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Medical surveillance is usually conducted by a qualified doctor and may involve clinical examinations and biological or biological effect monitoring, e.g. audiometry, spirometry or blood testing, chest X-rays, liver function tests, etc.

Audiometry

This involves the measurement of hearing performance in order to detect actual noise induced hearing loss. The most commonly used assessment of hearing is the determination of the threshold of audibility, i.e. the level of sound required to be just audible. This level can vary for an individual over a range of up to 5 dB from day to day and from determination to determination, but it provides an additional and useful tool in monitoring the potential ill effects of exposure to noise. Before carrying out a hearing test, it is important to obtain information about the person’s past medical history, not only concerning the ears but also other conditions which may have a bearing on possible hearing loss detected by an audiometric test.

Noise Induced Hearing Loss (NIHL)

Noise induced hearing loss is a condition which results from failure of the hair cells in the Organ of Corti to respond fully to sound intensities having frequencies within the speech range. The person does not necessarily lose the ability to hear sound, but is not able to distinguish the spoken word clearly, even if it is presented with a raised voice.

The hearing loss is usually bilateral, but variations in each ear have been observed. Wax in the ear can also cause hearing loss, so the ear should be examined to see if syringing is needed; also to determine if the eardrum has suffered any damage which may reduce the ability of sound to be transported to the cochlea.

The audiometric test can be carried out using automatic or manual audiometers, but the essential test procedure is the same:

 The subject is asked to remove anything which might upset the test results, e.g. spectacles, earrings, hearing aids.

 Instructions are given about the test procedure and the subject is required to indicate whether he/she can just hear or cannot hear a certain sound (the sound level may be increased from a very low level or reduced from a high level).

 Earphones are fitted carefully over the ears and the test is then carried out on each ear.

 Firstly, a threshold test is undertaken in which each ear is subjected to sound at a

The procedure is repeated several times so that an average threshold can be derived for the test. Thresholds can vary due to slight changes in the procedures adopted in setting up the test, e.g. variation of the position of the earphone on the ear.

 Following this pre-check, both of the subject’s ears are tested through a range of frequencies (usually 0.5, 1, 2, 3, 4 6 and 8 kHz) and hearing loss recorded for each frequency, again via a series of sound exposures. From them an average result can be computed.

 When the test is completed, a second threshold check should be carried out to see that no errors have crept in during the test. Both threshold checks should agree within a

maximum of 10 dB. If they do not, a re-test must be performed.

The accuracy of audiometry can be affected by a number of factors, including:

 Equipment limitations - how accurately can either the frequency or the hearing level be determined?

 Learning effect - the first ear tested sometimes appears worse than the second one since the individual becomes more proficient at detecting the threshold.

 Headphone fit - some of the variation in threshold measurement has been attributed to differences in the location of the headphones, which in turn affect the detection of the threshold.

 Background noise – audiometric tests should be carried out in a sound-proof chamber to eliminate external sounds from influencing the test.

A further complication of audiometric testing is that it is subjective and relies on the co- operation of the subject. If the subject is unable or unwilling to co-operate with the test then unrepresentative results will be obtained.

The technique described above enables us to compare the threshold of hearing of the

individual undergoing audiometry with a reference value at a range of octave band frequencies (125, 250, 500, 1000, 2000, 4000, 8000 Hz). From this data a pictorial representation, an audiogram, of hearing loss at various frequencies is produced.

Some examples of audiograms reflecting different levels of NIHL are shown below.

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Temporary Threshold Shift (TTS)

A temporary threshold shift occurs after exposure to a high noise level; hearing acuity returns with time. The condition has been described as a fatigue of the hair cells in the Organ of Corti. The level of the shift is expressed in terms of the raising of sound intensity required to hear a given sound level, e.g. a 20 dB shift means the sound pressure level has been increased by a value of 20 dB.

If a person is subjected to a high sound level, say 85 dB or over for a short period, and then has an audiometer test, a dip in hearing acuity occurs at 4000 Hz. It is often described as the “4 kHz dip” for acoustic trauma. The amount of “dip” from the 0 dB average level is used to specify the amount of threshold shift. For TTS, the amount of 4 kHz dip lessens with time as recovery from exposure occurs. An audiogram illustrating the 4 kHz dip is given in the following figure.

The recovery time from a TTS is illustrated in the next figure. Note the recovery is first rapid and then proceeds at a much reduced rate. The higher the noise exposure, the longer the recovery time. 80 70 60 50 40 30 20 10 0 -10 -20 125 250 500 1000 2000 4000 8000 TEMPORARY THRESHOLD SHIFT LEVEL OF SHIFT NORMAL HEARING THRESHOLD SHIFT (dB) FREQUENCY (Hz)

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Permanent Threshold Shift (PTS)

Permanent threshold shift is the term used to describe the condition where there is a permanent 4 kHz dip in a person’s audiogram. It is a non-reversible condition where the threshold shift does not return to the accepted norm as in TTS. It is generally accepted that PTS is a condition which follows from continual TTS exposures. PTS is illustrated in the figure which follows.

TIME TEMPORARY THRESHOLD SHIFT (dB) 0 10 20 30 40 105 Db 93 Db

A common use of audiometric testing is at the pre-employment stage. This serves two

purposes: the first is that it enables an initial assessment of hearing ability to be made in order to establish a base-line, which can be measured by future audiometric tests.

The other purpose is to detect any signs of noise induced hearing loss arising from previous employment. If this is detected and documented it can serve to safeguard the employer against any false accusations that hearing loss might have been due to this employment rather than previous ones.

Spirometry

A spirometer is a device that measures the amount of air that you exhale. The spirometer is concerned with lung function and involves the employee breathing in fully and sealing their lips around the mouthpiece of the spirometer. The most common measurements made are:

 FEV1 – Forced Expiratory Volume in one second. This is the amount of air you can blow out from your lungs in one second. Normal healthy lungs should be able to blow out most of the air in that time.

 FVC – Forced Vital Capacity. This is the total amount of air that can be blown.

 FEV1/FVC. This is the proportion of air in your lungs which can be blown out in one second.

A graphic representation in the form of a spirogram is usually included in the results. 80 70 60 50 40 30 20 10 0 -10 -20 125 250 500 1000 2000 4000 8000 PERMANENT THRESHOLD SHIFT LEVEL OF SHIFT NORMAL HEARING THRESHOLD SHIFT (dB) FREQUENCY (Hz)

Spirometry readings will vary according to age, size and sex, medical conditions, previous employment experience (e.g. inhalation of dust) etc., and charts are available with normal lung function readings against which a comparison can be made.

Where an individual has narrowed (obstructive) airways, e.g. due to asthma, or chronic obstructive pulmonary disease, the FEV1 will be low but the FVC will be relatively normal, i.e. the amount of air that can be blown out quickly is reduced. Conditions such as fibrosis or pneumoconiosis that affect the lung tissue itself or affect the capacity of the lungs create a restrictive pattern and will reduce the FVC, but the FEV1 will remain normal. It follows that where both a restrictive and obstructive pattern exist, both the FEV1 and FVC will be reduced. Spirometry is an effort-dependent test that requires careful instruction and the co-operation of the test subject. Therefore, like audiometry, if the subject is unable or unwilling to co-operate with the test then unrepresentative results will be obtained. Also, like audiometry, lung function tests may be carried out at the pre-employment stage and can also be used as a benchmark for future comparison.