Why Hearing Today May Be Gone Tomorrow
The human ear is an incredible piece of engineering and exquisitely sensitive. Calculations show that the quietest sound we can hear vibrates the eardrum by less than the diameter of a hydrogen atom.  Before those vibrations are analysed by the brain they pass through several stages of mechanical and neural processing that select the sounds we want to hear， adjust their level for comfort and intelligibility， and turn down the volume of distracting sounds.
 But for 8.5m people in the UK something goes wrong with one of the stages. Different parts of the processing chain are vulnerable at different ages. Some of the causes are avoidable and many are treatable. Treatments for problems that occur early in the processing chain are more straightforward and more effective.
 The easiest problem to treat is the one that occurs earliest in the chain. Obstruction of the ear canal by wax causes a relatively mild hearing loss. The loss is negligible unless the canal is completely blocked， which is rare， and is easily restored by cleaning out the wax.
 Things get more complicated behind the eardrum， in the airfilled middle-ear cavity.
 Middle-ear problems are common， treatable and the subject of intense debate about who should be treated and how. Anything that impairs transmission across the middle ear——even if it is only the pressure changes in an aircraft cabin， or blockage of the eustachian tube by a cold——causes hearing loss.
 At the centre of the debate over treatment is the common childhood condition known in the medical profession as otitis media with effusion or OME.
 This is usually caused by an infection of the middle ear， often in the after-math of a cold， in which the middle ear cavity fills up with a liquid effusion. The effusion can be thick and sticky， giving the condition its colloquial name of "glue ear". The liquid causes variable degrees of hearing loss by impairing the transmission of sound through to the inner ear.
 Glue ear can be treated surgically by inserting a tiny tube——a grommet——in the eardrum. This allows the glue to drain away， and ventilates the middle ear， which reduces the effusion of glue. There is evidence that removing the adenoids at the same time as inserting the grommet also helps by reducing the probability of future infections.
 Treatment of glue ear is controversial. The condition affects millions of children between the ages of one and four， at the time they are learning to speak.
 Evidence that it impairs the development of language and other cognitive functions led to a huge enthusiasm for grommet operations——which are the most common surgical operation in children——in the 1980s. Since 1992， doubt about whether the benefits of treatment would always justify the risk of surgery has swung the pendulum the other way.
 There are two problems that complicate the assessment， according to Mark Haggard， director of the Medical Research Council Institute for Hearing Research at Nottingham University （MRCIHR）。 He is conducting a large-scale evaluation of the effectiveness of the different treatments for glue ear， which will be completed in the autumn.
 First， the long-term developmental effects of glue ear are not very severe. Language development is slightly delayed in children under four， and in children between four and seven there are "modest but definite adverse effects on anxiety， social confidence and general coordination of behaviour， Haggard says.
 The second problem is more difficult. Although glue ear affects 83 per cent of children at some time in their lives， it fluctuates enormously in its severity and most children recover quickly and spontaneously. Nobody can identify in advance the children who will have persistent glue ear.
 "Deciding when to intervene is difficult because it is only when it's too late that you know you should have intervened. Another problem is that non-treatment also works， so it's a finely balanced issue，" Haggard says.
 In contrast， even the best treatments do not cure the deafness caused by prolonged exposure to noise. According to Adrian Davis， also of the MRCIHR， deafness is the commonest disability in the UK and affects 8.5m people.
 Noise exposure in the work-place is the most common preventable hazard to hearing. The hazard depends both on the intensity of the noise and the duration of the exposure.
 The UK Health and Safety Executive recommends that precautions should be taken by those who work 40 hours a week in sound levels of 85 decibels， and requires an annual check for those who work in noise levels between 85db and 90db. In working environments where the noise is above 90db， ear protection is mandatory.
 "For every 3db increase in intensity you halve the exposure limit，" says Davis， "30 minutes' exposure to l00db can produce deafness that lasts several days." These levels make leisure almost as risky as work： many personal stereo players can produce l00db， and disco dance-floor levels are usually about 103db.
 Noise damages the mechanisms in the inner ear that amplify quiet sounds and damp down loud sounds. It particularly affects sensitivity to the high-frequency components of the sound spectrum carrying the information that distinguishes different consonant sounds such as K， S and T. So the intelligibility of speech is severely affected. Just amplifying the sound with a traditional hearing aid doesn't help because it makes the unaffected frequencies too loud.
 Sophisticated hearing aids that selectively amplify affected frequencies are better， but even they are limited by the fact that the amplifiers in the healthy inner-ear turn loud sounds down as well as turning quiet sounds up. Without them， many sounds become uncomfortably loud almost as soon as they can be detected.
 This narrowing of the gap between detectability and discomfort is the reason most hearing aid users spend large amounts of time adjusting the volume. In principle， it is possible to produce a hearing aid that adjusts itself according to the user and to his situation， but the cost of such a device would be more than 10 times the cost of a National Health Service hearing aid.
 Davis is part of a consortium to evaluate the cost-effectiveness of leading-edge technology in the NHS. In doing so， they hope to exploit economies of scale： there are 500，000 hearing-aid wearers in the UK. However， even the best hearing aid will be no match for an undamaged ear， so keep the music turned down！