Long-term exposure to loud noise (in excess of 80 dB) leads to hearing loss. That is caused by local damage to the sensory cells in the cochlea. The cochlea is the part of the hearing organ in which external sounds are converted into nerve impulses that are sent to the brain.

Deterioration in sensitivity results in mounting hearing loss in the high tone area, especially around the 4000Hz frequency. The damage to the cochlea is irreversible. The individual sensitivity of people exposed to noise differs from person to person, but those over the age of 40 are more sensitive to noise because the nerves begin to show signs of ageing. People already suffering from disorders of the inner ear are also sensitive to further damage being caused by noise. The cochlea is affected by the noise to the same extent, but the hearing loss already suffered causes this to go almost unnoticed.

The noise damage is determined by the duration, rhythm and level of the noise and individual sensitivity among different people.  Factory noise is often above the 80dB limit. Noise at pop concerts and motorcycle races can be as loud as 120dB. Continuous noise is much more harmful than intermittent noise because in the latter case the individual nerve cells have the chance to recover. Apart from the fact that people working in excessively noisy environments suffer hearing loss, they also get tired more quickly and become more irritable, which has an adverse effect on their performance.  

Noise damage is irreversible: prevention is the only cure. The best approach is to try to achieve working conditions where the average noise level is well under 80 dB. But that is not always possible. In that case, the solution will have to be sought in hearing protection. The hearing protector must attenuate the noise level to such an extent that the noise reaching the inner ear is under the 80 dB limit. That means that with a noise level around 100 dB, the noise will have to be reduced by more than 20 dB.  

Ear caps and solid ear plugs reduce noise over all sound frequencies, which also reduces people's ability to hear what others are saying. There are however ear protectors that use filters to selectively reduce the high tone noises, such as the EARfoons, which as well as effectively reducing noise make it possible to clearly understand speech.