10 World Health • SlstYear, No. 5, September-October 1998
Noise can damage your hearing
Andrew W. Smith
A young man has his hearing tested in India. In developing countries, noisy occupations and the din of big cities are increasing risk factors for hearing damage. Photo WHO/A. Smith
The Romans compensated their armourers because they recognized that the noise of working with metal produced deafness. Today, noise-
induced hearing loss is still the most prevalent irreversible industrial disease in many countries.
I
t's an increasingly noisy world, what with traffic in cities, aircraft, construction work, mechanized industries and occupations, home appliances, rock concerts, personal stereos, motor racing and rowdy toys. Everyone is aware of the increase in noise in our daily lives, and of the effect it may have on our sleep patterns and working abilities, but fewer people are aware that prolonged exposure to harmful levels of noise can permanently damage hearing. In fact, in industri- alized countries, excessive noise is at least partially the cause of one-third of the total cases of hearing impairment. In developing coun- tries, especially those that are in the process of rapid industrialization, even more persons may be affected, and in those countries noisy occupa- tions and the din of big cities are increasing risk factors for hearing loss.
How does noise damage hearing?
In the cochlea, the organ of hearing in the inner ear, a large number of tiny hair cells respond to sound frequencies by producing nerve impulses which travel along the auditory nerve to the brain, where the sound is perceived. Hair cells are very sensitive to sound and they are easily damaged by excessive sound levels. At first the damage is temporary; this is a common experi- ence after exposure to loud noise - at a pop music concert perhaps - when one may have a mild hearing loss and ringing in the ears ("tinnitus") for a few hours.
As exposure to excessive sound levels increases, more and more hair cells, and also the surrounding structures, suffer permanent damage and eventually cell death. Hair cells that have died cannot be replaced at
present, and the hearing loss be- comes progressively worse if the exposure continues. The part of the cochlea that transmits high-
frequency sounds is particularly vulnerable. As a result, speech frequencies, especially consonants such as S, F, Sh and H which have a higher frequency than vowels, be- come difficult to distinguish. High- pitched children's voices are harder to hear. Increasing deafness leads to severe difficulties at work and in social interactions. Distressing tinnitus may occur. Extremely loud sounds, such as those from artillery gunfire or near to a jet aircraft taking off, may cause acoustic trauma which produces worse damage to many of the structures of the ear.
Decibel levels
What level of noise is dangerous?
A simple rule of thumb is that noise may damage your hearing if you have to shout above the background noise to make yourself heard. The intensity of sound is measured on a scale of decibels. People exposed to sound levels above 85 dB for eight hours a day over many years will be at significant risk for developing hearing damage. As the intensity increases, the duration of exposure that can produce damage decreases.
For every three to five dB increase in sound intensity, the allowable daily duration of exposure is halved.
However, people vary widely in their sensitivity to any given noise exposure. This is partly due to indi- vidual differences in anatomy and physiology, but may also relate to exposure to drugs or chemicals that damage the hearing (certain antibi- otics or certain industrial solvents, for instance). Age may be a factor.
Newborn infants, especially those
World Health • SlstYear, No. 5, September-October 1998
Damaged-ear cell cilia in the ear of a cat which has been exposed for two hours to a loud noise.
Photo by courtesy of Ors M.). Mulroy and M. C. Liberman, Massachusetts Eye and Ear Infirmary, HaNard Medical School, USA ©
that are premature, have increased susceptibility to noise exposure, yet may be placed in noisy incubators.
Elderly people suffer from a large burden of hearing loss; this is gener- ally due to presbyacusis (deafness of ageing), but can be made worse by previous occupational noise- exposure. This is one reason why men generally have worse hearing than women in later life.
Noise-induced hearing loss has been known about for a long time.
The Romans compensated their armourers because they recognized that the noise of working with metal produced deafness. Today, noise- induced hearing loss (NIHL) is still the most prevalent irreversible indus- trial disease in many countries, and more compensation is paid for noise than for any other occupational hazard.
There is no known cure for NIHL, although researchers are looking for one, and the only strategy is to prevent it. This is done through health education, individual protec- tion strategies, occupational safety measures and reduction in sources of excessive noise.
Reducing the risks
Health education on this subject needs to be targeted at the general public, those in influential positions,
health professionals, and those in high-risk situations. The last group would include workers in noisy occupations, such as firefighters, police officers, factory workers, farmers, construction workers, military personnel, heavy industry workers, musicians and entertain- ment industry personnel, but it also includes young people who are exposed to high levels of "leisure noise". It is vital to promote the value of conserving normal hearing,
NOISE CAN DAMAGE YOUR HEARING Noise Sources
Jet take-off Live rock music Steel mill Busy street Factory Conversation in office Whisper
WHO/A. Smith
Decibel Effects level
Eardrum rupture
110 Pain threshold 90
Hearing damage 80
Annoying 60 Quiet 20
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the dangers of excessive noise, the need to avoid prolonged exposure and the proper use of hearing protec- tors.
Noisy industries should develop hearing conservation programmes for their workforce. These pro- grammes should include regular inspection and monitoring of sound levels, strategies for noise reduction through engineering solutions and ear protection, regular screening of employees for hearing impairment, health education for the workforce, counselling, and compensation schemes for those affected. All this should be backed up by government legislation, which should apply to all industries, not just those believed to be noisy. If uniform regulations are enacted to award compensation for occupational hearing loss, this would stimulate employers' interest in developing effective hearing conservation programmes.
Noisy products need to be la- belled as such, and people should have greater access to hearing pro- tectors which are cosmetically and functionally acceptable - just as is the case with sunglasses. There should also be wider availability of basic hearing tests - again, just as many people readily have their eyesight tested.
Much can be done to conserve our wonderful and valuable sense of hearing in our increasingly noisy world. The main tasks are to raise public awareness and to provide more opportunities for noise reduc- tion and hearing conservation.
Accurate data are needed on the size and sources of the problem, espe- cially in developing countries. And more research is needed into the basic mechanisms of noise-induced hearing loss and into ways, firstly, of preventing it and then, eventually, of treating it. •
Dr Andrew W. Smith is Medical Officer, Prevention of Blindness and Deafness, World Health Organization, I 2 I I Geneva 27, Switzerland, and unti/January 1996 was convenor of the Hearing Impairment Research Group at the Liverpool School of Tropical Medicine.
