Noise-induced hearing loss (NIHL) is one of the most common occupational conditions (Stanbury et al., 2008; Tak et al., 2008) and globally accounts for a significant fraction of acquired hearing loss in adults (Nelson et al., 2005). OSHA has mandated hearing conservation programs for workers exposed at noise levels of 85dBA or greater (OSHA, 1983), but NIHL continues to occur in noise-exposed workforces (Verbeek et al., 2009). These noise-exposed workers are required to enroll in hearing conservation programs (OSHA, 1983), which includes the use of hearing protection devices (HPDs). Current conventional methods of assessing occupational noise exposures involve placing a personal dosimeter on a worker's shoulder and measuring the time-weighted ambient noise levels outside of the hearing protection (ANSI, 2007).

To predict the exposure of an individual worker wearing hearing protection, a de-rating factor is sometimes applied to the noise reduction rating (NRR) allowing for an estimate of the attenuation provided by their HPD (Neitzel et al., 2006). At the same time, studies have found that the field effectiveness of hearing protection is highly variable, and that laboratory measurements of “noise reduction rating” (NRR) may not correlate well with measures of attenuation in the field (Berger et al., 1998). Studies have also shown that HPDs are underused (Tak et al., 2009), that the effectiveness may vary widely between individuals, and that use may vary with ambient noise exposure levels, as suggested by the finding of higher hearing loss rates among workers with moderate ambient noise exposures compared to those with higher estimated ambient noise exposures (Rabinowitz et al., 2007). Because of these critical barriers to understanding true noise exposures, a principal way that worker overexposure to noise is detected is when periodic audiometric testing finds that the worker has had a substantial shift in their hearing threshold levels, by which time a significant amount of hearing has been irretrievably lost.

There have been a number of published intervention studies to promote use of hearing protective devices among noise exposed workers. Some of these, including multicomponent, longitudinal educational efforts, have shown success at increasing the rate of self-reported use of HPDs (El Dib et al., 2011; Seixas et al., 2011). To date, however, it has not been possible to assess the impact of such interventions on actual noise exposures experienced by the participating workers.

New advances in hearing protector technology include a device that allows for the collection of occupational noise exposure data under the hearing protector on a continuous basis (Michael et al., 2011). A study of noise-exposed workers using such devices on a mandatory basis reported that such use may reduce their risk of occupational hearing loss compared to matched controls (Rabinowitz et al., 2011). In that study it was noted that some of the beneficial effect could have resulted from the ongoing involvement of company personnel in overseeing the noise exposures of the individuals using the monitoring devices, or other contemporaneous changes. It was also pointed out that further studies were needed to determine whether the monitoring devices could have a beneficial impact on noise exposure and hearing loss risk when used in a voluntary program, as such trials might be necessary to validate the efficacy of these devices.

We report on the initial experience of a cohort of industrial workers enrolled in a voluntary study of the use of a device allowing them to perform daily monitoring of occupational noise exposure under their hearing protective devices.

The study took place at three company locations of Alcoa Inc., a worldwide producer of aluminum and other industrial products. Ongoing research collaboration agreements between Alcoa, the Yale University School of Medicine, and Stanford University School of Medicine have resulted in the analysis of audiometric data collected for hearing conservation purposes.⁵ As part of this research collaboration, an intervention study of voluntary use of daily noise exposure monitoring was initiated in 2007 at two aluminum smelters with funding from the National Institute for Occupational Safety and Health. The two smelters are located in geographical proximity to each other, but differ somewhat in production process, with one employing a prebake process and the other a Soderbergh smelting process. In 2010, additional volunteers were recruited at a smaller company location with a different production process (a turbine component factory).

The results of this study to date indicate that workers will voluntarily monitor their noise exposures on a daily basis, and that the impact of such daily exposure monitoring can lead to a reduction in personal noise exposures as well as a group decrease in the percentage of daily noise exposures in excess of 85dBA (8 hour time weighted average).

