Chapter 7. Research Needs

Considerable progress has been made in our understanding of occupational hearing loss prevention. However, additional research is needed to clarify the risks associated with various noise and ototoxic exposures and to reduce the incidence of hearing loss among workers. Furthermore, investigations of possible biological indicators of susceptibility to NIHL would be welcome. For example, although tinnitus is a frequent complaint of the noise-exposed worker, its relationship to permanent hearing loss is not well understood. The additional topics listed in the sections below do not include all areas that would benefit from further investigations, but they represent persistent problems or emerging trends.

7.1 Noise Control

Research is needed to reduce noise exposures through engineering controls in workplaces where the noise exposures are still being controlled primarily by hearing protectors. An HLPP is complex and difficult to manage effectively, and the need for one can be obviated by noise control procedures that reduce noise levels to less than 85 dBA. As important as such noise reduction technologies are, it is equally important to apply traditional noise control engineering concepts to the building of new facilities and equipment. Research also is needed to improve the retrofitting of noise controls to existing operations. A database of effective solutions (best practices) should be created and made accessible to the public.

7.2 Impulsive Noise

Research is needed to define the hazardous parameters of impulsive noise and their interrelationships. These parameters should include amplitude, duration, rise time, number of impulses, repetition rate, and crest factor. In the absence of any other option, impulsive noise is integrated with continuous noise to determine the hazard. Laboratory research with animals and retrospective studies of workers indicate that impulsive noise is more hazardous to hearing than continuous noise of the same spectrum and intensity. However, sufficient data are not available to support the development of damage risk criteria for impulsive noises.

7.3 Nonauditory Effects

Research is needed to define dose-response relationships between noise and nonauditory effects such as hypertension and psychological stress. Studies of hypertension conducted on noise-exposed workers have established a relationship between hypertension and NIHL but have not established a relationship between noise exposure and hypertension. Workplace accidents need to be analyzed to determine whether noise interference with oral communication or audio alarms has been a contributing factor. Technologies must be developed to allow easy identification of warning signals and efficient communication in noisy environments while providing effective hearing protection.

7.4 Auditory Effects of Ototoxic Chemical Exposures

The ototoxic properties of industrial chemicals and their interaction with noise have been investigated for only a few substances. Research in animals is needed to investigate the range of chemicals known to be ototoxic or neurotoxic and to appraise the risk of hearing loss from exposures to these chemicals alone or in combination with noise. Research is needed to support damage risk criteria for combined exposure.

7.5 Exposure Monitoring

NIOSH has been a pioneer in developing an exposure monitoring strategy for air contaminants based on the application of statistical methods [NIOSH 1977]. However, the appropriateness of the strategy for occupational noise exposure has not been determined, and not much research has been conducted in this area since 1977. Limited studies have indicated that a different strategy for monitoring occupational noise exposure may be required [Behar and Plenar 1984; Henry 1992]. Worker exposures to noise must be accurately monitored and appropriate control measures must be implemented when necessary. Several individuals and organizations have proposed different approaches to monitoring noise exposures [Behar and Plenar 1984; CSA 1986; Royster et al. 1986; Hawkins et al. 1991; Henry 1992; Simpson and Berninger 1992; Stephenson 1995]. NIOSH acknowledges the contributions of these individuals and organizations to this important subject and encourages continued effort in the development of exposure monitoring strategies applicable to occupational noise exposure. An important component of HearSaf 2000 is being codeveloped by NIOSH, the United Auto Workers-Ford National Joint Committee on Health and Safety, Hawkwa Group, and James, Anderson and Associates: noise monitoring with emphasis on noise exposure characterizations based on the principles of a task-based exposure assessment model (T-BEAM). The T-BEAM approach stresses the identification of all hazards (including noise) that may be associated with a particular work task. This approach may be especially suitable for mobile or itinerant workers. Additional research is needed to compare these monitoring approaches (including T-BEAM) to determine the best technique for a particular type of worker or work environment.

7.6 Hearing Protectors

The noise attenuation of hearing protectors as they are worn in the occupational environment is usually quite different from that realized in the laboratory. The manufacturer's labeled NRRs (which are currently used by OSHA in determining compliance with the PEL when engineering controls are being implemented or are not feasible) usually do not reflect actual experiences. Thus a pressing need exists for a laboratory method to estimate the noise attenuation obtained with hearing protectors worn in the field. Field research is now needed to validate the new laboratory subject-fit method with onsite fit-testing methods. Research should also lead to the development of hearing protectors that eliminate troublesome barriers by providing increased comfort to wearers as well as improved speech intelligibility and audibility of warning signals. In addition, as new technologies such as active-level dependency and active noise reduction are introduced into personal hearing protection, methods must be developed to describe the effectiveness of these methods alone and when built into passive hearing protectors.

7.7 Training and Motivation

Research is needed in using behavioral survey tools as resources for developing training and education programs that address workers' beliefs, attitudes, and intentions about hearing loss prevention. To date, research in training and motivation has focused on materials and their delivery, with the worker considered the passive receptacle. Research is needed to develop materials and programs that more fully involve the worker in the process and give the worker ownership in the HLPP. Additional methods are also needed to improve the training and motivation of workers who must depend on hearing protection.

7.8 Program Evaluation

Several methods for evaluating the effectiveness of an HLPP are discussed in Chapter 5. No single method is generally accepted as being superior to the rest. Further research and development of methods for evaluating the effectiveness of HLPPs are needed, and the method deemed to have the best balance between accuracy and ease of use should be adopted. All existing methods rely on the results of audiometric testing for evaluating effectiveness of the HLPP. Although audiometric data are crucial for managing an HLPP and evaluating the status of each worker, too much time must pass to build a database of audiograms that can support queries about overall program effectiveness. Methods that do not rely on serial audiograms need to be considered for immediate assessment of program effectiveness. Examples of such methods are observed behaviors that predict the success of a program or questionnaire-type surveys that evaluate workers' beliefs and intents (and correlate with actual behaviors).

7.9 Rehabilitation

Noise and hearing conservation regulations fail to deal with the worker who has developed NIHL. This failure affects policies regarding hearing protector use when speech communication is necessary, the use of hearing aids by hearing-impaired workers in noisy areas, and the use of hearing aids with hearing protectors such as earmuffs. Thus the worker with acquired NIHL is often managed as a casualty who is no longer in the HLPP management system.

Management procedures for workers identified with substantial hearing impairment need to be studied. They would include training in listening strategies, speech reading, and optimal utilization of hearing aids. Research also needs to be directed at developing hearing instruments designed to help workers continue to function in noise while protecting hearing and enhancing communication.

Rehabilitation communication strategies need to be studied. Currently, if hearing-loss-prevention service providers were to suggest that noise-exposed workers with NIHL could benefit from amplification, they would be fired. In such a hostile environment, it is very difficult to define, develop, deliver, and evaluate a rehabilitation program.