Health Department figures indicate that approximately 500,000 New Zealanders work in environments which are potentially hazardous to hearing health.

Workers at special risk of hearing damage are usually those in the heavy productive industry, such as metal work, drilling and quarrying, stone cutting, or the use of noisy machinery, as in textiles, printing, wood cutting, transportation and agriculture.

Noises above 85dB, are likely to cause damage to a proportion of those facing continued exposure. Very high levels may cause damage after relatively short periods, even when the noise is intermittent. This may be illustrated by the frequent finding of hearing loss in people who have fired guns as an occasional hobby, as well as in people who are exposed to noise of lower levels but more constantly, such as those working on construction sites.

The harmful effects of noise are cumulative and not, of course, confined to the workplace. The use of personal stereos and frequenting of discos has resulted in young people having some early damage to hearing before they even start work.

WHAT IS NIHL?

The answer to this is not completely known. However we do know that the damage is caused to the sensitive cells in the cochlea. For reasons which are not entirely clear, cells part of the way along the sensory organ in the cochlea are more sensitive than others. Noise induced hearing loss will therefore, begin to affect the hearing of certain frequencies.

High noise levels damage the hair cells of the organ of Corti, affecting first those in the basal part of the cochlea concerned with reception of the highest frequency sounds and progressing through to those receiving lower frequencies. This is reflected in audiometric changes which show loss of sound perception first in the 4-5 kilohertz (kHz) range, progressing both in severity and into lower frequency ranges.

THE EFFECTS OF HEARING LOSS

The first symptom of noise-induced hearing loss is usually difficulty hearing a conversation against a noisy background. The sufferer comes to dislike parties where everyone is apparently chattering away happily, yet he or she hears just a jumble of noise. Consonants seem to be lost first. Often he or she will mention intermittent high-pitched ringing in the ears, though this is rarely sufficient to be more than an irritant. By the time these symptoms have become sufficient to force medical consultation, the damage as measured by audiometry will be severe and, even with the cessation of noise exposure. When hearing is reduced at 3kHz and below, conversation is significantly interfered with.

HOW CAN NIHL BE PREVENTED?

Several measures can and should be taken in a hierarchical order: Assessment of exposure, using tools and equipment which generate a lower level of noise, segregation and insulation, appropriate work practices and personal protection such as ear muffs and ear plugs.

As well as taking steps to protect workers from noise, many companies now carry out regular audiometry.

The most effective way to protect the ears is to generate less noise in the first place through the use of low noise equipment and machinery.

Secondly, steps should be taken to insulate the machinery to reduce the noise emitted, and to segregate people away from the noise source.

People should work in areas where they are not exposed to high levels of noise. The same applies for leisure activities (e.g. discos and raves etc).

Remember the 'two metre rule' - if you find it difficult to communicate with a workmate at a distance of 2 metres because of noise, then probably the intensity is high enough to damage your hearing.

At a personal level it is possible to protect the ears with ear muffs and/or ear plugs. If you must work in an excessively noisy environment, you should wear protectors. You should also wear them when you are using power tools, noisy yard equipment, or firearms.

Personal habits are also important especially in avoiding high exposures to noise resulting from the use of so called "personal stereos".

Ear plugs are small, often torpedo nose shaped synthetic inserts, that are fitted inside the outer ear (canal). For them to work properly they should be of the appropriate shape, and correctly inserted. They may cause irritation especially if dirty. They are usually disposable and will not fit properly if repeatedly re-used.

Ear muffs  fit over and around the outer ear. They too, need to be appropriately fitted with correct headband tension, and avoiding glasses or hair which will impair the seal and hence the protection.

Hearing protection may attenuate noise by between 15 and 30 dBA.

Personal protection is the last line in the protection hierarchy. Appropriate low noise machinery/ processes must be in place, followed by insulation and segregation to attenuate the workers' exposure to noise.


PROBLEMS WITH EAR PROTECTION

The main problems are:

1) Undue reliance on personal protection without adequate steps to reduce noise exposure at source.

2) Inappropriate personal protection - because of poor choice, incorrect fitting or inadequate maintenance.

3) Hearing protection not worn as constantly and consistently as it should be. (Remember, from the physics of noise, and the logarithmic function of the decibel scale, that even if optimal hearing protection is worn, producing an average of say 30dBA of noise reduction over an eight hour shift, this will fall to less than 10dBA protection if the ears are unprotected for even one-eighth of that shift).


© 2005 Medica Consulting Limited.