Any OH practitioner who is likely to find themselves working in an environment where noise is a risk needs an understanding of how hearing loss can come about. Susanna Everton provides an overview.
High levels of noise can cause permanent irreversible hearing loss, but this can be prevented. OH nurses play a vital part in hearing conservation in the workplace and should work with OH and safety colleagues to advise and guide managers and employees in noise control measures.
It has been known for centuries that excessive noise can cause hearing damage, but it was first described as an occupational hazard in the seminal work “De Morbis Artificium” by Ramazzini, published in 1713. In this, he described copper workers who hammered metal as having “ears so injured by that perpetual din that [they] become hard of hearing and, if they grow old at this work, completely deaf” (Bell, 1966).
The first British study of occupational noise-induced hearing loss (ONIHL) was published in 1866, but it was not until the 1940s that work on hearing, testing and the relationship between work and ONIHL became more widespread.
While it is generally acknowledged that workers in heavy industrial settings are exposed to loud noises, and much research has been done on the risks of hearing damage, it is only recently that other occupational settings have been demonstrated as a cause of noise-induced hearing loss (NIHL), including call centres, pubs, restaurants and clubs. It is also becoming apparent that the noise we expose ourselves to in our ordinary lives, such as in-ear headphones with MP3 players, will probably damage our hearing permanently.
Defining sound
In order to understand noise damage, it is necessary to understand sound. Sound is defined as the mechanical vibration of a gaseous, liquid or solid medium through which energy is transferred away from the source by progressive waves.
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A vibrating object loses some of its energy to surrounding areas as sound – think of a washing machine on the spin cycle. Noise has been described as a sound without musical quality, or as unwanted or undesired sound.
Measurement of sound is based on the pressure the sound wave exerts in the surrounding medium, usually air. Sound is therefore a moving series of pressure fluctuations. Pressure is measured in Pascals (Pa). Amplitude is the degree of displacement of the molecules in the surrounding medium. Wavelength is the interval between successive compressions and the frequency of the disturbance. Frequency is the rate of movement in the number of times a complete cycle occurs in one second, measured in hertz (Hz).
The quietest sound the human ear is capable of hearing is recognised to be at 20 micro Pa, and the threshold of pain at 100Pa, an impossible scale to measure in the normal way. Acousticians therefore use a scale based on the bel – a unit used to express the ratio of two powers, usually electric or acoustic powers. An increase of one bel in intensity approximately doubles loudness of most sounds. However, for ease of understanding and representation on a chart, sound intensity is measured in decibels (dB), a logarithmic scale devised to relate to the hearing of the human ear. This ranges from zero to 120, with zero being the minimum threshold and 120 the threshold of pain.
The principle range for human hearing is between 250Hz and 4,000Hz, although we can still “hear” up to 16,000Hz. Young people can hear from 20Hz to 20,000Hz, dogs can hear from 40Hz to 60,000Hz, bats 2,000Hz to 110,000Hz, and elephants can hear from 16Hz to 12,000Hz, often communicating at infrasonic levels.
Human hearing
The auricle, or pinna, of the ear is designed to collect sound waves and transmit them along the meatus (the ear canal) to the tympanic membrane (the eardrum). The meatus acts as a resonator that amplifies the sound at certain frequencies. The sound then sets up a vibration in the taut skin of the tympanic membrane, which is picked up by the ossicles, the tiny bones in the middle ear. The sound wave intensifies by a factor of 18 when it then passes through the oval window and into the liquid within the cochlea. The sound waves set up vibrations, which are in turn felt by tiny “hairs” that pass the “message” to the nerve to be interpreted by the brain. These hairs are specific to each sound frequency and if they are damaged by repeated sound exposure they cease to function.
Temporary threshold shift is the term used to describe the ear’s response to a short duration high-volume noise exposure that produces a temporary loss of hearing, but which will usually recover to normal given a suitable period of quiet. This is commonly experienced in leisure activities, such as after visiting a club with loud music, but is also common following work activities that leave hearing feeling a little “woolly”. If these exposures recur and there is insufficient recovery time, a permanent and irreversible threshold shift can occur.
Tinnitus is sound “heard” in the ear that does not have an external cause. A ringing, tone or other sound is perceived that can be of any pitch, last for seconds or be continuous, in one or both ears and can occur at any age. However, tinnitus can also be associated with nerve damage in the cochlea as a result of noise exposure and as a side effect of some medication.
Presbyacusis is the term used to describe the hearing loss caused by the degenerative effects of ageing and is characterised by high-frequency hearing loss. When combined with NIHL at the middle frequencies, the individual will experience a debilitating hearing problem.
What is NIHL?
NIHL is a sensorineural hearing loss – a type in which the root cause lies in the vestibulo-cochlear nerve (cranial nerve VIII), the inner ear, or central processing centres of the brain – where repeated exposure to noise is experienced. The hair cells within the cochlea move in response to the sound pressure and are responsive to their own specific frequency. Picture barley in a field when the wind blows; if it is a breeze the stems will sway, if it is a strong force some stems get broken and will not recover.
An individual with NIHL develops a sensitivity to sound between three and six kiloHertz (kHz), and particularly at the 4kHz mark which can be demonstrated by a dip on an audiogram. With continued exposure, this sensitivity can spread to the adjacent frequencies and produce a chronic and permanent hearing loss. It is especially unfortunate that the frequencies of 3-6kHz are the ones that can define our speech – where the sound of the hard consonants are heard. If you have lost the ability to hear these, speech becomes difficult to understand.
