Chapter 58. Occupational Hearing Loss

The hearing mechanism, being very sensitive to sound stimuli, is also very susceptible to injury. Many workplaces are hazardous to hearing health due to exposure to (1) noise, (2) physical trauma, and/or (3) toxic materials. Each of these elements comprises a separate subchapter in this review. The majority of the review is focused on noise damage as it is far and away the most prevalent type of occupational auditory injury. The final section discusses some medicolegal aspects of occupational hearing loss.

Occupational hearing loss is noise-induced hearing loss (NIHL) due to chronic overexposure to hazardous levels of noise in the workplace. NIHL may also occur from nonworkplace (eg, recreational) noise overexposure. Because the pattern of hearing loss is essentially the same, allocating occupational versus recreational noise damage remains a challenge. NIHL represents about 15% of the total societal burden of hearing loss among American adults. People with occupational NIHL represent approximately half that total, or about 2.5 million adults; another 2 million suffer NIHLs from nonoccupational or leisure activities such as hunting and target shooting, listening to loud music, or engaging in noisy hobbies or recreational activities.

In spite of considerable attention aimed at limiting noise overexposure–through wearing of personal hearing protection, engineered reductions in noise levels, and industrial hygiene measures–loss of hearing still occurs. The additive effect of aging upon the auditory system complicates the evaluative process even if serial examinations are available. Traumatic hearing loss in the military–both NIHL and acoustic trauma–is an increasing source of disability and compensation in the Veteran's administration.

The goal of this review is to provide guidance for physicians who evaluate and treat people with suspected NIHL. We will focus primarily on diagnosis and evaluation, with updated information on prevention and remediation.

Background

Noise may be defined as unwanted, undesirable, or excessively loud sounds as experienced by an individual. The effects of chronic noise exposure vary with the characteristics of the sound: damage is related to intensity, exposure duration, and exposure pattern (continuous exposures are more damaging than interrupted exposures for the same overall duration and intensity). Daily exposure to hazardous noise over years produces the characteristic loss of high-frequency sensitivity in the 4–6 kHz range (Noise notch–See Figure 58–1).

A less common but potentially more devastating form of occupational hearing loss results from acoustic trauma, wherein high-intensity impulse noises (eg, explosions) physically disrupt any or all parts of the ear resulting in immediate and irreversible hearing loss. Blast damage generally follows energy levels of >140 dB on the A scale (dBA), and increases as the intensity increases. The damage from the improvised explosive devices (IED) being used in the current military conflicts is often total owing to the confinement of the explosive energy within the armored vehicle. IED trauma in the military is often associated with brain injury as well.

The stapedius muscle contracts reflexively (acoustic reflex) in response to noise >90 dB. Although the acoustic reflex dampens sound transmission, it is most effective against low frequency noise. The delay between the noise exposure to onset of the reflex is 25 to 150 ms, which renders it less effective against impulse noise as opposed to continuous noise. People without a stapedius reflex (about 1–2 % of the population) are more vulnerable to noise damage than those with the reflex. There are few if any ways to use the acoustic reflex in NIHL abatement plans. One potential example might be to trigger the reflex by providing background sound during magnetic resonance imaging as an attempt to attenuate the noise exposure from energizing the magnets.

Pathology

NIHL is thought to result from metabolic depletion of the sensory epithelium of the cochlea, mainly the outer hair cells and associated neurons. The damage begins first in the 4–6 kHz region of the cochlea depending more on the resonance characteristics of the ear canal than on the frequency of the noise. NIHL has two aspects: temporary threshold shift (TTS) and permanent threshold shift (PTS). TTS is commonly experienced after intense short-term exposures (such as a rock concert) wherein the hearing loss recovers in a few days. Repeated TTS exposures may result in PTS over time.

Early studies of TTS were directed toward determining whether TTS might predict susceptibility for PTS (which it does not) and the mechanism of effect. Pujol and Puel described the changes in spiral ganglion neurons wherein the synaptic face of the neuron physically withdraws from the inner hair cell and reattaches in a few days. This process coincides with the time course of TTS and is thought to result from glutamate toxicity secondary to overstimulation.

In classic PTS, the changes become irreversible and include loss of outer hair cells, degeneration of cochlear nerve fibers, and scar formation (dead zones) in the Organ of Corti. However, Kujawa and Liberman have shown that irreversible noise-induced neural degeneration may occur in the absence of changes in auditory thresholds, and, further, without loss of outer hair cells in some cases.

In contrast to NIHL, acoustic trauma causes immediate physical damage to the ear in proportion to the intensity of the overpressure. High-intensity impulse noises can physically damage tympanic membrane, ossicles, inner ear membranes and the Organ of Corti. Indeed, rupture of the TM may absorb some of the energy that would have otherwise been transferred to the inner ear. These types of blast injury are increasingly common in war injuries secondary to IED used widely in the current Mideast conflicts. There is evidence from animal studies that such damage initiates apoptotic cell death and the otoprotectants may theoretically limit the damage somewhat. Human studies to evaluate these possibilities are underway.

