For clinical purposes, noise is measured in decibels weighted according to the sensitivity of the human ear (dB(A)). Regardless of source, the effects of overexposure to noise are similar. Initially there is a temporary threshold shift, where reversibility of hearing loss is possible with removal away from further noise. Permanent threshold shift occurs following prolonged and/or intense exposure, with poor prospects for improvement of hearing. The classical audiogram for noise-induced hearing loss shows a 4 kHz dip. Prevention is by reducing exposure to noise at source, and in the United Kingdom a limit for exposure has been set at 87 dB(A) averaged over an 8-h day or 140 dB(A) for any instantaneous impulse noise.
Noise is any unwanted sound. Excessive noise damages the cochlear outer hair cells, breaking and disrupting the cilia, which act as local electromechanical amplifiers. This can result in physical and psychological harm. In addition to sensorineural noise-induced hearing loss (NIHL), noise contributes to stress and accidents in the workplace.
The two important characteristics of sound are its intensity and frequency. The human audible sound intensity range is 0 to 120 decibels. The decibel (dB) scale is logarithmic rather than linear, therefore every increase in sound intensity of 3 dB is equivalent to a doubling of sound intensity. In young adults, the ear sound frequency ranges from 20 Hz to 20 kHz, but its sensitivity is not equal across this range. To mimic the response of the human ear and to allow for the variation in ear sensitivity to different frequencies, noise-meters apply a weighting to the sound intensities, and express the readings as dB(A), i.e. decibels weighted by the A scale (as defined by international standards). Typical sound levels are 65 dB(A) for normal conversation at a distance of 1 m; 140 dB(A) for a jet aircraft taking off 25 m away; and 160 dB(A) for a rivet gun near the ear.
Noisy industries include manufacturing, construction, engineering, printing, motor sports, the military, and entertainment industries. There is a tendency to use more powerful equipment, with increasing potential for generating harmful noise levels. Instantaneous noise levels can be assessed using a noise-meter. For cumulative noise exposure, a personal noise dosimeter provides an ‘equivalent noise dose’ by averaging the frequencies and intensities over an 8-h shift. In the United Kingdom, the Control of Noise at Work Regulations 2005 stipulate an exposure limit of 87 dB(A) averaged over 8 h/day or 140 dB(A) for any instantaneous impulse noise.
Exposure to loud noise can cause auditory and nonauditory effects. There is wide variation in individual susceptibility. Massive impulse pressures, e.g. from bomb blasts, can cause perforation of the tympanic membrane. The extent of resulting conductive hearing loss depends on the perforation size. There may be associated otorrhoea or pain. Acute perforations usually heal spontaneously over several weeks unless they are large or complicated by infection. If healing has not occurred by 6 weeks, myringoplasty may be indicated. Attic tympanic membrane perforations need urgent referral to exclude cholesteatoma.
An early response to noise exposure is temporarily increased hearing threshold (temporary threshold shift) often with accompanying tinnitus. This may cause transient dullness of hearing, common in those who work in noisy environments, or those attending loud musical events, and typically lasts up to 24 h after which hearing thresholds return to normal. With continuing exposure, the magnitude of this temporary sensorineural hearing loss and the recovery time increase until, after months or years, there is a permanent shift in threshold accompanied by tinnitus. On audiograms, it is detected as a dip at 4 kHz (Fig. 18.104.22.168). Affected people find it difficult to distinguish between similar sounds, particularly consonants, in the presence of moderate background noise. With severe hearing loss, the listener may experience ‘loudness recruitment’, a rapid, uncomfortable increase in sound perceived when intensity increases beyond the already abnormal hearing thresholds. Hearing damage from noise exposure may not be apparent until early middle age. With continued noise exposure, the 4-kHz dip on audiograms extends to lower frequencies and hearing thresholds worsen. This may be combined with presbyacusis (age-related hearing loss) in later years.
The nonauditory effects of noise on health include increased blood pressure, ineffective performance of mental tasks, and symptoms of annoyance, distraction, fatigue, sleep disturbance, and feelings of isolation. These may combine to reduce work output and efficiency. Accidents in noisy workplaces have been attributed partly to inability to hear verbal warnings or instructions clearly.
A diagnosis of NIHL is from noise exposure assessment, a history of hearing difficulty that may be accompanied by tinnitus, and an audiogram showing the classical 4-kHz dip. Where abnormalities are detected, it is important to establish the history of occupational and leisure noise exposure, exposure to ototoxic drugs and chemicals, previous ear pathology or surgery, other relevant medical history, and the compliance with use of hearing protection. Otoscopic examination, tuning fork tests, and bone-conduction audiometry, should be carried out to exclude conductive hearing loss. Unusual asymmetrical audiograms with vertigo or unilateral tinnitus require otorhinolaryngologist referral to exclude cerebellopontine angle pathology (e.g. acoustic neuroma).
Management, prevention, and surveillance
People with hearing loss and tinnitus may benefit from using hearing aids and a tinnitus masking device.
The emphasis in dealing with noise-induced deafness must be on prevention. Employers whose workplaces are noisy should establish a hearing conservation programme, with commitment to a robust ‘noise policy’, at the highest level of management. Essential components of such a programme are measures to control noise at source, delineating and controlling access to noisy areas, providing and ensuring use of hearing defenders, and educating and training employees. To control noise at source, less-noisy equipment and engineering controls must be purchased, processes redesigned to reduce noise output. Exposure to noise can be limited by separating its source from the workers using soundproof enclosures or shelters. Hearing protection using ear plugs, ear muffs, or canal caps can reduce noise exposure at the ear by 3 dB(A) to 15 dB(A), but it must be fitted and used correctly to be effective.
The diagnosis of workplace NIHL in a worker should be treated as a sentinel event indicating that other workers were at similar risk and that prompt preventive measures should be implemented. After reduction of noise at source, any residual noise exposure of workers (exceeding 85 dB(A) as defined by the United Kingdom Control of Noise at Work Regulations 2005) warrants health surveillance. This involves symptom review and annual audiometry. In some cases, it may be necessary to consider changing jobs. Clinicians should also advise affected people about benefits available from state compensation schemes.
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