There are a number of possible explanations for the observed decrease in excessive noise levels. By receiving daily feedback about their daily noise “dose”, workers can respond by taking steps to use their hearing protection more effectively. This may involve wearing the hearing protective device more consistently (for example, not removing the device to communicate in noise), and also ensuring that the hearing protector is correctly seated in the ear canal. Workers can also reduce noise exposure through avoidance of noise sources or reducing the amount of time they spend doing noisy job tasks. Another way they might reduce exposures is to bring an excessive noise source to the attention of supervisors or other responsible plant personnel so that noise-engineering controls can be implemented and the noise source controlled.

While we found that the number of noise readings in excess of occupational exposure levels decreased over the first six months of use, and average noise exposure levels also decreased, we were unable to identify individual level factors associated with the change in noise exposure over time. The decrease in noise exposure was not monotonic- and the group of workers downloading over six months saw a collective spike in exposure levels during the fourth month of the intervention followed by a subsequent decline. Since the study is ongoing it will be possible to examine the long term trends in noise exposure in subsequent analyses. Further exploration of the factors that would lead an individual to take steps to reduce exposure appears warranted.

In addition, it appears that the major reductions in noise exposures occurred in the study smelter location where the greatest number of volunteers was located. The reasons why more reduction was found in one location compared to the two others are not clear. However, in the other smelter location, there were a significant number of layoffs and production slowdown due to the economic downturn, and this could have affected both volunteer retention as well as the efficacy of the intervention. Noise levels were generally low in that facility during the work slowdown period, making it more difficult to demonstrate a reduction in exposures. Differences in noise exposure types between the smelter and the turbine component environment may partially explain why certain individuals in the turbine component factory were having persistently high noise exposure readings. These individuals have been counseled to take steps to reduce their exposures. Further examination of such location area effects is planned.

Results from the annual questionnaire and the focus groups have been previously reported (Williams & Rabinowitz, 2011). Of individuals who consistently used the device, 89% believed it helped them control their noise exposure and 96% say it helped preserve their hearing. In focus groups, volunteers reported they were often surprised at their level of noise exposures and that the daily monitoring led them to use their HPDs more conscientiously or change the way they performed a task, and that this has improved their hearing protection. One group of workers reported being able to identify a noisy area in the factory and worked with management to make hearing protection mandatory in that area.

This longitudinal study also demonstrates some of the challenges of intervention trials to prevent noise induced hearing loss in workplaces. Since the monitoring device requires daily action by the worker to start and stop the device, wear the monitoring device consistently during the work shift, and download into the infrared reader at the end of the shift, it is highly likely that the workers who both enlist and persist in using the device are more motivated and perhaps more interested in preserving their hearing than their counterparts who either did not sign up for the study or who dropped out. The magnitude of this selection bias or “volunteerism” is difficult to assess given the non blinded and non-randomized nature of the intervention. It is also difficult to assess the impact of the financial incentives including the point earning system for daily downloads, although anecdotal reports from subjects indicated both that some felt the financial incentives were important, and others did not.

At the same time, the fact that more than 60% of the enrollees continued to use the device over a period of more than six months indicates that voluntary use of the monitoring devices may be possible in industrial worksites. There were key aspects of the study that differ from usual use of hearing protection in hearing conservation programs (aside from the daily exposure monitoring feature). These differences include the use of cash incentives to encourage use of the device, and allowing the noise exposure information to be kept confidential and not shared with management.

Since noise induced hearing loss is a cumulative disorder resulting from noise exposure over time, any preventive intervention to be effective should lead to a lasting change in exposure level, and may take several years for the preventive effect to be seen. Over the observation period reported on in this study, some persons have withdrawn from the intervention citing difficulties with the equipment and protocols, including interference of the wires with job function, ear discomfort, and the inconvenience of daily downloading. The study is now ongoing, and future studies of this cohort will examine the impact of noise monitoring on rates of hearing loss in the volunteers, as well as the long-term acceptance and perception of usability of the daily noise exposure-monitoring device.