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NIOSH Hearing Loss Research Program Review NIOSH Publications on Noise and Hearing The National Academies - Advisors to the Nation on Science, Engineering, and Medicine

What does the HLR Program Do?

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Research Goal 1

Research Goal 2

Research Goal 3

Research Goal 4

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R&D Portfolio - Research Goal 4.1:
Determine occupational noise exposure and hearing loss through national surveillance

“There can be no effective prevention or control of disease without knowledge of when, where, and under what conditions cases occur.”[1] Efforts to prevent occupational noise-induced hearing loss have been hampered by the lack of surveillance data to systematically identify sub-populations at risk, evaluate the effectiveness of intervention strategies, and monitor progress in prevention.

There is no consensus on the number of workers who are exposed to agents that may contribute to hearing loss. Estimates of the number of noise-exposed workers in the U.S. vary from 5 million to 30 million. However, these estimates are based on data collected at least twenty years ago.[2] Prevalence estimates of noise-induced hearing loss also vary widely (from 1 million to 10 million) and are difficult to substantiate.[3],[4] The number of workers exposed to other potentially ototoxic agents, such as solvents, heavy metals, and carbon monoxide has not been estimated; nor has the number of workers who have suffered hearing loss that developed from such exposures.

Valid and reliable surveillance data are vital for charting progress in prevention and appropriately targeting intervention programs and regulations. In the 1988 “Proposed National Strategy for the Prevention of Noise-Induced Hearing Loss,” the first research recommendation cited by NIOSH was the collection of “regular and accurate statistics to assess the magnitude of the problem and to monitor the effect of various prevention/intervention efforts.”3 In 1996, NIOSH reiterated the need for surveillance data, citing the lack of a national surveillance or injury-reporting system as a hindrance to the ability to assess the impact of occupational hearing loss.[5], [6] In its 1998 revised criteria document on occupational noise exposure, NIOSH proposed several methods for evaluating the effectiveness of hearing conservation programs in populations of workers, stating that such evaluation must occur on a continual basis.[7]

To date, the HLR program has not had the resources to establish a national surveillance program for noise exposure and/or occupational hearing loss. However, we implemented several programs to collect limited data, develop surveillance methods, and establish data sources to serve as a surrogate for a national system.

From 1992-2000, and beginning again in 2002, the HLR program provided technical and financial support to the State of Michigan through the Sentinel Event Notification System for Occupational Risk (SENSOR) program for a noise-induced hearing loss surveillance and prevention system. Through this program, Michigan and the HLR program established a case ascertainment system using reports from employers, audiologists, and otolaryngologists, developed a follow-up system to characterize reported cases, and instituted referral procedures to provide assistance to companies that do not have hearing conservation programs.

Distribution of NIHL cases in Michigan

Figure 4.1. Distribution and number of noise-induced hearing loss cases in Michigan by source of reports, 1991–2000.
During 1991–2000, 17,683 noise-induced hearing loss cases were reported in Michigan. Companies with hearing conservation programs reported the largest number of workers with occupational noise-induced hearing loss. Typically, companies reported standard threshold shifts, whereas the other three sources reported fixed losses of at least 25 dB.

Figure 4.1 summarizes the recorded annual incidence of occupational hearing loss in Michigan over seventeen years. It illustrates the effect of the SENSOR surveillance program on identification of cases. Although Michigan researchers believe that there are still a substantial number of cases which are not reported, the data provide a point from which progress can be measured. Additionally, as company-reported cases are more likely to represent new instances of occupational hearing loss than those reported by health providers (non-company), there may be some evidence that the ascertainment system and subsequent follow-up and referral activities are reducing the incidence of occupational hearing loss.

Although the need for data is still great, we have not yet been able to implement surveillance in any other state. National surveillance data is the ideal. The purpose of the OSHA regulations for recording and reporting occupational illnesses and injuries was precisely that – to generate surveillance data that would allow businesses and regulators to monitor occupational disease and injury rates and intervene with necessary standards and prevention programs. Until recently, the OSHA forms for recording occupational injuries defined a recordable hearing loss as an average threshold shift of 25 dB or more at 2000, 3000, and 4000 Hertz (Hz) from baseline thresholds obtained at the start of employment within a particular company. Such a large shift in hearing was not useful in monitoring incident changes in worker hearing. Additionally, hearing losses were recorded as repetitive trauma disorders, and there was no way to sort these out from other repeated trauma injuries, such as musculo-skeletal problems. Therefore, the old OSHA recordability system had no utility as a mechanism for surveillance of occupational hearing loss.

