Prevention of Noise Induced Hearing Loss in the workplace (Occupational Hearing Loss)

An effective hearing loss prevention program (HLPP) involves a comprehensive effort consisting of the following elements: (1) performing initial and annual audits of the work environment, labor and management needs, and HLPP procedures; (2) assessment of noise exposures; (3) engineering and administrative control of noise exposures; (4) audiometric evaluation and monitoring of hearing; (5) appropriate use of personal hearing protection devices; (6) education and motivation; (7) record keeping; and (8) program evaluation for effectiveness (NIOSH, 1996). But even when a comprehensive program is in place, noise-induced hearing loss can and does occur (Ohlin, 2000). Unless best practices have been adopted, people exposed to hazardous noise are at risk of unnecessary hearing loss.

The best way to prevent noise-induced hearing loss is to eliminate the hazard.

When engineering and administrative controls have not eliminated the hazard, best practices mandate six components for hearing loss prevention. Each component is described below.

1. The noise hazard must be realistically defined. The American Academy of Audiology promotes the use of a 3-dB exchange rate (Suter, 1992) in conjunction with an 85 dBA permissible exposure limit (PEL) (NIOSH, 1998). This constitutes the best practice for defining a noise hazard. Thus, any daily noise exposure should be controlled so that an individual’s occupational exposure would be less than the combination of exposure level (L) and duration (T), as calculated by the following equation:

Equation 1

T (min) = 480/2(L-85)/3

Furthermore, when the daily exposure consists of periods of different noise

levels, the daily dose (D) should not equal or exceed 100, as calculated

according to the following equation:

Equation 2

D = [C1/T1 + C2/T2 + … + Cn/Tn]

where

Cn = total time of exposure at a specified noise level, and

Tn = exposure duration for which noise at this level becomes hazardous.

When using equation 1, above (i.e., an 85 dB PEL with a 3-dB exchange rate), and when using A-weighting with Aslow@ exponential averaging to measure continuous-type noise, the American Academy of Audiology recognizes a ceiling limit of 129 dB for 1 second. Exposure to continuous-type sounds above this limit, even for brief instances of less than 1 second are considered hazardous. For impulsive-type sounds, exposures that exceed 140 dBC, peak SPL for any duration (no matter how brief) should be considered hazardous. Impulsive-type sounds are generally considered more hazardous than continuous-type sounds. Therefore, the American Academy of Audiology concurs with the ANSI S3.44 (1996) provision that a 5-dB “penalty” may be added to time-weighted averages derived from exposures to impulsive sounds.

2. Annual monitoring air conduction audiometry must be performed with methodology appropriate to the goal of accurately measuring hearing threshold levels. Best practice dictates that anyone exposed to hazardous noise should have a baseline as well as annual monitoring hearing tests.

A. Audiometric tests should be performed by an audiologist, physician, or technician with appropriate credentials. If audiometric tests are performed by a technician, all tests must be conducted under the supervision of an audiologist or physician.

B. All audiometry is to be conducted with audiometers that meet the specifications of and are maintained and used in accordance with the American National Standard Specifications for Audiometers (ANSI S3.6-1996, 1996b).

C. Audiometric tests must be conducted in a test space where background noise levels do not interfere with valid measures of hearing thresholds. Ambient noise levels should conform to all requirements of the American National Standard Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms ( ANSI S3.1- 1999).

D. At a minimum, audiometry should consist of pure-tone air-conduction threshold testing of each ear at 500, 1000, 2000, 3000, 4000, and 6000 Hz. To enhance the decision about probable etiology, testing at 8000 Hz is strongly recommended.

E. (1) Baseline air conduction audiogram should be obtained within 6 months of employment unless exposures are expected to periodically equal or exceed a time-weighted average (TWA) of 100 dBA, in which case a baseline should be obtained within 30 days. All baseline tests must be preceded by 12 hours of effective quiet. Hearing protectors should not be used as a substitute for quiet.

(2) Subsequent hearing thresholds measured during annual monitoring may show improvement or decrements in hearing. When and how such changes warrant revising the baseline audiogram is subject to many considerations. The National Hearing Conservation Association has developed a Professional Guide for Audiometric Baseline Revision (NHCA, 2001). The American Academy of Audiology endorses this guide for use within the context of administering a hearing conservation program that is compliant with the OSHA Hearing Conservation Amendment (CFR 1910.95).

F. A monitoring air conduction audiogram should be obtained annually. If feasible, these should be scheduled well into a work shift so that temporary changes in hearing due to insufficient noise controls or inadequate use of hearing protection can be observed. The results should be compared immediately with baseline hearing levels. The availability of audiometric database management systems makes such comparisons feasible, and also makes it possible to provide patients with timely feedback regarding the presence or absence of hearing changes. This is significant because timely feedback is an important factor in promoting increased use of hearing protectors (Zohar, Cohen, and Azar, 1980).

G. A confirmation hearing test to determine the presence/absence of a significant change in hearing threshold should be obtained within 30 days of a monitoring audiogram that detects a significant change.

