Irreversible Hearing Loss
Published: 19th Mar 2014
Barry Holt addresses the issue of noise induced hearing loss.
It was well known for many years that exposure to sudden, very loud noises such as gun fire or explosions could cause hearing loss as a result of damage to the ear drum. The industrial revolution, however, saw the establishment of the textile industry, where spinning and weaving operations produced lower noise levels to which workers were exposed for extended periods. This exposure was still found to cause hearing loss. In fact, this problem has become the most common form of occupational hearing loss and according to the UK’s Health and Safety Executive (HSE) it accounts for 75% of occupational disease claims.
An important factor regarding noise induced hearing loss (NIHL) is that it is irreversible. Once a worker’s hearing has deteriorated as a result of extended exposure to workplace noise, the loss cannot be repaired.
Unfortunately, for many years in sectors such as the textile industry, the severity and extent of NIHL was not fully understood as workers developed alternative ways of communicating; for example, lip reading.
To recognise how hearing damage can arise, we need to first understand the nature of sound transmission and the structure of the ear.
Sound is transmitted through the air in the form of compression waves. These consist of alternating areas of higher and lower pressure, as shown.
The frequency is the number of vibrations per second and is important because the ear responds differently at certain frequencies. The frequency or wavelength is the characteristic that allows us to recognise the pitch of a sound; for example, whether it is high or low.
The frequencies to which the human ear responds normally range from 20Hz to 20kHz. Within this range, hearing responds differently at different frequencies. As we get older the range, particularly at higher frequencies, is reduced. The response to different frequencies becomes important when we are selecting control measures, as we shall see later.
The amplitude is a measure of how loud the sound is and this is the factor on which legislative standards are based.
How noise leads to hearing loss
To understand the mechanism behind NIHL, we need to consider the structure of the ear and the way in which sound pressure waves are transformed into signals that are transmitted to the brain.
The ear can be divided into three main regions: the outer, middle and inner ear. The outer ear, which is the part that we can all see, is the part that collects the sound pressure waves and channels them into the middle and inner ears.
The middle ear is comprised of the ear drum and a sequence of bone linkages. These transform the air movements into physical motion, which is in turn transmitted to the inner ear.
The middle ear itself can be the cause of problems such as Meniere’s disease, which causes symptoms such as tinnitus and loss of balance. It can also lead to hearing loss, but the loss from this cause is normally temporary.
From the perspective of extended exposure to noise, the key component is the cochlea. This is the spiral, snail like structure in the inner ear. The cochlea is filled with fluid in which there are nerve cells called cilea. These transform mechanical motions into minute electrical signals, which are then transmitted to the brain via the auditory nerve.
In the event of exposure to high noise levels over an extended period, these cells are damaged irreversibly.
If we are to understand the levels at which NIHL can occur we have to understand how sound pressure levels are measured. Pressure levels are measured in a unit called decibels (dB). When considering the effect on hearing we normally use a scale known as the A weighted scale, or dB(A). This responds most closely to the frequency response of the human ear, which detects sound most sensitively within the middle of the hearing range, between one and eight kilohertz (Khz).
The A weighting is mandated in IEC 61672 to be fitted on all sound level meters, with precision meters also being fitted with C weighting mainly for testing purposes. Use of B and D weighting scales has largely ceased, with D weighting now only used for measuring noise levels of military aircraft engines.
As discussed in the introduction, it had long been recognised that exposure to sudden loud noises can lead to hearing loss. The sound pressure level of a gunshot, for example, is likely to be around 140dB(A) and can cause hearing loss as a result of rupture of the eardrum. Interestingly, however, workers encountering sustained levels much lower than this also suffer from NIHL.
When countries began to realise this and introduce statutory controls of noise exposure, it was usually on the basis of a 90dB(A) time weighted average (TWA) over an eight hour exposure period. This exposure period is critical owing to the nature of the decibel scale, which is logarithmic, as shown in the diagram, left. As a result of this, the actual sound pressure level to which the ear is subjected doubles from an increase of 3dB(A). This means that if the legal limit were an eight hour TWA of 90dB(A) and an employee would receive the same dose of noise if exposed to 93dB(A) for four hours, or 96dB(A) for two hours and so on. Conversely, at a level of 87dB(A) an exposure of period of 16 hours would be needed to achieve the same dose.
