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Demystifying noise induced hearing loss, Declan Chukwuma Umege looks at the anatomy and physiology of the auditory system.
Let me tell you a story about a man who worked offshore for an indigenous oil company in Nigeria and observed the popular 28 days on, 28 days off work rotation. It took his wife’s continuous nagging that the television volume was always too loud during family viewing for him to realise that all was not well with his sense of hearing. This man is a victim of noise induced hearing loss (NIHL) and could be gradually losing his sense of hearing.
Auditory anatomy
The auditory system is composed of the outer ear, middle ear and inner ear, as will be explained in more detail in the following sections.
Outer ear The outer ear includes the portion of the ear that we see – the pinna, or auricle, and the ear canal.
The pinna is a concave cartilaginous structure, which collects and directs sound waves traveling through air into the ear canal.
The ear canal, also referred to as the external auditory meatus, is approximately 1.25 inches long and 0.25 of an inch in diameter. The inner two-thirds of the ear canal are imbedded in the temporal bone. The outer third of the canal is cartilage. The ear canal directs airborne sound waves towards the tympanic membrane (eardrum). The ear canal resonates sound waves and increases the loudness of the tones in the 3,000-4,000 Hz range.
Middle ear The middle ear comprises the tympanic membrane and the cavity, which houses the ossicular chain.
The tympanic membrane or eardrum serves as a divider between the outer ear and the middle ear structures. It is a greyish pink colour when healthy and consists of three very thin layers of living tissue.
The eardrum is very sensitive to sound waves and vibrates back and forth as the sound waves strike it. The eardrum transmits the airborne vibrations from the outer to the middle ear and also assists in the protection of the delicate structures of the middle ear cavity and inner ear.
The middle ear cavity is located in the mastoid process of the temporal bone. The middle ear cavity extends from the tympanic membrane to the inner ear. It is approximately two cubic centimeters in volume and is lined with a mucous membrane. The middle ear cavity is actually an extension of the nasopharynx via the eustachian tube. The eustachian tube acts as an air pressure equaliser and ventilates the middle ear.
The middle ear is connected and transmits sound to the inner ear via the ossicular chain. The ossicular chain consists of the three smallest bones in the body: the malleus, incus and stapes. These are sometimes referred to as the hammer, anvil and stirrup, respectively.
The ossicular chain amplifies a signal of approximately 25 decibels as it transfers signals from the tympanic membrane to the inner ear.
Inner ear The inner ear comprises the sensory organs for hearing and balance: the cochlea and vestibular system. The systems are separate, yet both are encased in the same bony capsule and share the same fluid systems.
The vestibular system is, in part, made up of three semicircular canals located within the inner ear. It helps to maintain balance, regardless of head position or gravity, in conjunction with eye movement and somatosensory input. The semicircular canals are innervated by the eighth cranial nerve (VIII CN).
The cochlea, on the other hand, comprises three fluid-filled chambers that extend the length of the structure. The two outer chambers are filled with a fluid called perilymph. Perilymph acts as a cushioning agent for the delicate structures that occupy the centre chamber. The third fluid filled chamber is the centre chamber, called the cochlear duct. The cochlear duct secretes a fluid called endolymph, which fills this chamber.
The cochlear duct contains the basilar membrane upon which lies the organ of Corti, a sensory organ essential to hearing. It consists of approximately 30,000 finger-like projections of cilia, also referred to as hair cells, arranged in rows. Each hair cell is connected to a nerve fibre, which relays various impulses to the cochlear branch of the VIII CN or auditory nerve. The VIII CN or auditory cranial nerve carries the impulses generated from the organ of Corti to the brainstem. From the brainstem, nerve pathways extend through numerous nuclei to the cerebral cortex in the temporal lobes of the brain. It is in the temporal lobes of the brain that meaning is associated with the various patterns of nerve impulses.
Auditory physiology
The process of hearing begins with the occurrence of a sound. When this air movement stimulates the ear, a sound is heard.
Sound waves are transmitted through four separate mediums along the auditory system before a sound is perceived:
• Outer ear – air
• Middle ear – mechanical
• Inner ear liquid and to the brain – neural
Air transmitted sound waves are directed toward the delicate hearing mechanisms with the help of the outer ear, first by the pinna, which gently funnels sound waves into the ear canal, then by the ear canal.
When air movement strikes the eardrum, the eardrum moves. The energy generated through the sound wave is transferred from the medium of air to that which is solid in the middle ear. The ossicular chain of the middle ear connects to the eardrum via the malleus, so that any motion of the eardrum sets the three little bones of the ossicular chain into motion.
