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  Table of Contents    
Year : 2011  |  Volume : 13  |  Issue : 51  |  Page : 195-199
Role of continuous monitoring in a hearing conservation program

1 Michael and Associates Inc., State College, PA, USA
2 Dixie Industries, Chattanooga, TN, USA
3 Alcoa Intalco Works, Ferndale, WA, USA

Click here for correspondence address and email
Date of Web Publication1-Mar-2011
How to cite this article:
Michael K, Tougaw E, Wilkinson R. Role of continuous monitoring in a hearing conservation program. Noise Health 2011;13:195-9

How to cite this URL:
Michael K, Tougaw E, Wilkinson R. Role of continuous monitoring in a hearing conservation program. Noise Health [serial online] 2011 [cited 2023 Mar 22];13:195-9. Available from: https://www.noiseandhealth.org/text.asp?2011/13/51/195/77204

  Introduction Top

Continuous monitoring of personal noise exposure is a new approach to industrial hearing conservation. It is straightforward and simple solution to an often vague and difficult problem, based on a classic tenet of industrial hygiene, "if you prevent the exposure, you prevent the disease."

  Why are Workers Still Losing their Hearing? Top

Despite the best efforts of industrial hearing conservationists, occupational hearing loss remains prevalent worldwide. In most industries, noise control efforts reduce the overall levels, but personal hearing protection is still required to lower the exposure to a safe level. While hearing protective devices (HPDs) can be effective in reducing exposure by 30 dB or more, the protection afforded to an individual is highly variable depending on many factors. [1] These factors include the size and shape of the ear canal, the dexterity of the HPD wearer, and the motivation to fit and wear the HPD properly for the entire work shift. Recently, HPD fit-testing has been gaining more acceptance, but this only guarantees that the worker is capable of effectively wearing the device, not that the HPD is actually worn properly and for the entire duration of the noise exposure. Simply put, workers are still losing their hearing due to the fact that they are not wearing HPDs effectively during for the entire duration of the noise exposure. [2]

  What is Continuous Monitoring? Top

Continuous monitoring involves using a 2-channel noise dosimeter that samples the noise impinging on each ear. The dosimeter measures the noise level interior to the HPD and, therefore, the actual attenuation provided by the HPD is accounted for in the dosimeter measurement. In addition, during periods when the HPDs are removed from the ear, the hearing protectors (with embedded microphones) are to be worn in pre-defined secondary positions. The secondary position for earplug type devices is to have the plugs dangle on the chest and for earmuff type devices, the secondary position is to have the muffs rotate rearwards on the safety cap. When the HPDs are removed from the ear, the unprotected ear is exposed to the full ambient noise level, which is equivalent to the level that the secondary position microphones are sampling. Therefore, the full-shift cumulative noise exposure dose is comprised of periods when the HPDs are worn and of periods when the HPDs are removed from the ear. Byrne and Reeves [3] of the National Institute for Occupational Safety and Health (NIOSH) validated this approach as they found that the secondary microphone measurement positions provided a slightly conservative estimate of the level of noise impinging on the ear. For the insert-type and muff-type HPDs, the secondary position measurements were about 1 dBA high and 3 dBA high, respectively.

The protected noise dose is a new quantity, previously not available to the hearing conservationist. Both the quality of the HPD fitting and the duration of the wearing time are accounted for in the dose measurement. Both these characteristics must be considered to accurately quantify exposure over a work shift.

  How Does Continuous Monitoring Prevent Noise-induced Hearing Loss? Top

The elegance of continuous monitoring lies in the simplicity of the methodology. First, we must realize that typical occupational noise-induced hearing loss (NIHL) does not occur after a single overexposure. Instead, occupational NIHL occurs after months and years of daily or frequent overexposures.

Continuous monitoring provides safety personnel with full-shift employee exposure data on a daily basis. If an overexposure occurs, the safety representative must intervene appropriately. Intervention can take the form of a warning, such as "wear the HPD more effectively tomorrow…," or it can involve retraining or refitting with a different and more effective HPD. Then, the following daily exposures must be monitored to ensure that the intervention has been successful and that the noise dose is within acceptable limits. Timely intervention and supervisory monitoring is critical; the success of the program will be compromised if it is reliant on self-correction by the employee.

Experience in high-noise environments with this technology has indicated that virtually all US industrial noise exposures can be reduced to a safe level interior to the HPD using only single hearing protection, if it is fitted properly. While double-protection is an option with continuous monitoring, it is not required in a great majority of environments. The redundancy of double protection is not required as quantitative data is available that validates that the single HPD is sufficient.

