I have always wondered how hearing aids with MPO greater than 90 dB are being used for more than 8 hours a day and are allowed to be dispensed. How is it that power or moderately powered aids can exceed output levels set forth by OSHA?
This question does come up from time to time, and to my knowledge, there was at least one study conducted decades ago that suggested power analog hearing aids could have contributed to a progression of the hearing loss due to high sound exposure (sorry, can't find the reference for the life of me!). The question you pose essentially asks, "for ears that are already damaged, how do we know that our efforts to improve speech understanding and sound awareness, by boosting sound into the person's residual auditory area, are not causing further cochlear damage?" The answer is: they could. How likely that is needs to consider what the patient's typical sound environments entails. We need to do our best to estimate the patient's daily noise dose (sound level over time), and compare that to a damage risk estimate (like the more conservative damage risk criterion promoted by NIOSH: maximum exposure of 85 dBA for 8 hrs, with 3 dB trading ratio;or the less conservative damage risk criterion enforced by OSHA: maximum exposure of 90 dBA for 8 hrs, with 5 dB trading ratio).
As an example, let's consider two patients using similar hearing aid settings. First, a mainstreamed 12-year-old with a flat 70 dB HL loss using WDRC hearing aids that boost average level speech into 85-95 dB SPL (A-weighted;ie, 85-95 dBA), with MPO set at 130 dB peak. He's in class all day long (roughly 6 hours of instruction) and participates in an extracurricular activity for another 2 hours. He does his homework in quiet and watches 2 hours of television. We're thinking he gets roughly 10 hours of exposure to average level speech, but, as we know, not all 10 hours were constant at 85-95 dBA. The "duty cycle" of typical speech (how much "on time vs. off time") is likely to be less than 50% (depending on how "chatty" teachers and friends are during the day). I'm not an expert in what percentage of time a person listens to speech in a typical day but let's assume a worse-case scenario of 50% (half of the child's day involves exposure to typical speech sounds). This means this 12-year-old with WDRC hearing aids gets at most 90 dBA for 5 hours. This worst case scenario exceeds the NIOSH damage risk criterion by just a little bit, but does not exceed the more liberal OSHA damage-risk criterion. If the duty-cycle is much less than 50% (say 25% of his daily sound total is speech) then his exposure would be considerably less, and not exceed even the more conservative NIOSH damage-risk criterion. Is this 12-year-old at risk? Perhaps, which is why it's necessary to do at least annual audiograms. Is it worth the risk for this 12-year-old to have good audibility? That may be a judgment call, but likely it is, given that the risk is relatively small. You can see where datalogging offered by newer hearing instruments might be beneficial in helping to tease out daily sound exposures.
The second patient scenario may be more obvious: a 50-year-old car manufacturing worker with a 70 dB HL loss using the same WDRC hearing aids/program as the 12-year-old previously mentioned. He works in 88 dBA of noise on the job, and for an 8-hour shift, his exposure does not exceed the OSHA Permissible Exposure Level (8-hrs at 90 dBA), but does exceed the OSHA Action Level (8 hrs at 85 dBA). His co-workers use earplugs (as mandated by OSHA when the exposure exceeds the Action Level) but he does not, because he figures his hearing is "shot already" and he cannot hear speech or any other important sounds (like forklift back-up alarms) if he isn't using his hearing aids. So, his hearing aids are in serious compression because of the high level ambient noise, but even if the aids pass sound through only at unity gain (and likely are giving a few dB of gain even in full compression), this 50-year-old is regularly getting a sound overexposure. The duty-cycle of sound in his environment is much closer to 100%, so he could easily have 95 dBA exposure (just 7 dB of gain from the hearing aids) for his 8-hour work day. This patient is at risk for progressive hearing loss from noise, and the risk is possibly exacerbated by using hearing aids. He likely is not aware that his employer is responsible for ensuring a safe work environment that does not require him overexposing himself.
Thank you for your question!
Brian Fligor, Sc.D., is the Director of Diagnostic Audiology at Children's Hospital Boston and holds a faculty appointment of Instructor in the Department of Otology and Laryngology at Harvard Medical School. His primary research interests are investigating causes of acquired hearing loss, particularly in the pediatric population. Dr. Fligor's work on the potential for hearing loss from commercially available portable music players has been the subject of considerable popular media recently, including being spoofed on David Letterman in 2006.