Are hearing protection devices used in the workplace really efficient? A systematic review

Abstract

BACKGROUND:

The most common current hearing protection devices (HPDs) on the market include earplugs and earmuffs. A variety of materials can be used to manufacture these devices, and each offers a level of noise attenuation that is informed by the manufacturer although it does not always correspond to the attenuation observed in real-world use.

OBJECTIVE:

To evaluate the noise attenuation of HPDs available to workers exposed to noise.

METHODS:

The most relevant studies originally published in English, Portuguese, or Spanish that investigated the noise attenuation effectiveness of HPDs used by workers exposed to noise were analyzed. The following electronic databases were searched by 2 independent reviewers for studies published from 1999 to 2019: MEDLINE (PubMed), Scopus, Web of Science, EMBASE, Cochrane Library (OVID), ProQuest, and BVS-Bireme. Different combinations of the following search terms (MeSH terms) were used for all databases: “Hearing Loss, Noise-Induced”, “Ear Protective Devices” (Efficacy OR Effectiveness)”, “Noise, Occupational”.

RESULTS:

The search strategy yielded a total of 326 potentially relevant studies. After the removal of duplicates, 156 remained for the screening of titles and abstracts. After reviewing titles and abstracts, 46 studies were selected for full-text reading. Of these, six were included in this systematic review.

CONCLUSION:

Hearing protection devices reduced the noise exposure and were effective in all included studies in different countries, types of activity, and sound pressure exposure.

Keywords

Noise noise-induced hearing loss earmuff earplug

1 Introduction

Approximately 27.7 million adults 20–69 years old in the US live with a noise-induced hearing loss (NIHL) [1]. It is the second most common occupational disease or injury, after work-related musculoskeletal disorders, even after decades of study, regulation, and workplace interventions to prevent it [2]. NIHL is irreversible and tinnitus is a frequent auditory symptom [3]. It can negatively affect workers’ quality of life, potentially leading to psychosocial problems, such as stress and anxiety, difficulty understanding speech, and hyperacusis [4]. Sleep disorders and cardiovascular diseases are also common in affected workers [5].

Although other risk factors play a role in NIHL, the effect of noise on hearing is partially dependent on the overall noise exposure, which is determined by temporal pattern of noise, level of noise, frequency content of the noise, and overall duration of noise exposure [5, 6]. The temporal characteristics of noise partially determine the extent of damage to the auditory system. It can be continuous (steady state or stationary), time-varying (nonsteady or nonstationary), intermittent, and impulse/ impact transient noise [6].

A study on forestry workers reported NIHL among 23.8% of participants exposed to higher than 85 dB noise, with 5.5% of those exposed to 85–89.9 dB noise and 11% of those exposed to higher than 90 dB noise also experiencing NIHL [7]. Silva et al evaluated 748 metallurgical workers exposed to noise (mean of 85.61 dB) in the first 5 years of employment, they found a significant progressive worsening of audiometric thresholds after 3 years of employment, despite of the use of hearing protection devices (HPDs) [8].

The Occupational Safety and Health Administration (OSHA) requires the use of HPDs when the noise exposure levels are equal to or above 8-hour time weighted average (TWA) of 90 dBA. The use of HPDs is recommended when the noise exposure levels are at or above 85 dBA TWA. The attenuation provided by HPDs must be sufficient to lower exposure levels to at least 90 dBA TWA; in the case of a worker with a standard threshold shift, the attenuation should be sufficient to lower the levels to at least 85 dBA TWA [6].

Although noise control is the most effective means of hearing conservation, the knowledge of workers regarding the effects of noise, the need to conserve hearing, the ways of conserving hearing, and the consequences of carelessness or noncompliance is equally important. Effective education of workers regarding all aspects of hearing conservation is highly critical to the overall success of a hearing conservation program [3 , 6].

