5. How can sound exposure lead to hearing loss?
- 5.1 How many people are affected by sound-induced hearing loss?
- 5.2 What sound levels are we exposed to in our daily lives?
- 5.3 What is the relationship between sound exposure, hearing loss and age?
- 5.4 How do loud sounds affect the inner ear?
- 5.5 What factors can change the way sound exposure affects us?
Sound-induced hearing loss is irreversible and the main form of treatment is prevention. Commonly, damage to sensory cells of the inner ear builds up over time. The hearing loss goes unnoticed at first and increases until it reaches a certain degree where it becomes obvious to the affected person. In rare cases, exposure to very loud sounds can lead to immediate damage. More...
5.1 How many people are affected by sound-induced hearing loss?
Excessive exposure to loud sounds is a major cause of hearing disorders worldwide and is the main avoidable cause of permanent hearing loss.
Among workers, noise-induced hearing loss is the most common irreversible occupational disease. Worldwide, 16% of the disabling hearing loss in adults is caused by exposure to noise at work, although this proportion varies in different parts of the world from 7% to 21%.
Sound-induced hearing loss affects an estimated 10 to 15 million people in the USA. In the UK, about 350 000 people aged 35 to 64 years have serious hearing difficulties, including tinnitus, caused by exposure to noise at work. In France, a survey carried out in 2003 indicates that 7 % of employed workers were exposed to excessive sound levels above 85 dB(A) for at least 20 hours a week . Most exposed workers belonged to industry agriculture or the building sector.
In recent years, exposure to noise at work has decreased while exposure to loud sounds during leisure activities has become increasingly important, particularly for young people. In the USA, among a sample of children and teenagers aged 6 to 19 years 12.5% were found to have some hearing loss in one or both ears caused by exposure to loud sounds. The extent of the loss was larger for boys than girls, and for older children than younger ones. At present there is no equivalent data on the European population. More...
5.2 What sound levels are we exposed to in our daily lives?
Average noise levels in certain working environments can reach up to about 90-125 dB. People also expose themselves to loud sounds in their leisure activities. Outside the workplace, a high risk of hearing impairment arises for instance from attending rock concerts and discos, from practicing noisy sports such as shooting, and from exposure to military noise. Children could be exposed to noisy toys such as trumpets (92 to 125 dB SPL), whistles (107 to 129 dB SPL) and toy weapons (113 to over 135 dB SPL).
Listening to music played at high volumes can be as dangerous to hearing as industrial noise. This applies not only to rock concerts or nightclubs but also to personal music players (and mobile phones with music playing function) which can generate sounds across a broad frequency range at high volumes without distortion.
In our daily lives we are also exposed to environmental noise from traffic, construction, aircraft or various noises in the neighbourhood. These noises do not reach levels that can damage hearing but can be very irritating and cause other harmful effects. More...
5.3 What is the relationship between sound exposure, hearing loss and age?
Our current knowledge of the relationship between exposure to loud sounds and consecutive hearing loss is based on studies carried out several decades ago on individuals exposed continuously to high levels of noise at work. Tables compiled based on these results were used to establish the first international standards and are still widely used to predict hearing loss in populations exposed to noise while taking into account the effects of age. Based on the noise exposure and age of individuals, it is possible to estimate the likelihood that their hearing loss reaches or exceeds a certain level.
Predictions show that exposures at 80 dB(A) for 45 years (8 hours per day 5 days a week) have a minimal effect on hearing loss but exposures at higher levels result in hearing loss. For instance, exposure at 95 dB(A) for 45 years results in 26.5 dB of hearing loss for high-pitched sounds at a frequency of 4 kHz, which is the frequency at which hearing loss is most rapid. Most of the damage occurs in the early years of exposure, so preventive measures must be particularly aimed at those who start listening to personal music players when young.
Because potential consequences only become obvious many years later, most young people have normal hearing, whether or not they have been exposed to loud sounds.
Regularly listening to personal music players at high volume settings (above LEqu 80 dB(A)) at a young age will result in a larger proportion of people aged 18 to 40 with moderate levels of hearing loss at some frequencies; and the overall impact on hearing will be even larger in old age.