If the worker has only an occasional overexposure, NIHL is highly unlikely to occur on the job. This has been demonstrated in real-life application of the technology. For the first time, hearing conservationists can positively prevent the most common industrial disease, NIHL.

The hardware/software

Continuous monitoring is performed using a specially designed 2-channel noise dosimeter [Figure 1]. The proprietary hardware is marketed under the name QuietDose, and is manufactured by the Howard Leight Company (Howard Leight Industries, San Diego, CA, USA) a division of Sperian Protection. A dual microphone wiring harness is connected to the dosimeter, with one microphone earpiece going to each ear. The microphone earpiece connects to the eartips for the insert-type embodiment. The microphone earpiece is mounted inside the earcup for the cap-mounted muff-type embodiment of the system. The eartips/earmuffs are worn as any other HPD would be worn during the course of the work day. The microphones sample during the entire work shift, and the workers wear the device every day.
Figure 1: The Howard Leight QuietDose noise dosimeter

Click here to view

There are two warning light-emitting diodes (LEDs) on the dosimeter. A red LED flashes when the instantaneous level exceeds 85 dBA. This is an indication to the user that the level reaching the ear is potentially hazardous, and the HPDs should be worn more effectively. A yellow LED flashes when the cumulative dose for the shift has exceeded 45%. A practical goal for the workers is to limit the daily exposure to 50% of the U.S. Occupational Safety and Health Administration (OSHA) Permissible Exposure Level (PEL), or to 85 dBA time-weighted average (TWA). If the worker sees the yellow LED flash, he/she should limit the exposure as much as possible for the remainder of the work shift.

At the end of the work shift, the user places the dosimeter in an infrared reader download station and presses the DATA button for about 2 s. The data is read by the PC-based program, displayed to the wearer and stored in a database. If the daily exposure is higher than a programmed threshold level, e-mail alerts can be sent to appropriate safety personnel. Proper intervention is then required for the program to be successful.

With appropriate intervention when necessary, continuous monitoring virtually ensures that hearing loss does not occur at work; if hearing loss progresses for these individuals in a continuous monitoring program, it is likely that it was non-occupational in origin.

  Real-life experience - Dixie Industries, Chattanooga, TN Top

Dixie industries: The problem

Dixie Industries is a forging operation in Chattanooga, TN, USA. It has been operational for almost 60 years. The forging operation generates high-noise levels as large hammers generate over 50,000 tons of force in each of up to 80 strokes/min [Figure 2]. Noise levels are typically in the 110-112 dBA TWA range for the hammer operators.
Figure 2: Forging at Dixie industries

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Noise control efforts at Dixie

Obviously, the best way to avoid NIHL is to engineer-out the noise. Over the last 11 years, Dixie has invested over $1.8M across over 30 projects with the goal of lowering the noise levels in the forge shop. These efforts have included insulating the walls and installing noise-absorbing materials to reduce reflections within the shop. Also, a "buy quiet" initiative was enacted and several families of tools were replaced with less-noisy alternatives. Auxiliary equipment was also targeted for reducing noise levels. The air-control valves on the hammers were redesigned with the goal of minimizing noise.

Unfortunately, the results of these noise-control efforts were all too similar to the results found across many industries. This investment resulted in a lowering of the exposure levels by about 3 dB. While 3 dB is significant, unfortunately, there was still a personal exposure problem. The bottom line is that massive hammers are fundamental to the forging operation, and it is impossible to run these hammers quietly.

Eleven years of noise control engineering resulted in a decrease in the level of 3 dB. How can we reduce the personal exposure by another 30 dB to reach safe levels? The answer is simple - well-fitted personal hearing protection. However, it is well-known that the attenuation afforded to the individual wearer is highly variable and no laboratory measurement can provide an accurate estimation of the level of protection provided to a single worker. The complete answer is proper application of personal hearing protection combined with continuous monitoring, ensuring that the individual is protected to a safe level every day. This approach ensures that (1) effective hearing protection was selected, (2) the quality of the fit of the HPD was sufficient throughout the workday to reduce the exposure to a safe level and (3) the exposures during periods when the HPD was not worn were not enough to raise the cumulative noise dose above the "safe" threshold. For the first time, the effects of work activities and real-world wearing habits are quantified and included in personal noise exposure measurements.