The HPDs should allow sufficient attenuation without leading to overprotection. The attenuation must be sufficient to reduce the exposure levels to at least 85 dBA (OSHA, 1983) but preferably to 75 to 80 dBA. Noise levels should not be attenuated to below 70 dBA, as this will lead to overprotection. Overprotection can interfere with communication and the ability to hear warning signals. In many hearing critical jobs, the workers need to hear and localize some soft and moderate level sounds very clearly [9].

Several factors should be considered in selecting HPDs to ensure sufficient hearing protection to prevent hearing loss and to improve worker acceptance and continued use. Comfort and attenuation appear to be inversely related [10]. Thus, it is important to achieve a balance between attenuation characteristics and comfort to maximize HPD effectiveness [11]. Personal hearing protection depends on workers’ training and correct use of HPDs. However, environmental conditions such as excessive heat and humidity may cause workers not to use the devices correctly [12, 13]. In addition, communication in the workplace may be limited with the use of HPDs, which can make it difficult for workers to hear an audible alarm. Occlusion effects can also cause discomfort. Prolonged daily use of earplugs, for example, can trigger or aggravate local inflammatory processes that hinder their correct insertion [14].

HPDs are available in various physical styles. Four factors impose a limit on the amount of attenuation provided by HPDs, including sound transmission through the HPD barrier, the extent of the airtight seal against the ear canal wall for earplugs, or the circumaural area of earmuffs, HPD vibrations, and conduction of sounds through bone [6]. A variety of materials can be used to manufacture these devices, and each offers a level of noise attenuation that the manufacturer informs, although it does not always correspond to the attenuation observed in real-world use.

The U.S. Environmental Protection Agency (EPA) requires the single-number noise reduction rating (NRR) to be shown on the label of each hearing protector sold in the United States. The NRR is based upon human subject testing conducted in the laboratory with a panel of listeners. Hearing tests are administered to these individuals with and without the protectors in place. This test method is referred to as real-ear attenuation at threshold (REAT); the decibel difference between the open-ear threshold and the occluded-ear threshold indicates the amount of sound attenuation provided [15]. Because the real-world NRR is less than the labeled NRR, the Department of Labor/OSHA have instructed its inspectors to “derate” (reduce) a hearing protector’s labeled NRR by 50% for evaluating the relative efficacy of HPDs and engineering noise controls [6].

The REAT method is limited by physiological noise masking, which elevates the REAT values at 125 and 250 Hz, and REAT’s inherent variability, as it is the difference between the two subjective thresholds, each of which has its own uncertainty. Field use of REAT has its challenges as it can magnify these limitations that affect REAT even in controlled laboratory environments [16]. With the increasing popularity of individual “fit testing” and the advent of miniaturization of electronic components, the microphone-in-real-ear approach (MIRE) and its field counterpart Field-MIRE (F-MIRE) are becoming more appealing and well suited for estimating HPD attenuation both in laboratory and in “real world” occupational conditions [17].

The reviews published so far by El Dib et al. [18, 19] aim to promote the wearing of hearing protection and other reviews published by Loughran et al. [20] and Kraaijenga et al. [21] evaluate the recreational noise exposure. It is important to know whether the HPD used in the workplace are effective, so this systematic review is very timely.

Source noise control is the best strategy for protecting hearing, but it is not easy to perform in some settings. The HPDs are designed to reduce the noise level reaching the eardrum, minimize the possibility of hearing loss, and reduce other noise effects, including tinnitus, hyperacusis, annoyance, irritability, and hypertension. The objective of this systematic review was to evaluate the noise attenuation of HPDs available to workers exposed to noise.

2 Materials and methods

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines [22] and current recommendations of the Cochrane Collaboration.

The most relevant studies originally published in English, Portuguese, or Spanish that investigated the noise attenuation effectiveness of HPDs used by workers exposed to noise were analyzed. The following electronic databases were searched by 2 independent reviewers for studies published from 1999 to 2019: MEDLINE (via PubMed), Scopus, Web of Science, EMBASE, Cochrane Library (OVID), ProQuest, and BVS-Bireme. Different combinations of the following search terms (MeSH terms) were used for all databases: “Hearing Loss, Noise-Induced”, “Ear Protective Devices” (Efficacy OR Effectiveness)”, “Noise, Occupational”. The reference lists of the retrieved articles were also hand-searched to detect other potentially eligible studies. Two reference managers (Endnote and Rayyan) were used by the reviewers to consolidate the data extracted from the databases.

