Most people think that hearing loss is caused by exposure to very loud machinery in factories, or chain saws, or guns, or the music at rock concerts. However, hearing loss can result from any intense sound, even the music played at a symphony. The noise does not have to hurt the ears of the listener to be damaging. Sound levels above 85 to 90 decibels can be damaging depending on the duration of the exposure.
Temporary Threshold Shift
Many of us have experienced a Temporary Threshold Shift (TTS) following exposure to intense sound. People working in factories will often notice that at the end of their shift that their ears are ringing (tinnitus). They also notice a dull sense of hearing. Sound just isn’t as sharp as it usually is.
The next morning, the ringing may be gone and their hearing is sharp again. Things are much better. The day before, following their shift at work, they experienced TTS. TTS should be recognized as a warning sign from your auditory system. Your auditory system is telling you that you were exposed to too much sound and you better do something about it. You should either avoid the noise the next time you are aware that it will be present, or obtain hearing protection if you have to be exposed to the noise because of work.
Permanent Threshold Shift
If a person experiences Temporary Threshold Shift (TTS) day after day, that person will probably end up with a Permanent Threshold Shift (PTS). The inner ear hair cells can take only so much abuse until they die. In the past, it was thought that the hair cells were damaged by the “shearing” of the hair cells by the intense waves in the fluid in the inner ear (cochlea). For extremely loud sounds such as a blast from a bomb or explosion, the shearing or obliteration of the hair cells is probably true. Extreme blasts can cause sudden destruction of the hair cells.
With lower levels of noise, the hair cells may be destroyed by an increased level of free radical molecules. The free radicals can cause serious damage to cells unless they are “neutralized” by free radical scavengers. For noise exposures that are not damaging, it is hypothesized that free radical scavengers are produced at a rate that is adequate to neutralize the cytotoxic free radicals. For noise exposures that lead to permanent hearing loss, however, cytotoxic free radicals may be generated at a rate that exceeds the ear’s ability to neutralize them.
The bottom line of all this is that the damage to the hair cells may be a chemical problem and not always a shearing and tearing of the hair cells. In any event, there is no way in humans to regenerate the obliterated hair cells. In certain birds, researchers have discovered that they can obliterate the hair cells with intense noise and the birds can regenerate (grow) new hair cells. Researchers are looking for the reason that birds can do this and we humans cannot. It is possible that in the future there will be a pill or magic potion we can take to restore our hearing.
Permisisble Noise Levels
How long can you be exposed to a particular level of noise without risk of permanent hearing loss? According to the Occupational Safety and Health Administration (OSHA), when employees are subjected to sound exceeding those listed in the table below, feasible administrative or engineering controls shall be utilized. If such controls fail to reduce sound levels within the levels of the table, personal protective equipment shall be provided and used to reduce sound levels within the levels of the table.
OSHA further indicates that the employer shall administer a continuing, effective hearing conservation program whenever employee noise exposures equal or exceed an 8-hour time-weighted average sound level (TWA) of 85 decibels measured on the A scale (slow response). So OSHA is recommending a level of 85dB as a trigger to begin a hearing conservation program.
Those levels have been used in industry for several years. However, the knowledge from all the data contained in the above table can be used in areas outside the industrial realm. For example, if someone listens to and MP3 player at 110 dB for more than a half hour, the individual is running the risk of permanent hearing loss. If they turn the volume down to 92 dB, then could listen to it for six hours before possibly causing hearing loss.
Again, the sound does not have to be painful to cause damage. As the table above represents, chances of permanent hearing loss depends not only the level of the sound but also the duration. If you start to notice the symptoms of TTS discussed in the previous section, then you better beware!