Hazardous noise affects the functioning of the inner ear, which may cause temporary hearing loss. After a period of time away from noise, the hearing may be restored. With further exposure to hazardous noise, the ear will gradually lose its ability to recover and the hearing loss will become permanent. Permanent hearing loss can also occur suddenly if a person is exposed to very loud impact or explosive sounds. This type of damage is known as acoustic trauma.
A permanent hearing loss results from the destruction of hair cells in the inner ear. These cells cannot be replaced or repaired by any presently known medical treatments or technology. Usually, hazardous noise first affects the ability to hear high-frequency (high-pitched) sounds. This means that even though a person can still hear some sounds, the conversation will start to sound ‘muffled’ and a person may find it difficult to understand what is being said.
Communication difficulties occur especially when there are competing background noises. Modern hearing aids may improve the ability to hear speech but they are unable to completely restore the clarity of the full hearing function. Workers exposed to hazardous noise may also experience tinnitus, which could become permanent.
When severe, it may disrupt sleep, reduce concentration, make people extremely irritable and lead to depression. The degree of hearing loss that occurs is dependent on how loud the noise is, how long someone is exposed to it and, to some extent, individual susceptibility.
The frequency or pitch can also have some effect on hearing loss since high-pitched sounds are more damaging than low-pitched ones. Exposure to a number of common industrial chemicals and some medications can also cause hearing loss or exacerbate the effects of noise on hearing.
These substances are called ototoxic substances. Ototoxic substances absorbed into the bloodstream may damage the cochlea in the inner ear and/or the auditory pathways to the brain, leading to hearing loss and tinnitus. Hearing loss is more likely if exposure is to a combination of substances or a combination of the substance and noise.
There is also some evidence that exposure to hand-transmitted vibrations can exacerbate the effects of noise on hearing.