Clinical Characteristics of Occupational Noise-Induced Hearing Loss

According to the 1987 report of the Hearing Conservation Committee of the American College of Occupational Medicine (ACOM), the considerations that a physician should use in establishing a diagnosis of occupational NIHL are as follows: (1) significant exposure to hazardous occupational noise (time weighted average of 90 dB on the A scale (dBA); (2) gradual onset of hearing loss; (3) a symmetrical, or nearly symmetrical, loss in both ears; (4) a hearing loss at approximately 4000 Hz, commonly referred to as a hearing loss “notch”; (5) occupational hearing loss is not progressive after a maximum loss is incurred approximately 10 to 12 years after initial exposure; (6) normal or near normal speech discrimination scores ; (7) the maximum amount of hearing loss caused by occupational noise exposure to the loudest noises is 40 dB in the speech frequencies and 75 dB in the higher frequencies; (8) occupational hearing loss does not progress once the subject is removed from the noisy environment.

The presence of these elements does not necessarily lead to a conclusive diagnosis of occupational hearing loss since other causes may have similar characteristics. On the other hand, the absence of one, or more, of these factors is generally evidence of a cause other than occupational noise exposure. Not addressed by the ACOM report are: (1) the confounding issue of recreational noise exposure, which frequently co-occurs with occupational noise exposure and (2) the interaction of age changes superimposed on NIHL.

The profile of typical audiometric noise notches was compiled by Cooper and Owens (Figure 58–2) as the mean pure-tone thresholds from 450 ears of men with clear-cut histories of noise exposure. Note that the threshold at 8 kHz is better than at 4–6 kHz. With time (age), the depth of the notch is decreased by worsening of the 8 kHz threshold but is still discernable in the vast majority of cases. Figure 58–3 displays the net change in audiometric profile with time for women employed in jute manufacturing (as compared to age-matched controls). This classic epidemiologic study by Taylor et al. has yet to be surpassed.

In evaluating a patient claiming occupational hearing loss, the following conditions and factors must be considered either as alternative diagnosis or coexistent disorders:

1. presbycusis (ie, age-related hearing loss);

2. hereditary hearing impairment causing progressive degeneration;

3. metabolic disorders (eg, hypertension, diabetes mellitus, hypothyroidism, renal failure, autoimmune disease, hyperlipidemia, and hypercholesterolemia);

4. smoking;

5. hearing loss resulting from infectious origins (ie, bacterial or viral infections, including meningitis and encephalitis);

6. hearing loss resulting from central nervous system dysfunction;

7. nonorganic hearing loss (ie, functional hearing loss);

8. nonoccupational NIHL.

The following common otologic problems should be excluded in the differential diagnosis of NIHL even though they are predominately unilateral conditions: sudden sensorineural hearing loss, Meniere's disease, and cerebellopontine angle tumor. In general an accurate history will reasonably exclude the majority of these considerations but special testing may be necessary.

Evaluation of Hearing

In all cases of suspected occupational hearing loss, a complete pure-tone audiogram with speech reception thresholds (SRT) and word recognition scores (WRS) must be done. The audiometric equipment should have been calibrated annually to conform to American National Standards Institute standards. The testing should be done 48 hours or more after the last day at work.

Functional hearing loss should always be considered as an element in the evaluation of people seeking compensation for NIHL. Although the following tests may be used to help differentiate this from a genuine occupational hearing loss, the situation more often than not is not a clear choice between either functional or organic but, rather, a functional overlay on an organic loss. As such, the examiner has a challenge in separating the components.

If SRT scores diverge more than 10 dB from the pure-tone average (PTA) for speech frequency, additional testing should be done to exclude the possibility of a nonorganic component. Auditory brainstem response and otoacoustic emissions will be normal in pure functional hearing loss cases. The Stenger test is the classic behavioral test for estimating a functional unilateral hearing loss. It is based on the principle that if tones of the same frequency are presented to both ears, the patient can only perceive the louder tone.

Diagnostic Considerations

Patients with NIHL frequently complain of a gradual deterioration in hearing, in particular speech in the presence of competing background noise, and almost all note the presence of tinnitus. Background noise masks the better-preserved portion of the hearing spectrum and further exacerbates problems with speech comprehension. Because patients with NIHL have predominately high-frequency loss, they experience a decrease in high-frequency speech sounds (primarily the consonants) Figure 58–1 and when listening to people with particularly high-pitched voices (eg, women and children).

NIHL is frequently accompanied by tinnitus. Most often patients describe a high-frequency tonal sound (eg, ringing), but the sound is sometimes lower in tone (eg, buzzing, blowing, or hissing). Often, the tinnitus frequency matches the frequency of the hearing loss seen on the audiogram and is approximately 5 dB above that threshold in loudness. Tinnitus in the absence of threshold elevation in the 3–6 kHz region of the audiogram is unlikely to be related to noise exposure.

The clinical attributes of occupational NIHL were summarized in an evidence-based policy statement by the American College of Occupational and Environmental Medicine (2002) as follows.

