Hearing (ILO/IMO Guidelines Appendix B)

Impairment and risks

• Adequate hearing is essential for interpersonal and radio/telephone communications at sea. These communications often take place with background noise or interference and often have to transcend language barriers. Frequently failure to hear and respond to a message correctly is safety-critical.
• Audible alarms are widely used to signal safety information. They form the usual means of arousing off duty and sleeping seafarers if action is needed to safeguard the vessel or ensure personal survival.
• High noise levels are a feature of machinery spaces. Prolonged exposure to high noise levels leads to noise induced hearing loss, a recognised occupational disease. This can result in inability to hear the spoken word and in severe cases alarms.

Rationale and justification

• There is limited evidence about the degree of hearing loss needed to prevent correct understanding of the spoken word and it is highly dependent on signal to noise ratio.
• Understanding of face-to-face speech is normally better than for telecommunications as there are non-hearing clues about content. However technical improvements such as wider bandwidth and noise cancelling or excluding headsets and earpieces can improve reception of auditory information
• The middle frequencies of hearing are those most relevant to speech recognition.
• Unilateral hearing loss may slightly reduce a person's ability to identify the direction of a noise but it is the hearing in the best ear that determines ability to hear speech.
• The level of audible warning needed to wake a person from sleep is variable and so it is not possible to define objective levels that are needed to arouse those with hearing impairment. Vibrating alarms or bright flashing lights are suitable alternatives.
• Hearing aids will improve speech comprehension to varying extents depending on the nature of the hearing loss in the individual and any associated complications of deafness such as tinnitus. Individual assessment in noisy as well as quiet surroundings will be needed.
• There is good evidence that prolonged high levels of noise in ship machinery spaces cause noise induced hearing loss. There is also good evidence from onshore studies that links the severity and duration of exposure to the degree of hearing loss. This forms the basis for design standards of new vessels.
• Ear defenders: (muffs and plugs) produce some reduction in noise exposure and in the risk of damage. To be effective they must be worn all the time in noisy areas and fit well. Problems may arise from skin saturation leading to irritation or infection in hot and humid conditions. Ear defenders reduce both the levels of noise and of any communications by the same amount. As the signal to noise ratio is unchanged it is just as easy to hear conversation, alarms and abnormal sounds with and without defenders on. Incorporation of telecoms receivers within ear defenders can enhance reception while at the same time encouraging the use of the defenders
• Assessment of deafness by whisper tests is inherently subjective.
• Accounts of communication difficulties from other crew members are important evidence of impairment.
• Pure tone audiometry provides a valid and reliable way of measuring hearing loss in each ear across a range of frequencies. It is the method used for evaluating noise induced hearing loss but does not correlate well with subjective impairment when there is moderate hearing impairment.
• There are now well validated and simple to use screening tests of speech recognition.

Clinical assessment and decision taking

Note: Hearing aids

For a hearing aid or cochlear implant to be acceptable for use in a serving seafarer it must provide a level of hearing compatible with the minimum hearing standard required for a non-user. It is not recommended that hearing aids are worn 24 hours a day so any seafarer who sleeps aboard must either have sufficient residual hearing to be woken from sleep by an emergency alarm or have alternative arrangements made to ensure arousal. In general digital aids can be expected to perform better than analogue ones. In-ear aids can be expected to perform better under the adverse conditions met at sea but are more prone to breakdown than behind-ear models. Behind-ear aids are simpler to replace than in-ear ones during brief periods on shore

The seafarer must be responsible for carrying sufficient spares (including a spare hearing aid) to ensure that loss or malfunction does not impair their capabilities. Enough batteries and other consumables must be taken onboard to ensure that the device can operate for more than the maximum expected time before access to new supplies. The examining doctor should check that the seafarer is aware of this and understands that they have to ensure that they have they have made prior arrangements to have the equipment needed before they embark.

