Noise-induced hearing loss
Noise damage to hearing can be divided into two types: gradual and increasing loss of hearing, and the more extreme acoustic trauma.
Gradual and increasing loss of hearing comes from repeated exposure to loud noise, which can be with us throughout the day: a screeching train, a noisy office, listening to an MP3 player or frequently going to loud music gigs, nightclubs or pubs. At first, any hearing loss is temporary (a ‘temporary threshold shift’ of hearing). However, if the exposure continues or the ear is not given enough time to recover, the hearing loss becomes permanent and irreversible.
Acoustic trauma is an immediate loss of hearing after a sudden, exceptionally loud noise, such as an explosion.
Over the past ten years, scientists have made a lot of progress in understanding the exact process by which noise-induced hearing loss occurs. Very loud noises are thought to over-stimulate the sensory hair cells leading to the over-production of potentially damaging chemicals called free radicals. While cells can normally cope with a low level of free radicals, too high a level will damage the structure of the cell and eventually lead to its death. Exceptionally loud noise will burst the ear drum and cause extensive damage to the inner-ear.
How loud is too loud?
Noise is measured in dBA, which is a decibel scale modified to take into account the sensitivity of human ears to different pitches of sound. It is a logarithmic scale, which means that an increase or decrease of 3 dBA represents a doubling or halving of intensity, the energy it contains. So, for example, 73 dBA is twice as intense as 70 dBA. However, due to the way we hear sounds, a person with normal hearing will only think a sound has doubled in loudness when it is ten times more intense. For example, 80dBA will only sound twice as loud as 70dBA despite actually having ten times as much energy.
An average conversation will reach around 60 dBA, while a busy street can peak at 80 to 90 dBA. Generally, exposure to sound levels below 80 dBA are unlikely to cause any hearing damage. Prolonged exposure to sounds over 80 dBA can damage your hearing and the risk increases as the sound level increases. At 140 dBA, noise causes immediate injury to almost any unprotected ear.
As a general guide, if you have to shout to make yourself heard by someone two metres away the noise level could be dangerous.
Although there are laws about acceptable levels of noise in different situations, it is impossible to set an objective noise level that is safe for all. Provided the ear is allowed ample rest afterwards, a level of 80 dBA might be tolerated for up to 8 hours, but increase that level by just 3 dBA and the time is reduced to just 4 hours. By 95 dBA the tolerance is less than 15 minutes.
However, no two people will have an identical tolerance to noise. Research suggests that a genetic predisposition towards hearing loss is an important factor.
How does noise damage hearing?
Repeated exposure to excessive noise can kill hair cells and damage the hearing nerve making them unable to work properly, resulting in a permanent hearing loss. This is called a ‘sensorineural’ hearing loss.
Someone with noise-induced hearing loss will first experience a difficulty hearing the high-pitched sounds that are important in being able to hear people talk. So although some sounds will remain reasonably clear - like people’s actual voices - the words they are saying will be distorted. For example, you may be watching a TV programme and be able to hear that the characters are talking, but what they are saying will be hard to understand. Naturally, this makes trying to understand a single person speaking in a noisy environment or among a babble of other voices very difficult and frustrating.
However, it is possible to recognise signs of hearing damage quite early. If you have difficulty hearing people, or you suffer tinnitus (noises in the ear or head) on leaving a noisy environment, your hearing may well be damaged. Repeated exposure to noise at a high level can leave you with a permanent hearing loss or tinnitus.
If you do suspect your hearing is damaged, make an appointment to see your GP immediately. Usually, s/he will refer you to an audiologist or ENT (ear, nose and throat) specialist who will carry out tests.
If you are diagnosed with either a hearing loss or tinnitus, contact Deafness Research UK. We produce a wide range of literature on all aspects of hearing loss and can offer information and advice on the options available to you.