1. It is always sensorineural, affecting hair cells in the inner ear.

2. Since most noise exposures are symmetric, the hearing loss is typically bilateral.

3. Typically, the first sign of hearing loss due to noise exposure is a “notching” of the audiogram at 3000, 4000, or 6000 Hz, with recovery at 8000 Hz. The exact location of the notch depends on multiple factors including the frequency of the damaging noise and the length of the ear canal. Therefore, in early noise-induced hearing loss, the average hearing thresholds at 500, 1000, and 2000 Hz are better than the average at 3000, 4000, and 6000 Hz, and the hearing level at 8000 Hz is usually better than the deepest part of the “notch.” This “notching” is in contrast to age-related hearing loss, which also produces high frequency hearing loss, but in a down-sloping pattern without recovery at 8000 Hz.

4. Noise exposure alone usually does not produce a loss greater than 75 dB in high frequencies, and 40 dB in lower frequencies. However, individuals with superimposed age-related losses may have hearing threshold levels in excess of these values.

5. The rate of hearing loss due to chronic noise exposure is greatest during the first 10–15 years of exposure, and decreases as the hearing threshold increases. This is in contrast to age-related loss, which accelerates over time.

6. Most studies suggest that previously noise-exposed ears are not more sensitive to future noise exposure and that hearing loss due to noise does not progress (in excess of what would be expected from the addition of age-related threshold shifts) once the exposure to noise is discontinued.

7. In obtaining a history of noise exposure, the clinician should keep in mind that the risk of NIHL is considered to increase significantly with chronic exposures above 85 dBA for an 8 hour time-weighted average (TWA). In general, continuous noise exposure over the years is more damaging than interrupted exposure to noise which permits the ear to have a rest period. However, short exposures to very high levels of noise in occupations such as construction or firefighting may produce significant loss and measures to estimate the health effects of such intermittent noise are lacking. When the noise exposure history indicates the use of hearing protective devices, the clinician should also keep in mind that the real world attenuation provided by hearing protectors may vary widely between individuals.

The typical “4000 Hz notch” (Figure 58–1) is thought to occur primarily as a result of the position of the stapes footplate over the high-frequency end of the basilar membrane and the resonance frequency of the ear canal, which amplifies high frequency sounds. Lower and higher frequencies become affected after many years of noise exposure and a significant decrease in WRS does not begin until frequencies <3000 Hz are affected (Figure 58–1). Mild asymmetry can exist in the audiogram, particularly when the source of the noise is lateralized (eg, a rifle or shotgun firing, driving with the window open). Typically, the left ear has poorer thresholds in right-handed people.

The ACOEM statement warrants re-examination as newer evidence calls attention to the issue of progression on NIHL after noise exposure ceases. Whether the progression is viewed as continuing “injury” or “accelerated aging” is not resolved. It is clear that noise damage does not recover and that effects of the damage are exacerbated over time. Further, the importance of primary neuronal injury is not addressed by this statement.

Essentials of Diagnosis of Occupational NIHL

• High-frequency notch in 4–6 kHz area in one or both ears
• Plausible history of hazardous noise exposure
• Rate of loss over time fits NIHL pattern
• Exclusion of other causes of high-frequency loss.

Susceptibility

There appears to be variable tolerance to high noise levels. The genetic basis of this variability has been studied in animal models with conflicting results. While the risk is likely an interaction between genetic susceptibility and the duration and intensity of the noise exposure, the parameters of this risk have not been fully determined.

Presbycusis

NIHL and presbycusis often coexist in our aging population. Although studies have shown that the combined effect is additive over time, it appears that the effect is not linear and animal models suggest that genetic variability is an important factor in determining whether aging makes the ear more or less susceptible to noise damage.

Ototoxicity

Concurrent exposure to noise and ototoxic medications may potentiate hearing loss. These effects have been shown for both cisplatin and aminoglycosides. Loop diuretics and salicylates, however, have not been definitively been shown to potentiate noise-induced hearing loss.

Vibration

There is recent evidence that vibration can interact with noise to cause both TTS and PTS. The mechanism of this effect is not well understood.

Treatment

As there are no medical or surgical treatments available to reverse the effects of NIHL, prevention is key. This may require a collaborative approach involving persons with backgrounds in acoustic engineering, industrial hygiene, otolaryngology, and audiology to examine ambient noise levels in the various work environments and design educational and monitoring programs for personal hearing protection. After the diagnosis has been established by otologic examination and the administration of an audiometric test battery, the physician should counsel the patient on the likely consequences of continued noise exposure.

Use of amplification is the customary recommendation for people who note difficulty in hearing. Individual needs and acoustic environments influence decisions for selecting the specific type of aid. In bilateral hearing losses, bilateral amplification usually provides more satisfactory rehabilitation, unless there is evidence of central auditory dysfunction, where a unilateral aid often gives better results. A reasonable criterion for referral for hearing aid evaluation is a SRT of greater than 25 dB or a WRS of less than 80% at presentation levels of 50 dB above threshold. There are some instances in which hearing aids may be recommended to assist the patient to hear in special circumstances, such as lectures or group situations. In patients with high-frequency hearing loss and relatively normal low-frequency hearing, hearing aids are generally the most helpful to those who have a significant loss at 2000 Hz.

