A Systematic Review of Hearing Loss and Its Associated Factors Among Workers in the Metal Industry

Abstract

Background/Objectives: Hearing loss is a disorder that develops because of being exposed to high noise levels affecting the quality of life among affected individuals. A review of the literature was conducted to explore the prevalence of hearing loss and its associated factors among workers in the metal industry. Methods: The literature search was conducted on ScienceDirect, Google Scholar, Pub Med, ResearchGate and African Journals Online databases to identify articles according to the Preferred Reporting Items for Systematic Reviews and Meta analyses (PRISMA) guidelines. The studies published in scientific journals between January 2014 and December 2024 describing hearing loss and its associated factors among workers in the metal industry were considered for inclusion in the review. The articles were screened by the author. The Critical Appraisal Skills Programme (CASP) quality assessment tool with modified checklist questions was used to evaluate the quality of studies. Results: Following the literature search and using the relevant inclusion criteria, a total of 127 articles were identified, and 8 articles with a total of 2605 participants were included in the review. The sample sizes ranged from 93 to 606. The participants’ age ranged from 19 to 65 years. A review of studies showed varying prevalence of hearing loss ranging from 13.8% to 59%. Furthermore, the studies have found working experience, advanced age, cigarette smoking, tinnitus, working in areas of high noise levels and not using hearing protective devices to be associated with a risk of developing hearing loss. Conclusions: The review found that workers in the metal industry are at risk of developing hearing loss and, therefore, implementation of control measures to prevent the occurrence of hearing loss is necessary.

1. Introduction

Hearing loss evolves slowly following continuous exposure to high level of noise and there are about 1.5 billion affected people around the world [1,2]. Noise-induced hearing loss occurs when the hair cells in the cochlea of the ear are harmed by excessive noise exposure and cannot be restored [3]. It has been estimated that around 1.1 billion young people who listen to music at a high volume for a long time may suffer from long-term hearing loss [1]. The World Health Organization (WHO) envisages that the number of people afflicted by hearing loss will rise to 2.5 billion by 2050 [1]. Around 18% of workers in the age range of 20–69 years who are exposed to high noise levels at work have noise-induced hearing loss (NIHL) [4].

According to the South African Department of Employment and Labour (DEL), 73.2% of mine workers are exposed to the noise levels that exceed the occupational exposure limit of 85 dBA [5]. Furthermore, 80% of mine workers in the United States are exposed to noise levels that exceed the 85 dBA level [6]. The report by the Occupational Health and Safety Administration (OHSA) indicates that more than 20 million workers are exposed to hazardous noise levels at the workplaces annually [7]. In South Africa, there has been a 5.2% increase in the number of NIHL cases in mines, as reported by the Department of Mineral Resources and Energy [8]. It has been reported that the number of NIHL cases reported to the compensation fund in South Africa increased from 738 in 2020 to 776 in 2021 [8].

It is evident that workers in the metal industry are exposed to high noise levels that increase the risk of NIHL [9]. A study conducted among industrial workers in Jordan found that hearing loss was associated with increased exposure to noise levels of more than 85 dBA [10]. In Tanzania, a study [11] was conducted to evaluate the noise exposure levels among workers at iron and steel factories. In the said study, the dosimeters and sound level metres were used to record personal and area measurements, respectively. The results of the said study demonstrated that the average personal noise exposure level was 92 dBA while the mean area noise exposure level was 90.5 dBA. The study [12] was conducted to assess the noise exposure levels among 468 workers in two steel factories in the United Arab Emirates. The results of the said study demonstrated that 89% of workers were exposed to noise levels above 85 dBA. Furthermore, 45% of workers were not using hearing protective devices [12]. Operations such as arcing, welding, grinding and crushing of metal objects are some of the causes of high levels of noise in the metal factories [13]. Moreover, cutting activities, rolling mills, extraction and vacuum systems, electrical transformers and welding processes are other sources of noise. The use of compressors, conveyors, induced draught fans, blowers and equipment causes high noise levels [13].

The quality of life of people suffering from NIHL is negatively affected in terms of disruption in communication and participation in social activities. Moreover, people who suffer from NIHL may also have headaches, sleep disorders, tinnitus and heart disorders [14]. There is insufficient information regarding the prevalence of hearing loss among workers in the metal industry and the recommendations are limited. The aim was to review the literature and explore the prevalence of hearing loss among workers in the metal industry. This systematic review was conducted to explore the knowledge gap regarding the prevalence of hearing loss among workers in the metal industry. An up-to-date knowledge about hearing loss in the metal industry is essential to maintain health and safety standards. This systematic review will contribute to the generation of up-to-date evidence on the prevalence of hearing loss and its associated factors in the metal industry, which is necessary for advancing knowledge about this health issue.