At typical sound levels, the hearing damage caused by noise exposure takes many years to become apparent. However, hearing loss for high-pitched sounds at 4 kHz can be detected at an earlier stage. Testing hearing at that frequency thus allows predicting future hearing problems. More...
5.4 How do loud sounds affect the inner ear?
Extremely loud sounds such as those produced by bomb blasts can cause small cracks in various parts of the ear which can be seen with a simple microscope. However, in most cases, the damage to the inner ear occurs at cellular level and is thus less visible.
Sensory cells in the inner ear (hair cells of the cochlea) convert sounds into signals that can be interpreted by the brain and losing these cells causes permanent hearing loss. Different hair cells are receptive to different sound frequencies. The wider the loss, the larger the number of sound frequencies that are affected. For each frequency, the greater the number of lost cells, the larger the hearing impairment. Because of the shape and the characteristics of the human outer and middle ear, excessive exposure to loud sounds makes individuals less sensitive to high-pitched sounds at frequencies of 4 to 6 kHz.
Losing one type of sensory cells completely (outer hair cells) results in a hearing impairment of 50 to 70 dB and also makes affected individuals less capable of focusing on a particular frequency and therefore less able to understand speech in noisy environments.
Recent advances made using more powerful microscopes show that losing or damaging hair cells is not the only factor that harms hearing. Loud sounds can also harm other types of cells, such as nerve cells, in the organ of the inner ear that converts sound into electrical impulses (cochlea). However, the chain of events that leads to cell damage and to the resulting hearing loss is not well understood at present.
Short exposures to steady loud sounds can damage the cochlea but this damage is usually reversible and the effect on hearing loss is temporary. Repeated exposures to very loud sounds can cause irreversible damage; in that case the hearing loss is permanent.
The likelihood that exposure to a particular sound will result in temporary or permanent hearing loss depends not only on loudness and exposure time but also on how quickly sound levels increase. The body has a reflex to contract certain muscles in order to protect the ear from excessively loud sounds. Sudden, very loud sounds such as explosions occur too quickly for the body to activate this reflex and are therefore a lot more harmful to hearing than steady sounds, in particular at high frequencies.
The inner ear of some people is more vulnerable to damage than that of others. Several factors – some of which are genetic – play a role, such as smoking, high blood pressure, fat levels, age, gender, as well as other anatomical characteristics. More...
5.5 What factors can change the way sound exposure affects us?
New research has uncovered a series of factors playing a role in either preventing or exacerbating sound-induced hearing loss.
Amongst the protective factors, exposure to safe levels of certain sounds could both protect the listener from damage caused by a later exposure to loud sounds and help them recover from previous excessive exposure. This observation is relevant to the treatment of sound-induced hearing damage.
Some nerves in the hearing system influence the way the inner ear responds to potentially harmful sounds and may therefore have a protective role, although these nerves seem to play no part in the normal functioning of the ear.
Other factors exacerbate the effects of loud sounds. For instance, exposure to loud sounds at a young age can make the ear more vulnerable to ageing. Over the years, there are small unnoticeable effects on the inner ear that only become evident many years later when the affected person develops hearing loss. Exposure to certain chemicals, smoking or lack of oxygen supply to the body also increase sound-induced hearing loss.
Several newly tested drugs have been shown to either prevent or repair sound-induced hearing loss. For instance some anti-inflammatory drugs help recover hearing after excessive exposure to loud sounds. Drugs with antioxidant properties protect the body from sound-induced hearing loss and some of these are already used in some countries.
Exposure to loud sounds can produce some initial damage to the inner ear, for instance by causing inflammation. This triggers processes that cause cells to destroy themselves and results in further damage through the loss of sensory and nervous cells. Several drugs can prevent this sound-induced cell death but are unlikely to be used in the near future because the doses required are too high. Drugs that promote the growth and repair of nervous cells and several other treatments, particularly the intake of magnesium, also protect against sound-induced hearing loss. More...