The same 2 reviewers independently screened the titles and abstracts identified by the initial search. Studies were eligible for full review if they presented objective measurements of noise attenuation by HPDs. The full-text articles were read by the 2 independent reviewers. Exclusion criteria were review articles without clinical cases, experimental animal studies, studies on the use of hearing protectors in the military workplace or in shooting practices, studies on the use of hearing protectors by musicians or sportspeople, and studies without objective quantification of noise attenuation. Disagreements between the 2 reviewers were resolved by consensus or by consulting a third reviewer (ad hoc) for arbitration.

The following parameters were evaluated in each study: sample size, study setting, HPD type, method used to measure noise attenuation, and degree of hearing protection provided by each HPD under study.

3 Results

The search strategy yielded a total of 326 potentially relevant studies. After removal of duplicates, 156 remained for the screening of titles and abstracts. After reviewing titles and abstracts, 46 studies were selected for full-text reading. Of these, 6 were included in this systematic review. Hand searches of reference lists yielded no additional studies. Figure 1 provides an overview of the study selection process.

Fig. 1

Flow diagram of the study selection process according to the PRISMA statement.

The characteristics of included studies are shown in Table 1. The 6 included studies enrolled a total of 166 workers from different industry sectors and production lines. In the different workplaces investigated, the noise levels to which workers were exposed ranged from 67.38 dBA for kitchen assistants to above 120 dBA for workers in the food, petrochemical, wood furniture, aluminum, motorized products, and aeronautical industries. Earplugs and earmuffs were the most commonly used HPDs. REAT and MIRE were the methods most commonly used to measure noise attenuation. The level of noise attenuation provided by the HPD (measured as noise reduction and/or insertion loss) varied across studies, depending on the HPD type used and the frequency assessed. The “real noise exposure” (RNE) was calculated between noise exposure in the workplace and HPD’s noise attenuation. It can estimate the inner ear noise exposure in the studies evaluated.

Table 1

Characteristics of included studies (n = 6)

First author, year	Sample size	Study setting	Noise in the workplace (dB A)	Measurement method	HPD type	Noise attenuation (dB A)	Real noise exposure
Lutz, 2015	22	Mining	87.4–92.8	MIRE	Earplugs	7.1–9.3	78.1–85.7
Michael, 2011	6	Metallurgy	109.4	REAT	Earplug	29.4–32.0	77.4–80
Neitzel, 2006	20	Packaging plant	86.0±6.1	REAT	Earplugs (custom-molded/foam)	17.6–27.5 (custom-molded); 14.9–20.4 (foam)	58.5–68.4 (custom-molded); 65.6–71.1 (foam)
Nélisse, 2011	24	Food transformation, petrochemicals, wood furniture, aluminum transformation, motorized products assembly, and aeronautics	90.0–110.0	MIRE	Earplugs and earmuffs	20–40	50–90
Rocha, 2016	18	Kitchen assistants	67.38–123.79	MIRE	Earplugs	6.2–17.8	50–117.59
Tsukada, 2008	76	Electronics	>80.0	REAT	Earplugs	5.2±9.0–29.8±6.0	–

HPD, hearing protection device; MIRE, microphone in real ear; REAT, real-ear attenuation at threshold; NW, noise in the workplace; NA, noise attenuation; NW–NA, real noise exposure.

