In the western world unwanted noise can be found everywhere. The noise from the loud partying neighbors, works in the streets, car alarms and barking dogs in the early morning hours can cause severe discomfort to anyone who is forced to hear these shrill noises. Hearing is one of the early warning systems and the discomfort from unexpected or unusual sounds brings us to limit state, ready to react to potential danger.
Despite the fact that such sounds are annoying they are rarely organically harmful and very loud noise can damage the cochlea. Prolonged overload exhausts outer hair cells and the inner cochlear amplifier – the source of audioacoustic emissions – inadequate. A temporary hearing loss – that is, a change in auditory threshold – can be developed until the external acoustic cells can recover through the tranquility of nighttime sleep. This symptom is often the first thing that occurs in people exposed to noise, although they cannot appreciate its importance. Former employees of printing newspapers, with its noisy presses of their time, found that their cars were very noisy when going to work, but when they returned from their work they found their cars to be “quiet” and “nice” because their hearing had been influenced by the sound of the presses. Nowadays we see similar hearing loss in conjunction with the emergence of disruptive acclamation, after a night out in clubs in which the decibel produced touches and often exceeds the permissible limits.
This temporary shift in the threshold of hearing becomes permanent by continuous exposure to such an extent that it requires several months of abstinence from noise to achieve recovery. With continued exposure to the sound a permanent shift in hearing threshold is developed. The first frequencies to be affected seem to be the high frequencies, usually 4 or 6 kHz. Thus, there is a notch in the audiogram of such person when examined.
Why the injury must be seen in this area is not known with certainty, but this change in a person’s audiogram with a history of exposure to noise is almost on its own sufficient to make the diagnosis of occupational hearing loss caused by noise. If the patient is still exposed to noise, the groove deepens and expands, and the patient now begins to perceive the decrease of the ability to hear normally. You have trouble hearing in bars or nightclubs and later cannot hear the television clearly.
There are several factors that determine whether someone has suffered hearing damage from noise. The first and most important is the total exposure to sound that the cochlea has received. This is a combination of volume – i.e. the sound effects which were accepted by ear – and length of exposure. Doubling the power of sound, achieved by increasing the sound level by 3dB, reduces in half the time needed to cause an injury of the same severity. Thus, very high volume levels, like those experienced by soldiers of the artillery or those close to exploding missiles, can cause permanent deafness too fast for most people.
The second important factor is the individual sensitivity of each person. Almost no one shows loss of hearing to sounds in the environment of less than 80 dB, regardless of duration of exposure. For tones of 90 dB for eight hours work after a period of one year, it is typical for the 12% of workers to develop occupational hearing loss, although the severity varies between individuals. In an intensity of 85 dB, only 3% will suffer some damage. At present there is no way of regenerating outer hair cells and thus the loss of hearing is permanent.
The hearing damage due to noise has existed for many years but only recently has the law been strengthened in order to prevent this treatable problem. The reasons for this delay are simultaneously economic, technical and practical. The conversion of machines into non-noisy ones is very expensive. Using instruments that reduce the energy of the sound that reaches the ear is relatively simple, but makes it difficult to communicate and this can cause problems where there are other more obvious hazards in the workplace, such as moving vehicles and cranes.
Parents often ask about the risks children face from headphones and noisy popular music. Certainly in clubs and concerts there is a risk (especially for those located near the speakers) to damage hearing. Most patrons of the clubs are finding that they develop tinnitus and their hearing is reduced after a “good” night, but eventually that feeling disappears. However, ENT doctors still have patients with some permanent damage, which in some cases is extensive. This is most likely when they have taken part in an intense dance and there is some limited evidence that exercise and the reduction of oxygen make the cochlea more sensitive to noise.