TOSHA and Dixie industries

The Tennessee office of the Occupational Safety and Health Administration (TOSHA) visited the Dixie facility to assess the adequacy of hearing protection in this noisy environment. Compounding the situation was a recorded Standard Threshold Shift (STS) at Dixie on a hammer operator. TOSHA proceeded according to the regulations and applied the potentially inaccurate "subtract seven and divide by two" derating scheme to the Noise Reduction Rating (NRR) of the HPDs used by Dixie. Dixie mandates double protection in their hammer areas, using high NRR (33 dB) earplugs in combination with high NRR (29 dB) earmuffs. The derating scheme therefore calculated attenuation as ((NRR = 33) minus 7) divided by 2 equals 13 dB plus 5 dB for double protection, for a total attenuation of 18 dB. With an exposure level of 110 dB, the result was a calculated personal exposure of 92 dBA, which is in excess of the OSHA PEL of 90 dBA. Dixie was cited for noise violations in the September of 2005.

TOSHA was cooperative, forward-thinking and open to new technical developments in hearing conservation; however, they were adamant that Dixie had to prove that they were protecting their worst-case workers to an exposure level below the OSHA action level (85 dBA TWA). In May 2006, Dixie met with TOSHA to discuss the addition of new administrative controls. the In January of 2007, Dixie summarized their history of noise control efforts that had resulted in expenses of almost $2M. Finally, TOSHA agreed to review the continuous monitoring data of Dixie hammer operators in the April of 2007.

Sampling results

Dixie performed an initial sampling of employees working in the hammer area that involved six employees across 65 downloads and 600 h of noise exposure data. Concurrent to these measurements, ambient noise monitoring was performed so that the effectiveness of the HPD could be readily evaluated. [Figure 3] displays the levels under the HPD (labeled as ESP data), the on-the-shoulder ambient noise levels (labeled as CEL 460 dosimeter) and the calculated attenuation provided by the HPD.
Figure 3: Box and whisker display of initial measurements at Dixie industries, including median, mean (red-dashed line), upper and lower quartiles and extreme values

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During the initial measurement period, the following data were obtained:

Direct comparison data

  • Average ambient noise level = 109.4 dBA
  • Average protected noise level = 77.4 dBA
  • Calculated level of attenuation= 32.0 dBA

Data collection was then continued over a period of several months. The consistency of data is evident from the following data:

Extended sample data

  • Average ambient noise level = 109.0 dBA
  • Average protected noise level = 79.6 dBA
  • Calculated level of attenuation = 29.4 dBA

Overall, [Figure 4] displays the distribution of data measurements, with the majority of the data being in the 76-82 dBA range. Only five of the 65 measurements were above 83 dBA, and 100% of the measurements were below 86 dBA.
Figure 4: Distribution of measurements interior to hearing protection, Dixie industries

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TOSHA recognized the value of the continuous monitoring measurements and abated the noise citation in June 2007.

  Real-life Experience: Intalco, Ferndale, WA Top

The most extensive and ongoing continuous monitoring program is currently in place at the Alcoa Intalco Works, in Ferndale, WA, USA. [4],[5],[6] Intalco is an aluminum smelter with moderate noise exposure levels at approximately 95 dBA TWA. Intalco has a history of implementing an aggressive hearing conservation program. For over 10 years, the safety department has been fit-testing all employees that are using insert-type HPDs. In fact, the program goes beyond fit-testing. Every noise-exposed employee must achieve a fit-test Personal Attenuation Rating (PAR) of >25 dB during the annual fit-testing session and a photograph of the plug that was used during the fit-test is affixed to the workers safety cap. Therefore, the compliance officer can (1) ensure that the employee is properly wearing an earplug and (2) ensure that the plug had provided >25 dB of attenuation during the fit-testing session. Despite the progressive efforts of this hearing conservation program, there were an unacceptably high number of standard threshold shifts at the plant.

In the March of 2005, Intalco took another progressive step to prevent NIHL at the smelter. In an effort to prevent additional hearing loss injuries, the management decided to begin continuous monitoring on all employees that had progressed 5 dB or more toward an OSHA recordable STS. Participation in the program was not voluntary; it was a condition of employment at the smelter. This was a worst-case group of workers in terms of hearing loss prevention. Because they had successfully been fit-tested, yet were still progressing toward a recordable STS, obviously they were experiencing on-going non-occupational hearing loss, or they were poor-hearing protector wearers while on the job, or possibly both.

With the continuous monitoring system in place, both the employees and the management were aware of the personal protected exposure levels on a daily basis. To maintain exposures below a TWA of 85 dBA (50% of the OSHA PEL), these workers were required to select HPDs that fit effectively, and they had to wear them consistently during all periods of noise exposure. The continuous monitoring system software notifies designated safety officers following every full-shift measurement of >50% dose. At Intalco, these safety officers have the responsibility of documenting the intervention after every overexposure. This policy ensures that the frequency of the overexposures is minimized.