4 Discussion

Attenuation characteristics, comfort, ease of fit, convenience and availability, ability to hear important signals, compatibility with other work or protective gear, and many other factors that are specific to individual workers and specific work settings should be considered in selecting HPDs to ensure sufficient hearing protection to prevent hearing loss and to improve worker acceptance and continued use. Comfort and attenuation appear to be inversely related [10]. It is important to achieve a balance between attenuation characteristics and comfort to maximize HPD effectiveness [11]. The environmental conditions such as excessive heat and humidity may cause discomfort and communication during tasks may be limited with the use of earplugs.

Hearing protectors have had a decisive impact on preventing the worsening of hearing loss. A Canadian study, evaluating 22 376 workers and data from 316 476 audiometric tests performed between 1979 and 1996, showed a 30% reduction in the trend of hearing loss among workers after 1988, when the use of HPDs was introduced [23]. The use of HPDs is mandatory for workers in any company with levels of noise potentially harmful to hearing. Only a few studies, however, have evaluated the effectiveness of HPDs, and review studies are even scarcer. The 6 studies included in the present review evaluated different companies with a variety of occupational activities.

Only a few studies included in this review reported the level of noise attenuation informed by the manufacturer of the HPD under study. This information could be relevant for the companies, but it is not important for a review study due to the wide variety of manufacturers of HPDs around the world. The real-world NRR is less than the labeled NRR [6].

4.1 Techniques used to measure the real-world attenuation performance of HPDs

The test methods account for the acoustical interaction between the wearer and the device using measurements of passive (REAT) and active microphone-in-real-ear (MIRE) measurements as specified in ISO 4869-1 and ISO 11904-1, respectively.

The REAT technique, through psychoacoustic attenuation measurements, evaluates audiometric thresholds on an individual with (occluded) and without (non-occluded) HPDs. The difference between the occluded and non-occluded thresholds is equivalent to the minimum attenuation values provided by the HPD. It is considered the gold-standard technique since 1957, it is part of many worldwide standards and takes into account all relevant sound paths to the inner ear because it evaluates all the sound paths that are significant for the inner ear [24, 25].

The MIRE technique objectively measures noise attenuation by using either 1 microphone (placed in the ear canal during 2 separate measurements with and without HPDs) or 2 microphones (one placed inside the ear canal underneath an earplug, and the other simultaneously placed outside the ear canal); the latter is termed field-MIRE (F-MIRE). The difference between the sound levels measured simultaneously by the internal and external microphones is termed noise reduction (also measured in dB) [17]. After the use of fitting equations, MIRE has shown a good correlation with REAT, with the advantage that results can be obtained with just one measurement and it can be conducted in the workplace [19]. In the present study, a comparison of the measurement methods was not possible due to poor data consistency.

4.2 HPD type

The studies evaluated earplugs. It was not possible to determine whether one material or molding type (premolded or moldable) was superior to other, as most included studies did not provide this information. The high variability in the levels of noise attenuation between studies indicates that external factors, such as training on how to properly wear the HPD, can influence the level of protection obtained. Murphy et al. [26] showed an improvement of 10-20 dB in noise attenuation when workers were trained on how to wear the earplug correctly. Only the study of Nélisse et al. [27] evaluated double hearing protection, when both earplugs and earmuffs are worn simultaneously, achieving high level of sound attenuation (20–40 dB); however, it was not possible to conclude that such practice provides additional gain in the level of protection.

4.3 Type of HPD used and type of occupational noise exposure

Rocha et al. [28] reported the highest sound exposure level, up to 123 dB, and the highest protection achieved in their study was 17.8 dB, as measured by the MIRE technique. Therefore, these workers were exposed to very high sound pressure levels with HPDs unsuitable for such an exposure, placing the workers at increased risk of developing NIHL, although the duration of daily exposures to noise in the workplace was not detailed.

Michel et al. [29] evaluated six workers in the metallurgical industry. Considering the noise average of 109.4 dBA and the protection between 29.4 and 32.0 dBA, the RNE was 72.4 to 80 dBA, and it provided effective protection. Lutz et al. [30] evaluated 22 workers in the mining sector, and considering the highest protection, the noise attenuation (7.1–9.3 dBA) was the lowest compared to the other studies (Table 1). But this protection was effective, because the RNE was 80.1 to 81.5 dBA – exposure levels allowed by OSHA.