2. Materials and Methods

2.1. Search Strategy

To identify the sources of information, the literature search was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta analyses (PRISMA) guidelines [15]. The comprehensive search strategy is indicated under supplementary material additional File S1. The study was conducted in compliance with the PRISMA guidelines. The PRISMA checklist is available under supplementary material Table S1. Moreover, the search was conducted on different databases that include ScienceDirect, Google Scholar, Pub Med, ResearchGate and African Journals Online. Publications from scientific journals that used sound methodology were included in the review to ensure the accuracy and validity of the findings. The systematic search was conducted by combining search terms: ‘noise-induced hearing loss’, ‘workers’, ‘associated factors’, ‘metal industry’ as well as names of different continents and using Boolean operators ‘AND’, ‘OR’ according to the databases.

The PEO (Population, Exposure/Environment and Outcome) framework was used to develop the following review question: “What are the factors associated with the prevalence of NIHL among workers in the metal industry?” The titles and abstracts were evaluated, and suitable articles were identified and reviewed. During the literature search, duplicate articles were eliminated, and articles not complying with the inclusion criteria were also removed. Moreover, articles not aligning with the scope of the study were also eliminated. All selected articles were reviewed by the author (FR) independently to assess their eligibility and eight complied with the inclusion criteria. The results were synthesized by integrating the information according to the country, publication year, study design, sample size, study design, participants, participants’ age and outcomes (prevalence and associated factors) of all included studies. Only English-language articles were included in the analysis. This systematic review on hearing loss in the metal industry was not registered. The body of evidence is viewed at level 4.

2.2. Eligibility Criteria: Inclusion and Exclusion Criteria

The studies suitable for inclusion in the review had to comply with the requirements for the inclusion criteria. Furthermore, the studies that were considered for inclusion in the review were original research studies that focus on the prevalence of occupational NIHL among workers in the metal industry and published between January 2014 and December 2024. The search was conducted in April 2025. The selection of the studies was not restricted to the study design, research method and location. The studies that did not focus on the prevalence of occupational NIHL among workers in the metal industry and studies that were not published between January 2014 and December 2024 were excluded. In terms of the diagnostic criteria, occupational NIHL is a disorder that progresses steadily in one or both ears over many years of exposure to the noise level that is greater than the allowable noise limit of 85 decibel [dB(A)] during the daily working shift of 8 h [10]. Those articles for which the abstracts or full texts were not available were also excluded. Furthermore, review papers, abstracts and publications not reporting about the prevalence of hearing loss among workers in the metal industry were excluded.

2.3. Quality Assessment and Assessment for Risk of Bias

To evaluate the quality of the studies included in the review, the Critical Appraisal Skills Programme (CASP) quality assessment tool with modified checklist questions was used. The quality of the studies included in the review was evaluated in terms of the research method, recruitment of participants, data collection procedures, sample sizes, data analysis, outcomes and value of research. The studies were classified according to three categories (low, medium and high risk of bias) based on the findings of the quality assessment. The quality assessment displayed in

Table 1. Modified Critical Appraisal Skills Programme checklist [16] for quality assessment of included studies reporting hearing loss and its associated factors in the metal industry.

Study	Questions
	1	2	3	4	5	6	7	8	9	Total	%
Elshaer et al. [13]	1	1	1	1	1	1	1	1	1	9/9	100
Nyarubeli et al. [17]	1	1	1	1	1	1	1	1	1	9/9	100
Melese et al. [18]	1	1	1	1	1	1	1	1	1	9/9	100
Agarwal et al. [19]	1	1	1	1	1	1	1	1	1	9/9	100
Whittaker et al. [20]	1	1	1	1	1	1	1	1	1	9/9	100
Salih et al. [21]	1	1	1	1	1	1	1	1	1	9/9	100
Kerdonfag et al. [22]	1	1	1	1	1	0	1	1	1	8/9	89
Almaayeh et al. [10]	1	1	1	1	1	1	1	1	1	9/9	100

indicated that the studies had a low risk of bias. A modified checklist with a scoring system and questions, as shown in Table 1, was used to assess the quality of studies. The author (FR) evaluated the quality of each article, and no studies were excluded during the quality assessment.