The ossicular chain transfers energy from a solid medium to the fluid medium of the inner ear via the stapes. The stapes is attached to the oval window. Movement of the oval window creates motion in the cochlear fluid and along the basilar membrane. Motion along the basilar membrane excites frequency specific areas of the organ of Corti, which in turn stimulates a series of nerve endings.
With the initiation of the nerve impulses, another change in medium occurs: from fluid to neural. Nerve impulses are relayed through the VIII CN, which carries the impulses generated from the organ of Corti to the brainstem, through various nuclei along the auditory pathway to areas to the brain that interpret the neural impulses and create thoughts, pictures and other recognised symbols.
NIHL
Noise is unwanted sound. Noise induced hearing loss (NIHL) is hearing decrease caused by loud sound.
NIHL is one of the most common occupational illnesses, but it is often ignored because it displays no visible effects. It usually develops over a long period of time, and, except in very rare cases, there is no pain. What does occur, however, is a progressive isolation: loss of communication, socialisation and responsiveness to the environment.
Noise regulations
The Control of Noise at Work Regulations 2005 require employers to:
• Assess the risks to their employees from noise at work
• Take action to reduce the noise exposure that produces those risks
• Provide employees with hearing protection if they cannot reduce the noise exposure enough by using other methods
• Make sure the legal limits on noise exposure are not exceeded
• Provide their employees with information, instruction and training
• Carry out health surveillance where there is a risk to health
Again, the Mineral Oils (Safety) Regulations 1997, Part II, 16 (1-4), require the manager of any oil and gas facility to ensure that:
• Workers are provided with the appropriate hearing protection if noise levels are equal to or greater than 85 dBA for an eight hour time weighted average (TWA)
• No person should, unless appropriately protected, be exposed to noise levels equal to or greater than 115 dBA for any length of time, notwithstanding that the TWA is below the 85 dBA action level
• The sound pressure level at the edge of the nearest residential area should not exceed 50 dBA at night
• Annual audiometric tests should be conducted for all personnel working in high noise areas
Hearing conservation
In the oil and gas industry protecting workers’ hearing is a basic requirement. Companies are required to have in place a Hearing Conservation Programme (HCP), which requires that employees be protected from excessive noise exposure.
High noise levels usually result from transportation and operating process equipment such as helicopters, workboats, pumps, generators, compressors and cranes, and are also by products of streams moving through process equipment at high pressures and high velocities.
On identification of high noise level areas, employee monitoring will be conducted on all employees who work in such zones. Any employees who work in high noise areas on a regular basis should be enrolled in the HCP.
The key factor for enrolment into the HCP is the amount of time an employee works in a high noise area. When the daily noise exposure is made up of two or more periods of noise exposure of different levels, their combined effect should be considered, rather than the individual effect of each.
A typical HCP for the oil and gas industry is divided into seven sections:
1. Noise exposure monitoring
2. Audiometric testing
3. Noise exposure evaluation and employee enrolment in HCP
4. Notification procedure for employees
5. Noise exposure controls
6. Employee training
7. Programme evaluation
Monitoring exposure Noise exposure monitoring is usually done to properly determine whether engineering or administrative controls are necessary. Examples of engineering controls include the use of sound barriers, sound-absorbing enclosures, and low noise equipment. Locations where noise levels exceed 85 dB should be considered high noise level areas. Monitoring will be repeated when a change in production process, equipment, scope or duration of work results in increased noise exposures.
Noise level surveys will be conducted by the occupational health and safety department, according to the company’s exposure monitoring plan, and updated as the need arises. The exposure monitoring plan usually includes mapping and posting colour coded plot plans of facilities indicating the noise range of various sections and posting warning signs in high noise areas, as well as ensuring that phase frequency detectors (PFDs) are used in high noise areas.
Audiometric testing Audiometric testing allows for significant hearing loss to be identified. Additionally, the testing allows for the identification of personnel who are in the process of losing their hearing.
During the often annual audiometric testing, employees are usually tested with properly maintained and calibrated equipment for exposure and results are kept for at least two years. Such records will be maintained and stored by the occupational health and safety or medical department as the case may be.
Baseline audiogram A baseline audiogram, against which subsequent audiograms can be compared, is established within six months of an employee’s first exposure at or above the action level (an eight hour TWA of 85 dB or a dose of 100% of allowable daily dose (ADD). TWA is a noise exposure that is averaged over a certain period of time, usually eight or 12 hours.