To complicate matters, the ANSI standard in the USA is based on a 5dB(A) increase or decrease being required to double or halve the exposure. For the purposes of this article, however, we will instead consider the IEC standard which applies throughout the rest of the world.
We have looked at the statutory level of 90 dB(A), which was the most common standard when legal limits were first introduced. Further research, however, began to indicate that 90dB(A) was too high a limit and that noise induced hearing loss was still a major problem. Most national standards now, including those of EU member states, have set a limit of an eight hour TWA of 85dB(A), although cases of NIHL still occur at levels below this.
In the UK, the Noise at Work Regulations were amended to introduce a two tier standard. This placed 85dB(A) as the level at which reduction measures must be put in place and included a lower action level of 80dB(A), above which employees must be made aware of the risks and provided with hearing protection should they wish.
Statutory requirements for controlling exposure to potential hearing loss generally follow a risk management approach. This is the approach required by the EU Directive 2003/10/EC, which requires employers to “assess and, if necessary, measure” levels of employee exposure. Where this exceeds either the 80dB(A) or 85dB(A) stipulated levels, employers are then required to take the necessary action. In the case of the higher action level, the action should be based on the general principles of prevention set out in Directive 89/391/EEC.
How do we know whether there is a need to carry out a programme of noise measurement? The following is a much used rule of thumb: should it be necessary for someone to raise their voice to be heard by another person at arm’s length,the ambient noise level is likely to be close to the statutory limit, in which case more detailed measurement should be made. So how do we measure the actual sound level?
There is a range of measuring devices that can be used, each of which is designed for a specific purpose. IEC 61672-1 specifies three types of sound measuring instrument: the conventional meter, the integrating averaging meter, and the integrating sound level meter.
For the purpose of assessing the risk of hearing damage, the conventional meter is of limited value as it simply gives a snapshot in time. Integrating meters, on the other hand, will give an estimated time averaged value known as Lat or Leq. This is a continuous (root mean square) exposure level, which is equivalent to the variations of the level over the reference period. In some cases, such as where the sound arises from impacts, it is also necessary to measure the peak sound pressure level; for example, in a metal stamping workshop. This is measured using the C weighting scale. In the EU, the maximum value for peak sound pressure level is 140dB(C), which corresponds to a peak pressure of 200Pa.
Sound level meters are classified under IEC 61672-1 into two types. These can both carry out the same functions. Class 1 meters have a wider frequency range and tighter tolerance for error but are, as a result, much more expensive. Normally these are only used for research or enforcement work and a Class 2 meter is normally sufficient under national standards for workplace noise assessment.
Whichever class of meter is to be used, it is essential that it is used with the equivalent type of calibrator. This is a device that couples with the meter and emits sound at a defined frequency and amplitude, allowing the calibration of the meter to be checked before and after use.
As the aim is to measure the exposure of an employee to excessive noise levels, in some cases it may be necessary to use a personal sound exposure meter, known as a dosemeter. This is a device that is worn by an employee who may be working in different areas where he/she is exposed to different sound levels during the course of the day. Ideally the microphone of the device should be worn close to the ear. Personal sound level meters are now subject to international standard IEC 61252:1993.
Another type of measuring device is the octave band analyser. This is useful for selecting correct hearing protection or noise attenuation measures, as different materials and devices are more affective at certain frequencies.
A compressor room in which the author carried out a noise survey had machines that produced noise levels of more than 95dB(A), but most of the noise was in the frequency range of less than 64Hz. Octave band analysis allows the spectrum to be divided into bands where the top of the range is double the lower limit, e.g. 32Hz, 64Hz, 128Hz and so on. In this case it is important to know this, as short term hearing protection was used when maintenance staff were working in the area and many forms of hearing protection are not effective at such low frequencies.
In all relevant standards including EU Directive 2003/10/EC, national standards in member states and the US OSHA standard 29 CFR 1910.95, the requirement is that exposure above the designated action levels must be controlled in accordance with good risk management practise. In the EU Directive this is defined in accordance with the general principles of prevention, set out in Directive 89/391/EEC.