Modified checklist questions [16] to assess the risk of bias for included studies

• Did the study address hearing loss and associated factors?
• Were the appropriate methods used to answer the research question?
• Were the participants recruited in an acceptable way?
• Were the variables related to hearing loss accurately measured to reduce bias?
• Were the data collected in a way that addresses the prevalence of hearing loss and associated factors?
• Were there enough participants in the study to reduce the play of chance?
• Are the results accurately presented?
• Was the data rigorously analyzed?
• Was the research valuable and did the study make contributions to existing knowledge?
  Yes—1
  No—0

2.4. Data Extraction

To extract data from the included studies, an excel spreadsheet was used. The author extracted data from the included studies in relation to characteristics such as country, publication year, study design, sample size, participants’ age and outcomes (prevalence and associated factors). The factors influencing the prevalence of NIHL were categorized as follows:

Personal factors: These factors include age, smoking, alcohol consumption, tinnitus and family history of hearing loss.

Work-related factors: These factors include the length of employment, the duration of exposure during the working shift, the noise level in the working area, nature of employment and the utilisation of hearing protective devices.

The results in relation to the prevalence rates of NIHL were presented in percentages. The A-weighted sound pressure levels, average personal noise exposure levels and mean area noise exposure levels were used to present the results in relation to workplace noise exposure levels. Moreover, the odds ratios and 95% confidence intervals were used to report on the association between personal- and work-related factors with NIHL.

3. Results

3.1. Study Selection

Figure 1 shows the PRISMA flow diagram that was used for the selection of the studies. Following a search strategy, a total of 127 publications were initially identified on the databases. A total of 13 duplicates were removed, and thereafter titles and abstracts of the publications were screened resulting in 96 publications being removed. Eighteen full-text articles were evaluated for eligibility and this led to a further removal of ten articles with reasons, resulting in a total of eight articles included for the review. Eight articles were removed as they were not published between 2014 and 2024, and two review papers were also removed.

3.2. Study Characteristics

The characteristics of the included articles are displayed in

Table 2. Characteristics of the included articles.

Study	Publication Year	Country	Study Design	Sample Size	Age (Mean or Median or Range) Years	Participants	Outcomes (Prevalence and Associated Factors)
Elshaer et al. [13]	2023	Egypt	Retrospective study design	606	Mean = 38	Steel workers	NIHL prevalence—47% Associated factors: Age, tinnitus and job duration (univariate analysis) Tinnitus (multivariate logistic regression
Nyarubeli et al. [17]	2019	Tanzania	Cross-sectional	Exposed group—221 Unexposed group—107	Mean = 32 years for exposed group; 42 years for unexposed group	Iron and steel workers; primary school teachers	NIHL prevalence—48% (exposed group); 31% (control group) Associated factors: high risk for NIHL among exposed workers
Melese et al. [18]	2022	Ethiopia	Cross-sectional	300	Median age = 35 years	Metal workers	NIHL prevalence—30.7% Associated factors: Advanced age (30 and 44 years and 45 and 65 years), cigarette smoking, increased noise level in the working area, work experience
Agarwal et al. [19]	2015	India	Cross-sectional	341	Age range: 27–45 years	Steel factory workers	NIHL prevalence—40.46% (right ear), 45.75 (left ear) Associated factors: exposure to occupational noise
Whittaker et al. [20]	2014	Nepal	Cross-sectional	115 (exposed group) 123 (unexposed group)	Median age—24 years	Metal workers and hotel workers	NIHL prevalence—30.4% (exposed group); 4.1% (unexposed group) Associated factors: High noise level in the workplace
Salih et al. [21]	2024	Iraq	Cross-sectional	503	Age range: 19–65 years	Metal industry workers	NIHL prevalence—59% Associated factors: duration of exposure to noise
Kerdonfag et al. [22]	2019	Thailand	Cross-sectional	93	Age range: 19–59 years	Steel industry workers	Associated factors: longer length of employment above 5 years, age over 40 years, high noise level in the workplace
Almaayeh et al. [10]	2018	Jordan	Cross-sectional	196	Age range: 20–54 years	Industrial workers	NIHL—13.8% both ears; 21% right ear; 21.4 left ears Associated factors: Period of exposure of 10 years or more; age—35 years or more. Exposure to high noise levels in the workplace; not using hearing protective devices

. From a total of eight studies included in the review, seven were cross-sectional studies [10,17,18,19,20,21,22]. Moreover, the factory medical records were reviewed retrospectively in one study [13]. Furthermore, the studies were conducted in different countries, including Egypt [13], Tanzania [17], Ethiopia [18], India [19], Nepal [20], Iraq [21], Thailand [22] and Jordan [10]. The dates of publication for the studies ranged from 2014 to 2024 and a quantitative research method was used in the studies with sample sizes varying from 93 to 606. The total number of participants from the included studies was 2605.