The basic hearing aid today is a digital programmable device with squeal suppression and an open ear mold. A hearing aid evaluation should be done to select from the variety of possible fittings and a trial usage period, with the patient wearing the aids in various circumstances, is recommended. Numerous assistive listening devices are available (FM and infrared) to enhance comprehension in specific situations. Aural rehabilitation classes designed to enhance the patient's ability to comprehend speech may also be helpful and are usually available in some urban areas.

There is no cure for tinnitus associated with NIHL although numerous ameliorative measures are available. In the absence of further inner ear injury, tinnitus usually diminishes with time. A variable degree of tinnitus often persists and is especially obvious in quiet. For the few patients who find this to be extremely troublesome, masking the tinnitus with music or some other pleasant sound is often helpful. In those with significant hearing loss, appropriate amplification is helpful. Modified hearing aids (tinnitus maskers) designed to produce masking noises have generally been of limited success. Tinnitus retraining therapy is now widely available to help people cope with their tinnitus trouble. Psychiatric referral may be necessary to manage associated depression and anxiety that is common in those “bothered” by their tinnitus.

Prognosis

The trajectory of NIHL shows the greatest change in the first 10 years of exposure with less and less additional loss as exposure continues. In contrast, age-related hearing loss has an increasing trajectory with time. The superimposition of age effects on prior noise damage effects remains as a controversial matter in compensation evaluations. The pattern of hearing loss in the Scottish jute weavers (See Fig 58-3) is the classic demonstration of NIHL over time.

The standard teaching is that NIHL does not progress after hazardous noise exposure ceases. The worsening of hearing after, say, retirement is usually attributed to aging (presbycusis), although other conditions may contribute to the change. Presbycusis can add to NIHL as the patient grows older. However, the effect is uneven. Since cells lost from one cause (eg, noise) cannot be “relost” (eg, aging), there is less change with time in the “notch” frequencies but slightly greater loss with time in the adjacent frequencies, in particular, 3 kHz. (Gates and Kujawa). Whether such loss should be attributed to the consequence of prior noise-damage or to accelerated aging is moot at present.

NIHL can be prevented by reducing noise at the source through engineering controls, limiting exposure by administrative controls, and employing effective hearing protection practices for exposures that cannot or are not avoided. The key component to all prevention efforts is education. Workers exposed to hazardous workplace noise need to understand that hearing can only be protected and preserved if efforts to reduce all hazardous exposures, not just those in the workplace, are undertaken.

Regulation of Occupational Noise Exposure

Noise exposure associated with the workplace has been known for centuries to produce hearing loss. In fact, “boilermakers' deafness” was the term coined to describe the now-familiar bilateral sensorineural hearing loss associated with excessive exposure to occupational noise. Largely on the basis of knowledge gained through field studies of hearing loss in industrial workers and military personnel, the U.S. Department of Labor promulgated regulations in the 1970s and 1980s designed to protect the hearing of employees who work in noisy environments. The principal regulation is the Occupational Noise Standard, promulgated by the Occupational Health and Safety Administration in 1972 and amended in 1983. This regulation covers workers in industry governed by the Department of Labor; other federal agencies (Federal Railroad Administration, Mine Safety and Health Administration, etc) have regulations that share key features with the OSHA rule.

The maximum exposure levels permitted by OSHA for various durations are shown in Table 58–1. The levels stated in the table represent the maximum allowable daily noise exposure, or the “permissible exposure limit” (PEL), as specified by OSHA and other federal agencies. The PEL for an 8 hours exposure is referred to as the “criterion;” it reflects the sound level in dBA (decibels measured with the A-weighting filter network in place, which reaches the PEL after 8 hours of exposure. Note that for exposures that differ from 8 hours the allowable daily exposure level is increased or decreased by 5 dB for each halving or doubling of exposure duration: ninety decibels are allowed for 8 hours daily, 95 dB for 4 hours daily, etc. Each of the exposures listed in the table represents an equivalent TWA exposure of 90 dBA for 8 hours. By definition an 8 hours TWA of 90 dB represents 100% of the allowable “dose.”

When the daily noise exposure is composed of two or more periods of exposure at different levels, their effects are combined by the following rule:

C1/T1 + C2/T2 + … + Cn/Tn

where C is the exposure duration at a given level; and T the allowable duration at that level.

“Percent allowable dose” is then calculated by multiplying the result by 100%. That is,

D = 100(C1/T1 + C2/T2 + … + Cn/Tn)

All exposures between 80 and 130 dBA are required to be integrated into the dose calculation. Exposures below the so-called threshold are not counted in the calculation of daily exposure; threshold values range from 80 to 90 dBA among various regulations. For example, consider the following daily noise exposure for a sheet metal worker:

Calculation of Daily Dose is Made as Follows.

Therefore, this employee's exposure would not exceed OSHA's PEL. The dose could also be expressed as a TWA level in decibels by calculating the 8 hours exposure level that would result in the same dose:

• TWA = 16.61log10(dose/100) + 90
•      = 16.61 Xlog (56.25/100) + 90 = 85.9dBA

It is important to remember that “dose” and “TWA” really refer to the same measurement: the 8-hours equivalent exposure for any measured duration or combination of levels and durations, expressed as percent or decibels.

