RELATIONSHIP BETWEEN NOISE EXPOSURE AND HEARING LOSS BASIC INFORMATION

WHAT IS THE RELATIONSHIP BETWEEN NOISE EXPOSURE AND HEARING LOSS
The relationship between noise exposure and hearing loss.

If hearing damage is to be prevented by limiting occupational noise exposure, then it is necessary to have some quantitative understanding of the relationships between sound pressure level, frequency, exposure time and the degree of damage caused.

Having established that there is a risk to hearing, though, it would be unethical to refrain from taking all reasonable measures to prevent it. During the 1960s a great deal of work was done in the UK and the rest of the world to establish the relationships between noise exposure and noise-induced hearing loss.

At that time it was relatively easy to find populations who had worked at one job, and been exposed to steady noise levels, for a number of years. Since then, social mobility, changing patterns of employment, and indeed government action to limit noise exposure, have made it much harder to find large groups of workers whose noise exposure can be logged over several years.

Information on the precise relationship between the various factors influencing hearing damage is therefore incomplete, and a full understanding of the subject will never be achieved. Full understanding is not required, though.

What is needed is sufficient information to frame legislation and advisory procedures which are capable of being put into practice in such a way that occupational hearing damage is reduced and eventually eliminated, without also making essential industrial processes impossible or uneconomical to carry out. This is itself quite a demanding objective.

In studying the relationship between noise exposure and hearing loss a range of questions can be asked. It can be assumed that louder sounds will result in more damage than quieter ones, but more detailed questions include:

. Is there a sound pressure level below which there is no contribution to hearing damage?
. If so, then what is this level?
. If all noise contributes to damage then what is the trade-off between level and damage?
. Does an extended period of noise exposure do the same amount of damage as a series of shorter exposures at the same level?
. Are particular frequencies or ranges of frequencies significantly more damaging than others?
. Is there a link between the frequencies to which the ear is exposed and the frequencies at which hearing loss occurs?

The answers to these questions and other questions will all have consequences for the way in which noise exposure must be measured. In the European Union, an approach to the assessment of noise exposure has emerged which uses the best available answers to these questions.

Each of the assumptions listed below can be challenged, and together they represent a gross simplification of a very complicated area of knowledge. For the time being, they seem to offer a practical way forward to those working to reduce occupational hearing loss, and as stated above, that is the most that can be asked for.

1. All sound energy received by the ear will, in some degree, contribute to hearing damage.
2. The degree of damage is proportional to the amount of sound energy deposited in the ear. That means that a doubling of exposure time is equivalent to a 3 dB increase in sound pressure level. It also means that the total exposure time at a given level is important; breaking the overall time up into shorter periods has no effect.
3. The A weighting system correctly evaluates the contribution of different frequencies to hearing loss.
4. Very high sound pressures can cause damage which may not be reflected in an equal-energy assessment as described above. An additional limit on peak sound exposure can be used to prevent this.

In the United States, rather different conclusions have been reached, and as a result a rather different trade-off between sound pressure level and exposure time is used. This is based on the assumption that a 5 dB increase in level (rather than 3 dB) is equivalent to a doubling of exposure time.

To add to the confusion, for some purposes in the United States 4 dB (rather than 3 or 5 dB) is assumed to be equivalent to a doubling of exposure time. Those carrying out noise exposure assessments in Europe need to be aware of these different practices in order to avoid being misled by procedures or instrumentation intended for American use.

The current European approach to the prevention of occupational hearing damage is based on the principles listed above. The issue is the subject of continuing debate as research into hearing damage continues. Given the difficulties of generating further large sets of data which can be used to refine our knowledge, it seems likely that for the foreseeable future this approach will continue.