the process) and reread it to make sure that what I thought I was reading, was indeed the case.
Well, this happened to me about 15 years ago (and many times since, but I didn’t always spill my coffee). There is an interesting article by Gates, Schmid, Kujawa, Nam, and D’Agostino published in 2000 in the journal Hearing Research. For those who are not aware of Hearing Research, it is as difficult to get an article published in that journal as it is to get something published in the Journal of the Acoustical Society of America (JASA). Without going in to too much detail, if an article is in Hearing Research or JASA, you can be darned sure that it has been peer reviewed by the very best, and what it has to say is not just sloppiness or opinion – the content is well packed with thoughtful and well-designed experimental protocols.
The Gates et al. article has the rather benign name “Longitudinal threshold changes in older men with audiometric notches”. This may seem rather insignificant, but the article is anything but.
The two major items deriving from this article (at least for me) were that (1) progression due to noise exposure in not necessarily the same as the progression observed from “merely” aging, and (2) noise-related hearing loss may continue to be observed after the worker has left the noisy environment.
Most regulatory agencies in the worker’s compensation area assume that once the worker is no longer exposed to noise, then no more hearing loss that may be observed in the future would be related to the previous exposure. This article suggests that that assumption may be erroneous, or at least over-stated.
Gates and colleagues looked at 203 men from the Framingham Heart Study cohort over a 15 year period and examined progressions in their audiograms. Most of the men were retired as the sample had a mean age of 64 (58-80 years old) so were not assumed to be affected by significant occupational noise exposure.
In this study, they divided the 203 men into three groups – no audiometric notch (N0), a small audiometric notch (N1) in the 3000-6000 Hz region of less than 35 dB HL, and a larger audiometric notch (N2) greater than 35 dB HL. As expected, the N0 group (no notch) correlated highly with the absence of noise exposure over their careers and the N2 group had the greatest amount of noise exposure.
That is, for some reason, those with a significant noise notch in the 3000-6000 Hz region, after retiring and being away from work, also continued to have a 2000 Hz hearing threshold decrease that was in excess of what would have been suspected for just age-related issues alone. Their conclusion that “… the noise-damaged ear does not ‘age’ at the same rate as the non-noise damaged ear” is actually quite astounding.
And the finding that the 2000 Hz region of hearing continues to decrease in excess of any age-correction, especially if the damage in the 3000-6000 Hz was significant enough (in excess of 35 dB HL), is quite telling as well.
This article,and others, state that the mechanism(s) are unknown that evidence does point towards the prior noise exposure to the cochlea.
As the conclusion, restating an above paragraph from this blog “Most regulatory agencies in the worker’s compensation area assume that once the worker is no longer exposed to noise, then no more hearing loss that may be observed in the future would be related to the previous exposure. This article suggests that that assumption may be erroneous, or at least over-stated.”