As amended in 1983, the current U.S. occupational noise exposure standard identifies a TWA of 85 dBA, or 50% dose, as an “action level.” Workers covered by the standard who are exposed above the action level must be provided an effective hearing conservation program, including annual audiometric evaluations, personal hearing protection if desired, and education programs. With a daily noise exposure of 56.25%, or a TWA above 85 dBA, the sheet metal worker described should be in a company hearing conservation program.

The exposure limits set by OSHA were empirically determined from epidemiological and laboratory data concerning hearing damage from noise exposure and were designed to protect employees against sustaining a material impairment in hearing after a working lifetime. They were derived by subtracting the percent of workers sustaining a material impairment in hearing as a function of exposure level from a control population without occupational exposure. The resultant percentage is the “percent risk” or “percent additional risk” of a material impairment in hearing after, say, 40 year of exposure, above that expected from presbycusis alone. Estimates of percent risk vary depending upon which criteria and databases are used; the estimates provided in the original standard have been revised downward on the basis or more modern statistical fitting methods by the National Institute for Occupational Safety and Health (NIOSH) and support a conclusion that that the PEL of 90 dBA, with the 85 dBA action level, if enforced, would protect 93–96% of the working population from sustaining occupational noise-induced hearing loss.

In 1998 NIOSH revised its original 1972 recommendation that the permissible exposure limit for occupational noise be set to a time-weighted exposure of 85 dBA, and added, further, that the exposures be calculated with a 3 dB exchange rate. In other words, daily exposure at 85 dBA represents a 100% dose and the dose is doubled or halved for every 3 dB increase or decrease (ie, 88 dBA= 200%, 82 dBA= 50%, etc). NIOSH's recommended exposure limit (REL) is much more conservative than the OSHA standard, particularly for workers exposed at high levels for relatively short durations. For instance, the sheet metal worker's OSHA PEL 56% exposure (TWA = 85.8 dBA) described above would be 292% of the REL (TWA = 89.7 dBA) using the NIOSH limits. Although various groups have recommended the NIOSH REL as the appropriate federal standard for many years, it has not been adopted by any federal agency as a national standard.

The American Conference of Government Industrial Hygienists has also listed guidelines for occupational exposure to noise. These limits are specified as “threshold limit values” (TLVs), and they use the same metrics as the NIOSH REL (85 dBA criterion; 3 dB exchange rate). However, the TLV is specified by ACGIH as the minimum exposure at which one should consider implementing a hearing conservation program; it does not imply an upper limit of tolerable exposure, as does the OSHA PEL. Viewed in this way, there is no conflict between the ACGIH TLV and the OSHA PEL; hearing conservation programs should be implemented if the exposure exceeds the ACGIH TLV, and workers should not be exposed above the OSHA PEL.

The OSHA noise standards are useful to the physician in arriving at a diagnosis, and in determining whether a recommendation about hearing protection should be made. First, because workers exposed to excessive occupational noise should be in a hearing conservation program, evidence of exposure history and prior company-obtained audiograms may be available for consideration (Table 58-2). If the worker is not in a hearing conservation program, he or she may not work in significant occupational noise. Unfortunately, because not all workers are covered by OSHA standards and because enforcement has been weak, lack of participation in a hearing conservation program by a worker does not guarantee he or she has not been exposed to excessive occupational noise. However, in evaluating a patient, if it is determined that exposure to occupational noise did not exceed a TWA of 85 dBA, or a dose of 50%, then exposure to occupational noise should be ruled out of the etiology.

Hearing Conservation Program

OSHA requires workers exposed above the action level (8 hours TWA of 85 dBA, or 50% dose) to be enrolled in a continuing, effective hearing conservation program. The major components of the program are summarized below.

Noise Monitoring

Two general types of exposure assessment are commonly used in industry: area noise surveys and personal noise dosimetry. Area surveys are used to identify job locations where the TWA exposure may exceed 85 dBA; they are conducted by placing a sound level meter in a specific location and sampling the noise field. Exposures are then calculated based upon the amount of time an employee works in that specific location. Area surveys are also useful in determining the sources of exposure in an industrial environment and for planning noise control engineering strategies for reducing exposure (Table 58-3).

Occupational exposures can also be measured directly by using personal noise dosimeters. These devices are small, computerized integrating sound level meters that can record the minute-by-minute sound exposure throughout the workday. They are worn on a belt or in a pocket, and the microphone is positioned on the shoulder, approximately 5 in. lateral to the ear. Technological advances in microprocessor design, incorporating low power consumption and component miniaturization, have led to sophisticated, small, lightweight dosimeters that can be worn unobtrusively. A distinct advantage to using dosimetry over other methods is that the measurement instrument travels with the worker and can therefore provide a more accurate assessment of exposure as he or she moves among different noise environments during the work day. Exposure assessment using dosimetry does have some disadvantages. Because the instrument is mounted on the shoulder, reflection of sound off the body adds about 2 dB to the exposure assessment. In addition, it is nearly impossible to prevent bumping or touching the surface of the microphone during a normal workday. Contact with the microphone, even with a windscreen in place, will always increase the dose measure, and for short-duration, high-level impacts, the exposure can be inflated dramatically. Finally, because the dosimeter travels with the worker rather than staying under the control of the professional assessing the exposure, errors of commission and omission can occur. For example, a dosimeter attached to a jacket, which is then deposited in a locker before the workshift begins, will record the exposure of the jacket, rather than that of the worker.