Tests to establish a baseline audiogram are usually preceded by at least 14 hours without exposure to workplace noise. Hearing protectors should be used as a substitute for this requirement where noise is unavoidable. The employee should be notified of the need to avoid non-occupational noise exposure during the 14 hour period immediately preceding the audiometric examination.
Annually, after the baseline audiogram, a new audiogram for each employee exposed at or above an eight hour TWA of 85 dB should be obtained.
As a standard, companies are expected to administer a continuing effective hearing conservation programme whenever employees noise exposure equals or exceeds an eight hour time weighted average (TWA) sound level of 85 dB measured on the A scale (slow response) or, equivalently, a dose of 100% of ADD.
Evaluating exposure Every employee’s audiogram should be compared with his/her baseline audiogram to determine if the audiogram is valid and if a standard threshold shift has occurred. A standard threshold shift (STS) is defined as the difference of the average of the sums of hearing threshold values, in decibels, of the current and baseline audiograms at frequencies of 2,000, 3,000 and 4,000hz. A factor for aging can be introduced into the STS calculation. Note that STS is recordable if it exceeds 25 dB.
Every worker exposed to noise levels in excess of 100% of ADD will be enrolled in a hearing conservation programme (HCP). The occupational health and safety or medical department will evaluate noise-monitoring data to determine which workers’ noise doses justify inclusion in the HCP. All affected workers will be officially notified of monitoring results after the results have been analysed and documented.
If it is noticed that an STS has occurred after the employee’s audiogram is compared with that employee’s baseline audiogram, the following actions should be taken:
1. Employees not using hearing protectors will be fitted with them, trained in their use and care, and be required to use them.
2. Employees already using hearing protectors will be refitted and retrained in the use of hearing protectors and if necessary will be provided with hearing protectors offering greater attenuation.
3. The employee should be referred for a clinic audiological evaluation or an otological examination, as appropriate, to see whether additional testing is necessary or if there is a suspicion that a medical pathology of the ear is caused or aggravated by the wearing of hearing protectors.
4. The employee should be informed of the need for an otological examination if a medical pathology of the ear that is unrelated to the use of hearing protectors is suspected.
Notification procedures Shifts in hearing sensitivities will normally be determined through the hearing test, conducted during an annual health examination. All affected workers will be notified in writing of monitoring results after the results have been analysed and documented. The departmental managers will also be notified of any confirmed STS of any personnel in his/her department. Exposure records will be kept for at least two years.
Exposure controls Here, the focus is to ensure that efforts are made to control noise exposures through the use of engineering or administrative controls, such as the use of sound barriers, sound-absorbing enclosures, low noise equipment and job rotation. If both types of control fail to reduce noise levels to less than 100% of ADD, hearing protection devices (HPDs) will be used. In order to immediately reduce noise exposure levels, HPDs will be used while engineering controls are installed or until administrative controls are fully implemented.
HPDs will be used by workers who possess one of three criteria:
• Eight hour TWA meets or exceeds 100% of ADD
• Confirmed STS and eight hour TWA at or above 100% of ADD
• No baseline audiogram and eight hour TWA at or above 100% of ADD
Noise exposure control plans often include posting colour coded plot plans of facilities following a sound monitoring survey to indicate the noise range of various sections, posting of warning signs in high noise areas to alert workers, as well as ensuring that appropriate PFDs are used in high noise areas. All entrances to high noise areas should be marked with orange coloured stripes.
Typically, the plot plans are colour coded as follows:
• Green represents up to 80dB
• Blue represents 80-84dB
• Yellow represents 85-90dB
• Red represents 89dB and above
Employee training All workers listed in the company’s HCP and their supervisors will receive hearing conservation training prior to their placement in a noise-exposed job. The purpose of this training is to explain the effects of noise exposure and how to properly protect oneself from them.
Refresher training will not be given annually unless an STS is confirmed by medical review. When an STS is confirmed by follow-up testing, updated training will be provided. Such training will focus on use of HPDs with attenuation greater than those used previously. Hearing conservation training received by eligible workers will be adequately documented.
Programme evaluation The occupational health and safety department will evaluate the HCP annually. The purpose is to assess all aspects of the programme and its effectiveness, ensure that employees are provided with proper hearing protection devices and/or controls, and that all occupational hearing losses are properly recorded.
Employees who use HPDs and their supervisors will be consulted concerning the performance of the devices. The information gathered will be analysed and improvements made to the programme where considered appropriate. The findings of the evaluation and recommended actions taken will be documented and maintained by the occupational health and safety department.
Published: 3rd Mar 2015 in Health and Safety Middle East
