It is not uncommon to have a worker experience a temporary spike and then recovery in audiogram levels. The official medical term for this condition is a Temporary Threshold Shift (TTS). TTS is defined as a temporary neurosensory hearing loss that recovers almost completely once the noxious stimulus is removed. The amount of time over which recovery occurs is unclear and controversial. Sixteen hours has been used in the past, but some people with TTS require longer periods to recover. Dobie (a respected authority on the interpretation of audiograms) uses a 24-hour threshold; however, some argue that days or months may be required to recover TTS, especially if the case is associated with acoustic trauma. A clinically important feature of TTS is that it is rarely apparent to the subject because of its relatively low magnitude and relatively high frequency. Repeated TTS over weeks, months, and years with a failure to recover completely can easily become a permanent threshold shift (PTS).
A temporary threshold shift which results in a temporary hearing loss is normally caused by exposure to intense and/or loud sounds or noise for a shorter or longer time. This could be e.g. an explosion or a concert. However there may be other factors dependent upon the individual patient (allergies, specific medications, recent cold or flu, etc). In general, structural damages to any anatomical part of the ear can cause hearing-related problems. Usually, minor bending of the stereocilia (inner ear) is associated with temporary hearing loss and is involved in auditory fatigue.
There are also several factors that may not be harmful to the auditory system by themselves, but when paired with an extended noise exposure duration have been shown to increase the risk of TTS. One such factor is physical exercise. Although this is generally good for the body, combined noise exposure during highly physical activities was shown to produce a greater TTS than just the noise exposure alone. Heat exposure is another risk factor. As blood temperature rises, TTS increases when paired with high-frequency noise exposure. Finally, contact with ototoxic chemicals such as styrene, toluene and carbon disulfide heighten the risk of auditory damages.
It is important to understand that noise exposure itself is the main influential factor in threshold shifts and auditory fatigue, but that individuals may be at greater risk when synergistic effects take place during interactions with the above factors. I trust the above information provides a sufficient explanation of some possible causes for a temporary threshold shift. If the patient has concerns regarding the state of his/her hearing, we would recommend they book a follow up appointment with their family physician or an audiologist. It the audiograms were completed through Health & Hearing Conservation Consultants, a letter detailing the audiometric history of the patient can be provided upon request.