Engineering Controls

Can be implemented to reduce employee exposure in many cases. Designers' conceptualize possible engineering solutions in terms of (1) the source (what is generating the noise), (2) the path (the routes the generated noise may travel, and (3) the receivers (the noise-exposed workers). To reduce noise exposure to workers such controls may involve (1) enclosures to isolate the sources or receivers, (2) barriers to reduce energy transmission along the path, and (3) distance to increase the path and ultimately to reduce acoustic energy at the receiver. Additional important engineering controls include the design of quieter manufacturing processing (low-acoustic emission saw blades).

Administrative Controls

Include (1) reducing the amount of time a given worker might be exposed to a noise source to prevent a TWA of noise exposure from reaching 85 dBA and (2) establishing purchasing guidelines to prevent the introduction of equipment that would increase the dose of noise to which workers are subjected. Though simple in principle, the implementation of administrative control requires management commitment and constant supervision, particularly in the absence of engineering or personal protection controls. In general, administrative controls are used as an adjunct to existing noise control strategies within a hearing conservation program rather than as an exclusive approach for controlling noise exposure.

Worker Education

Workers and management must understand the potentially harmful effects of noise in order to satisfy OSHA requirements and–most important–to ensure that the hearing conservation program is successful in preventing NIHL. A good worker education program describes (1) program objectives, (2) existing noise hazards, (3) how hearing loss occurs, (4) the purpose of audiometric testing, and (5) how workers can protect themselves. In addition, the roles and responsibilities of the employer and the workers should be clearly stated. Training is required to be provided annually to all workers included in the hearing conservation program. Opportunities for maintaining awareness occur during periodic safety meetings, as well as during audiometric testing appointments, when testing results are explained.

Hearing Protection

Workers in environments that exceed the OSHA PEL must wear hearing protectors that will reduce exposures to below the action level (85 dBA TWA). Hearing protectors must be offered to employees exposed at or above the action level but below the PEL, and hearing protector use is mandated if the employee exhibits a standard threshold shift (defined below). It is important that workers not be forced to wear hearing protectors with attenuation values significantly exceeding the attenuation needed to reduce the exposure to safe levels (ie, below the action level). There is no need to reduce exposures below the safe limit; doing so decreases workers compliance and decreases the ability of workers to communicate and hear warning sounds, which increases the probability of injury from accidental causes.

For many years OSHA has required employers to use the “noise reduction rating” (NRR), a method for rating hearing protectors approved by the Environmental Protection Agency in 1979. Because the NRR was developed as a laboratory standard to provide a benchmark comparison value among earplugs, it has long been recognized as an inadequate method for determining the real-world attenuation values that can be expected for a worker in a real-world environment. Recognizing the problems with the original NRR, OSHA provides, in a complicated appendix to the noise standard, three methods for derating the NRR values in order to arrive at a more “realistic” estimate of the actual attenuation that might be expected for a worker using the earplug. Discussion has been ongoing about a possible new rule, and a new one appears to be around the corner at the writing of this chapter.

The new rule, EPA Noise Labeling Standards for Hearing Protection Devices (40CFR 211 subpart B), is expected to be issued early in 2010. The rule provides a new method that gives NRRs determined by subjects who fit themselves without assistance. Further, a range of NRRs is provided rather than a single number. The range is designed to account for differences in individual subject fitting and also will address inherent differences in hearing protector type, as well as addressing modern hearing protectors that include active noise reduction circuitry. The low number in the range is the attenuation achieved by 80% of the subjects; the higher number reflects the attenuation achieved by 20% of the subjects. (Note in editing: there is considerable sentiment that the upper number should be 10%.) The new rule, when enacted, will form the basis of OSHAs hearing protector fitting methods.

Audiometric Evaluation

Provides the only quantitative means of assessing the overall effectiveness of a hearing conservation program. A properly managed audiometric testing program supervised by either a certified audiologist, physician, or other personnel trained and experienced in occupational hearing conservation can detect changes in hearing over time that might otherwise be overlooked. The results of audiometric testing must be shared with employees to ensure effectiveness. The overall results or trends noted in an audiometric testing program can be used to fine-tune the hearing conservation program, including determining what types of hearing protection devices to offer to employees and the location where additional employee training is needed.

Employers are required to obtain an initial audiogram, usually obtained within the first 6 months of employment to be used as a baseline to which subsequent audiograms are compared. If a “standard threshold shift” defined as a change in hearing averaging 10 dB or more at the audiometric test frequencies of 2, 3, and 4 kHz occurs in either ear, the individual employee is notified and further action is required that may necessitate both modifying the hearing conservation program and notifying the appropriate authorities (eg, the employer or the appropriate government agency). In some cases, a referral to an otologist is indicated to determine the work-relatedness of the shift and to evaluate other potential medical causes.