When OSHA proposed revising its rule for recording and reporting occupational illnesses and injuries in 1996, the HLR program recognized an opportunity to help formulate a system for recording noise-induced hearing threshold shifts that would allow surveillance of occupational hearing loss. The HLR program provided OSHA with an extensive analysis of various definitions of threshold shift in order to develop a recordability standard that would be useful in identifying early occupational hearing losses without mistakenly tagging workers whose tests merely reflected test-to-test variability. The HLR program recommended defining a recordable shift as an average threshold shift of 10 dB at 2000, 3000, and 4000 Hz. Ultimately, OSHA adopted this definition for its recordability standard, adding an additional criteria that the average hearing level exceed 25 dB (to avoid recording threshold shifts that remained within the range of normal hearing).

The HLR program analysis also enabled OSHA to justify adding a separate column for recording occupational hearing loss. The analysis indicated that rates of threshold shift were high enough to warrant a dedicated column for recording. The final OSHA recordability rule became effective January 1, 2004. Initial rates of noise-induced threshold shift for calendar year 2004 should be available from the Bureau of Labor Statistics in late 2005.[8]

Data from the OSHA 300 log will be a major step towards a national surveillance program for occupational hearing loss. However, the data represent only the numerator of the risk ratio. The HLR program recognizes that large amounts of additional audiometric data are collected through OSHA-mandated hearing conservation program activities. Moreover, noise exposure measurements and sometimes even information on hearing protector use and worker training is collected through these programs. However, there is no systematic national repository for these data. If the information already being collected could be standardized and organized, it would create a valuable surveillance program for noise and hearing loss.

The data management structure requisite for such a database already exists in the HearSā f  2000™ software package developed under a CRADA between NIOSH and several corporate partners. However, before the software could be utilized as the basis for a national database, a system for “cleaning” audiometric databases had to be developed to remove invalid tests which would give spurious results. In the past, it was necessary for an audiologist to review audiometric records individually in order to flag questionable results. Based on the experience of NIOSH audiologists in reviewing several large datasets, rules for a computerized system to review audiometric databases were developed. These rules were programmed as computer filters which tagged questionable audiograms for audiologist review. Such a system reduces the amount of time required to clean the threshold data, as audiologists do not have to find the problem exams, but only review the exams identified by the computer program.

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[1] Morbidity and Mortality Weekly Report (MMWR) [1996].  Historical Perspectives Notifiable Disease Surveillance and Notifiable Disease Statistics -- United States, June 1946 and June 1996.  45(25); 530-536  June 28, 1996.

[2] Berger EH [2003]. Noise control and hearing conservation: Why do it? In Berger EH, Royster LH, Royster JD, Driscoll DP, & Layne M (eds.), The Noise Manual, Revised, Fifth Edition. American Industrial Hygiene Association, Fairfax, VA, pp 1-17.

[3] NIOSH [1988]. Proposed national Strategies for the Prevention of Leading Work-Related Diseases and Injuries , Part 2. Assoc. School of Public Health.

[4]Noise and Hearing Loss [1990]. NIH Consensus Development Conference Consensus Statement. Jan 22-24, 8(1). [note: the suggested bibliographic format for this statement requests that there be no author attribution]

[5] NIOSH [1996]. Preventing Occupational Hearing Loss: A Practical Guide. DHHS (NIOSH) Publication No. 96-110.

[6] NIOSH [1996]. National Occupational Research Agenda. DHHS (NIOSH) Publication No. 96-115.

[7] NIOSH [1998] Criteria for a Recommended Standard Occupational Noise Exposure: Revised Criteria 1998 U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health , NIOSH Publication No. 98-126.

[8]OSHA, 29 CFR Part 1904.10 [2002]. Occupational Injury and Illness Recording and Reporting Requirements. Federal Register, 67:77165-77170.


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