Since the March of 2005, approximately 125 employees have been monitored on a daily basis. To date, approximately 100,000 full-shift measurements have been recorded. In summary, 95% of these measurements are <85 dBA TWA (50% of the OSHA PEL) and 99.5% of the measurements are <90 dBA TWA (100% of the OSHA PEL).

The overall average 8-h TWA as measured interior to the hearing protector across this population is 81.0 dBA. The predicted median (Q = 0.5) hearing loss for this population from program inception through 7/09 (4 years), according to ANSI S3.44-1996(R2006) [7] at 500, 1000, 2000, 3000, 4000 and 6000 Hz, is 0, 0, 0, 0.5, 1.4 and 0.5 dB, respectively.

Extrapolating these data to longer periods of time, up to a 40-year career with this level of average exposure, yields the following predicted NIHL [Table 1].
Table 1: Predicted median hearing loss levels at extended durations of exposure interior to the HPD. Predicted Hearing Loss, ANSI S3.44-1996 (2006),[7] in dB, Exposure = 81.0 dBA, interior to HPD, Q = 0.5

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In summary, ANSI S3.44-1996(R2006) [7] would predict that, across this group of smelter workers with an average protected noise exposure of 81 dBA, the median (Q = 0.5) NIHL would be about 2 dB at 4000 Hz over a 40-year career. The predicted loss at 3000 and 6000 Hz would be less than about 1.0 dB, and up to and including 2000 Hz, there would no predicted loss.

In the 4 years prior to continuous monitoring, the average audiometric thresholds at 3, 4 and 6 kHz across these workers were increasing by an average of 2 dB per year. Since the continuous monitoring program was initiated, protected exposure at work has been limited to 81 dBA. At this level, the predicted hearing loss accumulated across an entire career is minimal.

  Yale University School of Medicine Analysis Top

The Yale Occupational and Environmental Medicine Program has partnered with Alcoa/Intalco in the analysis of industrial hygiene data collection across many types of occupational hazards. Recent analysis by Yale of data from Intalco indicates that use of the QuietDose units, combined with proper intervention when overexposures occur, significantly reduces the risk of occupational hearing loss. [8]

  Summary: Continuous Monitoring - A Preventative Technology that is here Today Top

For the first time, the occupational hearing conservationist is provided with a quantitative assessment of personal exposure that accounts for the effectiveness of hearing protection across the entire work shift. Besides empowering the employee to better protect himself/herself, these data allow the safety officers to monitor noise exposures on a daily basis and intervene after occasional overexposures. With proper intervention and oversight of the program, a continuous monitoring program can absolutely prevent occupational hearing loss.

  References Top

1.Michael K. Measurement of insert-type hearing protector attenuation on the end user: A practical alternative to relying on the NRR. Spectrum 1998;15:24.  Back to cited text no. 1
2.Burks J, Michael K. A new best practice for hearing conservation: The exposure smart protector (ESP). Noise-Con, 2003, Cleveland, OH (2003).  Back to cited text no. 2
3.Byrne DC, Reeves ER. Analysis of non-standard dosimeter measurement locations. J Occup Environ Hyg 2008;5:197-209.  Back to cited text no. 3
4.Michael K. American Industrial Hygiene Association (AIHA). Continuous monitoring of protected noise exposure: A practical alternative. Platform presentation, Philadelphia, PA. 2007.  Back to cited text no. 4
5.Michael K, Wilkinson R, O'Sullivan M. American Industrial Hygiene Association (AIHA), An Aluminum Smelter's Aggressive Hearing Conservation Program: Daily, Continuous Personal Noise Monitoring. Platform presentation, Chicago, IL. 2006.  Back to cited text no. 5
6.Michael K, Wilkinson R. National Hearing Conservation Association (NHCA), Hearing Conservation without Assumptions: Using Daily MIRE (microphone-in-real-ear) Techniques to Verify Safe Exposures for Pot-line Workers in an Aluminum Smelter. Poster presented at Annual Conference, Seattle, WA. 2004.  Back to cited text no. 6
7.American National Standards Association (ANSI), ANSI S3.44-1996(R2006), Determination of Occupational Noise Exposure and Estimation of Noise-induced Hearing Impairment. Accredited Standards Committee, S3, Bioacoustics.  Back to cited text no. 7
8.Rabinowitz PM, Galusha D, Kirsche SR, Cullen MR, Slade MD, Dixon-Ernst C. Effect of daily noise exposure monitoring on annual rates of hearing loss in industrial workers. 0.1136/oem.2010.055905, Occup Environ Med,12/30/10.  Back to cited text no. 8

Correspondence Address:
Kevin Michael
Michael & Associates, Inc, 400 Long Lane, PA Furnace, PA 16865
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1463-1741.77204

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