Sensorineural loss is damage to the cochlea and the nerves. It is usually permanent, cannot be reversed or operated on, and at the moment is not treatable with medication. However, exciting developments in research demonstrate that the biological mechanisms of NIHL may be treatable or even prevented through the repair of damaged hair cells, the sensory cells that normally reside in the inner ear, with stem-cell treatment. Scientists from Stanford University have produced stem cells in the laboratory that look and act very much like hair cells. If they can generate hair cells in the millions, it could lead to significant scientific and clinical advances along the path to curing deafness.
Other research is looking at the use of antioxidants or pharmaceuticals to scavenge and eliminate the free radicals that may cause the damage (Henderson et al, 2006). In order to know what our noise tolerance level is, scientists have developed an “equal energy principle” (EEP). Evidence has shown that most human ears can tolerate sound levels below 85dB without experiencing damage, but it is noticeable that the majority will show damage if the ear is exposed to higher levels for specific periods.
The EEP states that for every doubling of sound intensity above 85dB, the exposure time must be halved. On a logarithmic scale, this means that every 3dB increase equals a doubling of intensity – for example, 85dB for eight hours, 88dB for four hours, 91dB for two hours and so on (McCombe, 2008). Knowing some common noise levels of different activities can help us to prevent our own ears from exposure. It should also be noted that sound is very rarely made up of a pure tone, but multiple tones at different frequencies.
The law and ONIHL
Regulations passed under the Health and Safety at Work Act 1974 placed general obligations on employers to safeguard workers’ hearing health, and specific laws came into force with the Noise at Work Regulations 1989. Following European Framework Directive 89/391 as part of Physical Hazards, the Control of Noise at Work Regulations were introduced for all industry sectors in 2005 and for the music and entertainment sectors in 2008.
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The Health and Safety Executive (HSE) states: “The aim of the Regulations is to ensure that workers’ hearing is protected from excessive noise at their place of work, which could cause them to lose their hearing and/or to suffer from tinnitus.”
The law states that employers must assess the noise hazard within their area of responsibility and provide information and training to their workers if they may be at risk to exposures greater than 80dB and, where the noise level is at 85dB averaged over the eight-hour daily or weekly exposure, provide hearing protection and hearing protection zones. There is also an exposure limit of 87dB, taking account of any reduction in exposure provided by hearing protection, above which workers must not be exposed.
OH and hearing conservation
The OH nurse is an important part of the team in the identification and management of noise hazards in the workplace, together with the manager, the individual, safety colleagues and specialists. The OH nurse’s role covers all aspects: risk assessment, control measures, consultation with professional colleagues and education of management and of workers.
It is therefore vital that the OH nurse knows the business they are working in and is familiar with the hazards that may affect health that are specific to that particular industry. Employers have a legal responsibility to assess those hazards, identify those at risk and control that risk.
Those individuals entering employment in an area known to be hazardous to hearing should have a baseline measurement of their hearing to use as a reference. This measurement should then be used as part of hearing surveillance as the control from their risk assessment. Full details of implementing a hearing health surveillance programme are available from the HSE in its “Controlling noise at work” guidance.
Even if an organisation has not considered the risk and has done nothing to protect their employees, a hearing conservation programme can be introduced and the hearing assessed and monitored from that time onward. The purpose of this is to identify if any employees are showing early signs of hearing damage and prevent the damage from getting worse.
A normal cochlea does not just receive sound – it also produces low-intensity sounds called otoacoustic emissions (OAEs). These sounds are produced specifically by the cochlea and, most probably, by the cochlear outer hair cells as they expand and contract.
In the majority of settings, pure tone audiometry is the standard measure of hearing. However, OAE tests are being used by some to enhance the hearing test through determining cochlear function, specifically hair cell function.
OAE testing is often used as a screening tool to determine the presence or absence of cochlear function (Campbell, 2012). Research is showing that distorted OAEs may indicate early occupational hearing damage before it is obvious on a standard audio-metry test (Marques et al, 2006).
Analysis of the exposed group’s data can provide information on the success or otherwise of the control measures, and also provide a basis for education of managers and staff. Workplace noise exposure is not different from home-based noise exposure as the ear does not differentiate, so educating staff about preventative measures for leisure activity where there is usually no control is of great importance.
Help is at hand: industrial injury disablement benefit
Occupational deafness is a recognised disease under the industrial injury disablement benefit system and will not be affected by the introduction of universal credit.
Awards are made to those with sensorineural hearing loss amounting to at least 50dB in each ear, being the average of hearing losses at 1kHz, 2kHz and 3kHz frequencies, and being caused in the case of at least one ear to occupational noise.
In 1995/96, there were more than 500 individuals receiving the industrial injury disablement benefit, compared with 150 in 2011. This group was mainly comprised of those working in the manufacturing, construction and extraction, energy and water supply industries.
This is a notable decline and probably reflects improvement in noise controls in the workplace and a reduction in the numbers employed in high-risk activities.
Susanna Everton RGN OHN CSP MSc CMIOSH is an OH and safety practitioner.
References
Bell A (1966). Noise. World Health Organisation.
Stanford University School of Medicine. Stanford initiative to cure hearing loss.
Henderson D et al (2006). “The role of oxidative stress in noise-induced hearing loss”. Ear & Hearing; vol.27, issue 1, pp.1-19.
The British Tinnitus Organisation.
Health and Safety Executive. Controlling noise at work.
Health and Safety Executive. Health surveillance.
Campbell K (2012). Otoacoustic emmissions.
Marques FP, da Costa EA (2006). “Exposure to occupational noise: Otoacoustic emissions test alterations”. Brazilian Journal of Otorhinolaryngology; vol.72, issue 3, pp.362-36.
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Seixas NS et al (2005). “Prospective noise induced changes to hearing among construction industry apprentices”. Occup Environ Med; vol.62, pp.309-317.
Health and Safety Executive. Noise-induced hearing loss in Great Britain.