Blunt head injury is by far the most common cause of traumatic hearing loss; motor vehicle accidents account for approximately 50% of temporal bone injuries. The cochlear injury observed following blunt head trauma closely resembles, both histologically and from an audiologic perspective, the trauma induced by high-intensity acoustic trauma. Such injury may be ameliorated if the middle ear structures absorb some of the energy and rupture.

Penetrating injuries of the temporal bone are relatively rare. Other occupational causes of ear injury include falls, explosions, burns from caustic chemicals, open flames, and welder's slag injuries.

In the conscious patient, the type of auditory injury may be suspected with a 512 Hz tuning fork. The sound will lateralize toward the damaged ear with a conductive hearing loss and away from a sensorineural one. In many cases the injury is mixed and bilateral so the tuning fork results may be ambiguous. Patients should also be checked for signs of vestibular injury (eg, nystagmus) and facial nerve trauma (eg, paralysis). Complete audiometric examinations can be performed after the patient has been stabilized.

Injuries Causing Conductive Hearing Loss

Blunt head trauma with or without temporal bone fracture may cause hemotympanum, a collection of blood in the middle ear. If this is the sole injury, hearing usually recovers over several weeks. Burns sustained when a piece of welder's slag may penetrate the eardrum. These often heal poorly, and chronic infection often results. A loud explosion with sound pressure levels exceeding 180 dB may cause rupture of the tympanic membrane. Traumatic membrane perforations usually heal spontaneously if secondary infection does not develop, although hearing loss may persist. Water precautions should be observed to avoid secondary infection.

A conductive hearing loss that persists for more than 3 months after injury is usually due to a tympanic membrane perforation or disruption of the ossicular chain. These lesions are generally suitable for surgical repair, which is detailed elsewhere in this book.

Injuries Causing Sensorineural or Mixed Hearing Loss

Trauma to the inner ear most commonly results from blunt head injury. Labyrinthine concussion frequently occurs with transient vertigo, potentially with permanent hearing loss and tinnitus. These patients may be treated with vestibular suppressants for short-term symptomatic relief of vertigo. However, vestibular rehabilitation is warranted for severe loss.

Chemicals in the workplace can be absorbed through the skin or inhaled, and secondarily reach inner ear fluids via the bloodstream. Industrial chemicals are thought to damage both the cochlea and central auditory structures via free-radical production.

According to Morata, the Human Field Studies Working Group has identified the following chemicals as having the highest priority for intervention in the workplace:

• Solvents–toluene, styrene, xylene, n-hexane, ethyl benzene, white spirits/Stoddard, carbon disulfide, fuels, and perchloroethylene.
• Asphyxiants–carbon monoxide and hydrogen cyanide.
• Metals–lead and mercury.
• Pesticides/herbicides–Paraquat and organophosphates.

Outside of the occupational setting, however, the majority of ototoxic hearing loss is secondary to medications including aminoglycosides, loop diurectics, antineoplastic agents, and salicylates.

Workplace controls must be in place to limit exposure to chemical ototoxins. High-risk workers should be identified based upon ototoxic exposure, preexisting sensory hearing loss, and compromised renal or hepatic function. Audiometric evaluation is appropriate to identify and monitor ototoxic exposure, and the addition of otoacoustic emissions, evoked auditory brainstem potentials, and behavioral audiometry have been proposed to examine central effects of industrial chemicals.

Workers taking potentially ototoxic medications are at an increased risk of hearing loss when placed in noisy environments, since the combination of some ototoxic drug treatments and noise trauma can lead to a greater degree of hearing loss than either of these would produce by itself. Conversely, patients with any type of preexisting sensorineural hearing loss, including NIHL, may be more susceptible to the ototoxic effects of medications. Aspirin, however, while known to cause reversible sensorineural hearing loss, is probably not associated with an increased likelihood of NIHL.

Medicinal ototoxins should be administered in the lowest dose compatible with therapeutic efficacy. Serum peak and trough levels should be monitored to reduce the risk of excessive dosages. The simultaneous administration of multiple ototoxic drugs (eg, furosemide and an aminoglycoside antibiotic) should also be avoided, when possible, to minimize synergistic effects.

Workers' Compensation

All states within the U.S. have workers' compensation programs to compensate the worker for injuries that arise from employment. Each state has developed its own method for handling the injured worker, and state statutes are not uniform across the United States. Prior to assessing a worker compensation case, it behooves the medical examiner to understand the appropriate statutes of the state in which the claim is being filed.

To make matters more complicated, cases that fall under the purview of the Federal government, such as civilian Federal employees under the Federal Employee Compensation Act (FECA) are handled differently than cases involving longshoreman under the Longshoreman and Harbor Workers' Compensation Act (LHWCA), despite the fact that both acts are adjudicated by the U.S. Department of Labor.

Cases involving maritime workers fall under the purview of the Jones Act, which covers workers such as merchant marines (seamen and ship crewmen), some divers, and pile drivers. Though Jones Act cases are under the auspices of the Federal Government, they are adjudicated differently than the Department of Labor cases.

Cases involving railroad workers involved in interstate commerce are handled by the Federal Employers Liability Act (FELA). Though the Jones Act and FELA are different from a practical point of view for the physician performing the evaluation, they are similar and are handled by medical examination and possible testimony in court rather than through a scheduled award (by a guideline that determines the percentage of hearing loss).