3. Protocols capable of identifying meaningful changes in hearing should be employed. The purpose for monitoring audiometry is to provide timely detection of significant changes from baseline hearing threshold levels. OSHA (1983) uses the term Standard Threshold Shift (STS) to describe significant changes from baseline hearing levels. The OSHA Standard (paragraph (g) (10)) defines STS as, “a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear” (OSHA,1983,). OSHA also permits the use of age corrections when computing threshold changes. Age corrections may be both suitable and useful for risk analyses of group data. However, age correction of individual audiograms before checking for threshold shifts is counterproductive to detecting temporary changes in hearing before they become permanent (Merry and Franks, 1995). “Many professionals feel that if intervention for threshold shifts is delayed until after age-corrected STS has occurred, then significant hearing changes will not receive needed follow-up attention” (NHCA, 2001).

Royster (1992, 1996) studied 8 criteria for detecting significant threshold shifts and applied each criterion to 15 different industrial hearing conservation databases. Royster demonstrated that the OSHA STS criterion identified true positives only 57% of the time. By comparison, the 15-dB TWICE method (a 15 dB shift at any test frequency affirmed by an immediate retest) identified true positives 71% of the time. When a 15-dB TWICE method has identified a suspected STS, a confirmation hearing test should be performed within 30 days. to determine whether the STS was a temporary or permanent threshold shift. This test should be performed when the patient has been in a quiet environment for at least 12 hours immediately prior to the test. If the confirmation test demonstrates a persistent STS, the change likely represents a permanent threshold shift. When an STS is confirmed, the audiologist should follow appropriate guidelines (e.g., 29 CFR 1910.95) for disposition of persons identified as having an STS, including (1) counseling the employee and notifying his/her employer, (2) retraining the worker to ensure he/she can properly use personal hearing protection, and (3) referring the worker for follow-up clinical audiological evaluation or otological examination, as appropriate. Additionally, the audiologist should determine if the hearing change must be recorded on an OSHA Log 300 per 29 CFR 1904.10.

The STS should function as a sentinel for identifying significant changes in hearing. Therefore, the American Academy of Audiology recognizes the 15-dB TWICE method, followed within 30 days by a confirmation audiogram as the best practice for identifying significant noise-induced threshold shifts.

4. Educational methods and materials should be tailored to the specific audience. The goal of education and training is not just to inform, but also to motivate. The success or failure of a hearing loss prevention program, including employee buy-in, depends upon effective education and training (Berger, 2001). Education and training must be relevant to a person’s specific needs if hearing health behaviors are to be influenced positively (Stephenson, 1996). For example, individual feedback can be given either to encourage workers to adopt better hearing loss prevention behaviors or to affirm existing behaviors, depending on the presence or absence of an STS. Workers may know loud noise can damage hearing, but they may be ill-informed about the hearing hazard inherent to the specific tools they use or the environment in which they must work. The hearing loss prevention program audit can provide opportunities to find out what particular hearing hazards are present and what resources labor and management are willing to bring to bear to address these hazards. Education and training can be tailored to address specific attitudes, beliefs, and behavioral intentions labor and management have about hearing loss prevention. In other words, education and training must consist of more than showing a film and passing out a pamphlet or it will be ineffective. Through effective training, individuals can become motivated to adopt hearing loss prevention behaviors (Berger, 2000). This means education and training content must be framed within the context of the needs of each audience. Dynamic, relevant training will imbue workers with a sense of personal control over their hearing health, lead to the development of intrinsic motivation to adopt positive hearing health behaviors, and diminish reliance on ineffective systems based on external rewards and punishment (Merry and Franks, 1995).

5. The attenuation ratings for hearing protectors must be based on methods that yield realistic estimates of the amount of protection provided as a device would be worn. The American Academy of Audiology endorses the use of the subject fit procedure, Method B, of ANSI S12.6-1997 to describe the amount of attenuation a personal hearing protection device (HPD) can be expected to provide as it would actually be worn.

Research has demonstrated that the amount of noise reduction provided by an HPD as it is actually worn bears little relationship to the noise reduction rating (NRR) shown on the HPD=s label (Berger, Franks, and Lindgren, 1996). Additionally, the NRR was intended to be used with C-weighted sound measures. Royster (1995) described how the subject fit method can be used to derive a noise reduction rating (NRRSF) that addresses both of these issues. The NRRSF provides both a simple, realistic estimate of the protection a user can expect to receive, as well as a measure designed to be used with A-weighted sound levels (Berger, 2000).

6. Hearing protector devices (HPDs) should be individually fit, or, at a minimum, fit in small groups. Failure to fit hearing protectors properly and to wear them consistently is probably the leading cause of occupational noise-induced hearing loss (Sweeney, et al., 2000). Studies show that hearing protectors use/non-use is determined by removing barriers to their use and by imparting users with skills needed to select and wear the right hearing protector for his/her needs (Lusk, et al., 1994; Lusk, et al., 1995). Audiologists can ensure that hearing protectors they recommend address barriers to their use by taking care of the 4-C’s: comfort, convenience, cost, and communication. There are hundreds of hearing protectors available (NIOSH, 1994). Without proper instruction in how to fit and use hearing protectors, people will get only a fraction of the available hearing protection (Berger, 2000). Each person who must be exposed to hazardous noise should receive individual or small group instruction on how to fit and use personal hearing protector devices** (Adapted from the position statement on Occupational Hearing Loss by the American Academy of Audiology).

AAA position statement on Preventing Noise-Induced hearing loss, 2003-2004, http://www.oem.msu.edu/userfiles/file/News/Hv6n4.pdf