The HPDs should allow sufficient attenuation without leading to overprotection. The noise exposure variable was 90–110 dBA in Nelisse et al. [27]. The workers used earplugs and earmuffs. Double protection usually adds only 5 to 10 dB to the maximum attenuation provided by the use of a single HPD [8]. Considering the highest protection, this study showed noise attenuation of up to 40 dBA. The RNE was 70 dBA. In the study by Neitzel et al. [31], the variation of exposure was 86.0±6.1. There is a risk of overprotection, especially in the “custom molded” model in which the RNE was 58.4–68.4 dBA.

Tsukada et al. [32] evaluated workers exposed to the noise of more than LAeq 80 dB in the workplaces of an electrical equipment parts manufacturing plant. There is no other information about noise exposure. Thus, the calculation of the RNE is unable to be performed. The authors consider as effective the noise attenuation of≥25 dB in both ears; this level of attenuation is enough if noise exposure is below 105 dBA. However, if exposure is below 95 dBA, there is also a risk of overprotection. The article is about one of the factors that can increase the effectiveness of the protection of HPDs, that is the individual training on the proper way of wearing earplugs. The rate of proper earplug use, judged from a sufficient noise attenuation effect of≥25 dB in both ears, improved from 46% of workers before the training to 72% immediately after the training and to 62% two months after the training.

4.4 Higher protection according to frequencies

The variability in noise attenuation levels between the different frequency bands was attributable mainly to differences in the techniques used and sound paths. REAT measurements, for example, tend to overestimate noise attenuation by up to approximately 6 dB at low frequencies due to physiological noise masking effects [31, 33]. In a study evaluating the use of earmuffs, higher levels of protection were identified at the frequencies of 2 and 4 kHz, related to sound transmission via bone conduction, not assessed by the MIRE technique, which could overestimate the values obtained in this frequency band [27]. This study also reported a large interindividual variation, suggesting that the HPD choice should be tailored to the individual worker.

Overall, the results of the included studies, which evaluated the use of earplugs and earmuffs in different occupational activities, confirm the noise attenuation effectiveness of HPDs, but hearing protection varies according to the frequency band assessed and type of HPD used. We observed variations in noise attenuation effectiveness within the same HPD type. Prevention is the best strategy to avoid NIHL, and the choice of hearing protector should be tailored to the individual worker. The effectiveness of HPDs could be assessed sporadically by companies as a measure of quality control of personal protective equipment provided to their employees. The present study shows that this assessment can be easily performed by companies through the MIRE method, without increasing costs. Controlling the effectiveness of HPDs is important not only for workers, who avoid NIHL and its irreversible health consequences, but also for companies by reducing lawsuits.

4.5 Limitations

The present study has reviewed the studies in the literature of HPDs. Although this is the first systematic review of this topic, and it was possible to gather many relevant data, the level of scientific evidence of the studies was low. There was significant heterogeneity among all studies. The measurement method was diverse. The level of noise in the workplace was not uniform, and the workers were from different sectors. Finally, the sample size was small in some studies. The heterogeneity makes the combined analysis less significant; on the other hand, it was possible to compile all data available in the literature.

5 Conclusion

Noise control is the most effective approach to hearing conservation. The hearing protection devices reduced the noise exposure and were effective in all included studies in different countries, types of activity, and sound pressure exposure. It is important to strike a balance between noise attenuation and comfort to maximize the effectiveness of HPDs. They would probably be better accepted if they were tailored to the individual worker.

Ethical approval

Not applicable.

Informed consent

Not applicable.

Conflict of interest

The authors have no conflicts of interest to disclose.

Footnotes

Acknowledgments

The authors thank Ana Paula de Morais Oliveira – Faculty of Medical Sciences, Universidade Estadual de Campinas Library.

Funding

The authors have no financial relationships relevant to this article to disclose.

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