This means that there is a hierarchy of controls to be followed:
• Eliminating the source of excessive noise • Replacing equipment with that which creates less noise • Using technical measures, e.g. enclosing the equipment or operators, or using noise absorbing mountings • Changing working methods, e.g. reducing duration of exposure or moving operators away from the noise source
Personal protective equipment (PPE) in the form of ear plugs and ear muffs can be used in certain circumstances:
• As a temporary measure until more effective measures can be put in place • Where noise reduction measures on their own are unable to reduce the exposure to an acceptable level
Where hearing protectors are provided as a control measure, this imposes certain duties on the employer, such as to provide information about the risk of NIHL, instruction about the control measures that have been put in place and training in their correct use.
Employers also have the duty to ensure that PPE is being regularly and correctly used. This can be part of a routine workplace inspection programme carried out by managers and supervisors. Where noise reduction measures including PPE are not being used correctly, managers and supervisors should ensure that the employee received the correct information and training and identify whether there is any practical reason, such as comfort, that needs addressing.
When selecting appropriate hearing protection, a number of factors must be taken into account:
1. It is important to ensure that devices are a good fit - This can be an issue in the use of ear muffs by employees who wear spectacles or require safety goggles.
2. Employee preference - Hearing protection is only of use if employees actually wear it. It is good practise to offer a choice that includes different styles, as some employees may prefer in-ear devices, whereas others are happier with over-ear protection.
3. Correct usage - If in-ear protection is provided, for example in the form of foam plastic inserts, it is essential that wearers are shown the correct way to insert the plugs. It may seem obvious, but they are designed to be compressed and then when inserted into the ear canal they expand to provide a good seal. If you see a piece of coloured foam plastic hanging from an employee’s ear, he/she will be receiving negligible protection.
4. Disposable or reusable - It is also important for employees to understand that some of these inserts are designed to be temporary and should not be reused, as they can become contaminated and cause ear infections. With over ear protection, these should be regularly cleaned and stored in a place where they are not likely to get contaminated. I’m sure we have all seen earmuffs lying on a work bench collecting dust.
5. Compatibility with other protection - In situations where safety helmets must be worn, the use of normal earmuffs can compromise the impact protection. Protectors can be provided that attach to the outside of the safety helmet, but the degree of attenuation may be less than with a standard version.
6. Measure distribution levels - When selecting hearing protection it may be necessary to measure the distribution of levels within different octave bands, as each device will provide a different attenuation profile across the audible frequency range. This data is provided with the devices and should be used in the selection process.
The identification, evaluation and control of exposure are not the end of the management process. These should form part of an ongoing programme through which noise levels are regularly checked. Levels are not a once and for all factor, as they can be affected by problems such as poor maintenance of machinery and equipment. Increased wear and tear leads to greater friction between components, which in turn can create higher noise levels.
As well as a programme of monitoring noise levels it may also be necessary to introduce audiometric assessment of employees. This is of particular importance where an employee may already have some deterioration, perhaps from a previous workplace or even from a totally unrelated condition. If this is the case, it is vital that the remaining hearing is protected.
In an audiometric test the subject will be exposed to sounds at different frequencies and amplitudes. This will establish the individual’s threshold of audibility. Although this can vary by as much a 5dB from day to day, regular monitoring will identify any downward trends that may be caused by exposure to noise. It can also be used as a diagnostic tool to identify whether hearing loss is caused by exposure to excess noise levels or whether it is a naturally occurring effect; for example, due to aging.
Although not understood by all employers, a very clear drop in the hearing level around a frequency of 6kHz is characteristic of NIHL. Often when considering the introduction of audiometry as an element of a hearing conservation programme, employers tend to bury their heads in the sand and feel that they will lay themselves open to claims for hearing loss that may not be occupational.
It is important to realise that it is better to identify a problem at an early stage and take action to prevent further deterioration, than to wait until the individual has noticed problems with day to day conversation or watching television. By using an audiometric test, the cause of the hearing loss can be identified if it is in fact noise induced. This does not, however, limit possible lifestyle factors, such as attending discos or indulging in leisure activities such as shooting.
This broad level review of the issues relating to noise induced hearing loss has shown that it is still an issue for organisations. The aim has been to show the elements of a risk management approach that can be applied to this problem and to draw attention to some of the issues that need to be addressed by implementing a hearing conservation programme.
In the past, provision of hearing protection has often been the knee jerk response, but while acknowledging this has a role to play, there are other elements that must be considered first. It is essential that the programme is seen as an ongoing part of occupational health management.
Published: 19th Mar 2014 in Health and Safety International