Calculation of Percentage of Hearing Loss

Several methods for calculating the percentage of hearing loss are in widespread use. The most current and frequently used method recommended by the American Academy of Otolaryngology (AAO; 1979) is as follows (Table 58–4).

Thresholds

The average hearing threshold level at 500, 1000, 2000, and 3000 Hz is calculated for each ear.

Monaural Impairment

The percentage of the impairment for each ear is calculated by taking the PTA (500–3000 Hz), subtracting 25 dB, and multiplying the result by 1.5. The maximum 100% monaural loss is reached at 92 dB (high fence). This is based on the assumption that hearing loss only becomes a handicap beyond 25 dB and that the handicap increases at a rate of 1.5% per decibel after this point.

Hearing Handicap

This is calculated by multiplying the smaller percentage (better hearing ear) by 5, adding this figure to the larger percentage (worse hearing ear), and dividing the total by 6. As unilateral deafness is only considered a mild handicap, a 5 to 1 weighting is used for the better ear.

The AAO method for calculating the percent hearing loss described above is identical to the hearing impairment guidelines developed by the American Medical Association (AMA Guidelines on Evaluation of Permanent Impairment). The AAO method is now used by the majority of states in local worker compensation programs and by the US Department of Labor (eg, FECA and LHWCA).

Some states use the 1959 American Academy of Ophthalmology and Otolaryngology rule. This method for calculating the percentage of hearing loss is similar to the AAO method, except that the 3000 Hz threshold is not included in the pure-tone average.

Generally, there is a statute of limitations that determines when an employee is eligible to apply for compensation. This statute varies from state to state and with the Federal government. In taking a history, the medical examiner should include a statement as to when the hearing loss occurred and when the employee may have realized that the hearing loss was related to noise.

Most states will apportion a pre-existing hearing loss; the U.S. Department of Labor does not deduct for pre-employment hearing loss. The U.S. Department of Labor, FECA, and LHWCA ask only if the hearing loss was precipitated, accelerated, aggravated, or proximately caused by the accepted conditions of employment.

Assessment of Impairment

The normal range of SRT is between 0 and 20 dB, with hearing losses designated according to the following measures: (1) mild (25–40 dB); (2) moderate (40–55 dB); (3) moderately severe (55–70 dB); severe (70–90 dB); (5) profound (>90 dB). Of course, the extent of the disability suffered by the patient depends on many psychological, social, and work-related factors. Disability is a relative term. The assessment of an individual's ability to do his or her job requires knowledge about the various duties performed by that individual. Some typical work-related issues for consideration include the amount of communication with coworkers and others that is required on the job, the type of communication (eg, in person or via the telephone), and the need to hear alerting signals or emergency warning alarms.

Police, firefighters, and other emergency and law enforcement personnel generally have to meet certain hearing requirements for employment. Guidelines for these occupations differ regionally; however, the guidelines for entry-level police officers and firefighters generally require a PTA at 500, 1000, 2000, and 3000 Hz of 25–30 dB. Postemployment requirements vary greatly. There is an effort in some states to quantify hearing in a noisy environment and California now conducts “hearing in noise” tests (HINT).

To meet the Social Security Administration guidelines for total disability due to hearing impairment, an individual must have either (1) an average hearing threshold of ⩾90 dB for the better-hearing ear based on both air and bone conduction at 500, 1000, and 2000 Hz, or (2) a speech discrimination score of 40% or less in the better-hearing ear. In both cases, hearing must not be restorable by hearing amplification devices.

In assessing cases of tinnitus, the otologist and audiologist may attempt to match the tinnitus with the intensity of the tinnitus in decibels and the frequency of the ringing in Hertz. Tinnitus is a solely subjective finding. Some states allow an award for tinnitus while other states do not.

The examining physician should include a statement regarding the claimant's ability to perform his usual and customary occupation.

Compensation for Occupational Hearing Loss

An example of how occupational hearing loss is compensated is provided by the statistics of the US Department of Labor (FECA). In the fiscal year 1999–2000, there were 6745 claims. The cost to the Federal government was $8,982,139 in medical costs and $30,925,247 in compensation for a total cost of $39,907,386. The average cost per claim was $5917. The general rise in costs per claim over the years reflects the rising costs of hearing aids. Many claimants are requesting newer digital hearing aids that cost $2500 or more each.

The relationship between NIHL and presbycusis remains incompletely understood. Many studies have tried to address the issue of workers exposed to hazardous noise for a long period of time and their “presumed” hearing losses based on their age (ie, presbycusis). The International Organization Standards (ISO) published a report that attempts to quantify that relationship. As with all large series, attempts to estimate hearing for individuals at certain ages are also based on determining the median or averages of large populations at a given age. There is much debate whether epidemiologic hearing loss data can be applied to individuals.

We would like to acknowledge Sumit K. Agrawal, MD, David N. Schindler, MD, Robert K. Jackler, MD, and Scott Robinson, MPH, CIH, CSP for their contribution to this chapter in the previous editions of CDT.