An audiological assessment of the adequacy of the aid to meet both the hearing requirements and the rigours of maritime work should be obtained in all seafarers whose hearing is likely to be relevant to the safe operation of the vessel, including emergency procedures. The hearing aid will need to meet standards of intrinsic safety (against ignition of fires and explosions) if it is to be worn in parts of the vessel where other electrical equipment must also meet these standards.

a) Methods

Audiometry

At the start of a seafaring career a baseline audiogram provides both information on any current hearing impairment and a benchmark against which future changes from noise exposure or other causes can be identified.
In addition loss greater than 30dB (unaided) in the better ear, taking the average of the frequencies 500, 1000, 2000, 3000 Hz, indicates that there are likely to be problems with hearing normal speech at a distance of 2-3 metres.

The recommended minimum hearing levels in the ILO/IMO Guidelines are 30dB best ear and 40dB worst ear (both unaided) averaged over the frequencies 500, 1000, 2000, 3000 Hz. Any national recommendations or requirements should be complied with (STCW Table B-1/9).

The need for health surveillance of seafarers exposed to noise using audiometry will be determined by the employer's assessment of noise levels and the length of exposure to them. As a minimum engineers regularly spending two or more hours a day in machinery spaces where a normal talking voice cannot be heard at 2 metres will need periodic audiometry examinations and these should be arranged by their employer. However audiometry should only form one part of a wider noise control and hearing conservation programme.

Tests of speech recognition

For all seafarers an ability to hear speech and warning signals is essential. Well-validated screening tests for speech reception ability have been developed. They may be used with the agreement of national maritime authorities as alternatives to audiometry for assessment of communication. They are not suitable for surveillance of those at risk of noise induced hearing loss. Tests are often based on responding to triplets of single digit numbers presented over the telephone or by loudspeaker that have varying levels of white sound masking applied to interfere with their clarity. If they are to be used to assess seafarers they should be able to provide a result that categorises the test results as:

1. Unimpaired: approximating to less that 25 dB loss,
2. Possibly impaired: approximating to 25-35 dB loss,
3. Definitely impaired: approximating to more than 35 dB loss.

These tests require a quiet room but do not require a soundproof booth and only take about five minutes to complete. Because they use correct reception of numbers they can be seen as immediately relevant to the sort of practical task for which hearing at sea is essential.

A speech recognition test using English is available at. This may be accessed for use on a PC at:
www.actiononhearingloss.org.uk/your-hearing/look-after-your-hearing/check-your-hearing/take-the-check.aspx  - A PC with a sound card and loudspeakers is required.

The whisper test

This test is highly subjective and is no longer recommended as a means of assessment in seafarers, except as an additional method of screening.

Testing – other aspects

The effectiveness of audible alarms is difficult to assess except on board a vessel. This may be a risk when there is definite impairment, as shown by audiometry or by speech recognition testing or less reliably by for, instance, an inability to hear a telephone ringing in the same room. A practical test based on the cabin occupied by a seafarer wit this level of impairment who is still employed at sea is recommended. Seafarers who use hearing aids may be at particular risk as these are usually removed at night.
Testing of those who wear a hearing aid is complex. A view from an audiologist on impairment with and without the aid should be obtained. A validated speech recognition test with and without the aid in place can be used but only as an initial screen.
An audiologist's assessment may be needed if there is defective hearing in one ear to establish that this is not caused by a condition that could later reduce hearing in the other ear. As decline in hearing is slow this is unlikely to influence immediate decisions about fitness but may be relevant when advising on future career options.

b) Decision taking – communication abilities

c) Noise induced hearing loss

See relevant national guidance on health surveillance for noise induced hearing loss by audiometry and the handling of results. If findings do not comply with capability requirements noted above similar decisions on fitness to those listed should be made. In addition the hearing protection used by the person needs to be reviewed and removal from noisy work should be considered. The individual should be informed that their hearing shows a pattern that is consistent with damage by noise exposure if the audiogram shows the characteristic pattern of noise induced hearing loss (initially a dip at 4000 Hz with a later reduction at all high frequencies that progressively encroaches on the speech frequencies). Where the hearing loss is considered to have arisen from current conditions of work the ship operator will need to reduce the risk to all those who work there.

d) Other ear conditions

See ILO/IMO Guidelines Appendix E H65-81.