3.3. Risk of Bias Within the Studies

Table 1 shows the results for the quality assessment of the studies. Seven studies [10,13,17,18,19,20,21] included in the review had scores of nine and one study had a score of eight [22].

3.4. Prevalence of Hearing Loss

The studies reported varying prevalence rates of NIHL. It was found that the prevalence of NIHL among workers ranged from 13.8% to 59% in all eight studies. Four studies reported the prevalence ranging from 13.8% to 30.7% [10,18,20,22]. The prevalence of NIHL that ranged from 45.75% to 47% was reported in three studies [13,17,19]. Only one study reported the prevalence of above 50% [21].

3.5. Factors Associated with NIHL

The factors associated with NIHL are shown in Table 2. Four studies (50%) found an association of advancing age with NIHL [10,13,18,22]. Moreover, working experience was found to be associated with NIHL in four studies [10,13,18,22]. Six studies (75%) reported an association of increased noise level in the working area with NIHL [10,17,18,19,20,22]. Other associated factors were reported as tinnitus [13], cigarette smoking [18] and not wearing hearing protective devices [10].Moreover, the logistic regression of factors associated with NIHL among workers in the metal industry is shown in

Table 3. Logistic regression of factors associated with hearing loss among workers in the metal industry.

Study	Associated Factors	OR (95% CI)	p Value
Elshaer et al. [13]	Age (>55 years)	7.0 (2.12, 23.11)	<0.001
	Working experience (20 years or more)	3.78 (1.81, 7.74)	<0.001
	Tinnitus	2.06 (1.01, 4.2)	0.045
Almaayeh et al. [10]	Age (>35 years)	2.7 (1.2, 6.1)	0.014
	Working experience or exposure (>10 years)	2.0 (1.1, 5.9)	0.037
	Working area with high noise level	4.2 (1.6, 10.5)	0.003
	Not using hearing protective devices	2.7 (1.1, 6.6)	0.033
Melese et al. [18]	Age between 45 and 65 years	3.8 (1.5, 9.5)
	Working experience (>10 years)	3.5 (1.3, 4.3)
	Working area with noise level >85 dB	2.2 (1.1, 6.5)
	Cigarette smoking	2.3 (1.2, 4.5)
Nyarubeli et al. [17]	Working area with high noise level	1.3 (1.10, 1.62)
Whittaker et al. [20]	Working area with high noise level	18.544 (5.621, 61.180)	<0.001

OR: odds ratio; CI: confidence interval.

.

4. Discussion

In this study, a systematic review was conducted to describe and report the factors associated with NHIL among workers in the metal industry. According to the findings, an association of advanced age with the risk of developing NIHL was found. It has been discovered that workers aged 40 years and above are at risk of acquiring NIHL [22]. There is a decrease in the number of nerve cells that function normally in the ears with ageing, and this could contribute to hearing loss. Presbycusis is a hearing loss condition related to ageing that is common among individuals aged 50 years and above [23]. In the case of workers in the metal industry, this condition may be worsened by exposure to high noise levels. In relation to the pathology of the temporal bone and the results of audiometric tests, the categories of presbycusis include sensory, neural, metabolic and cochlear conductive types [24]. The hearing loss that is mostly prevalent among middle-aged people is sensory presbycusis which originates from the deterioration of the organ of Corti and mainly occurs when the outer hair cells are harmed [25]. This is in line with the findings of the studies [18,22] included in the review which reported a high prevalence of hearing loss among middle-aged workers. Neural presbycusis is characterized by the loss of 50% or more than 35,500 neurons in the cochlear [26]. The third type of presbycusis is metabolic presbycusis which results from the degeneration of 30% or more of the stria vascularis tissue of the cochlea and may be due to ageing or excessive noise exposure [27]. In this context, middle-aged workers exposed to excessive noise in the metal industry are at risk of acquiring presbycusis due to degeneration of the cochlear. In addition, the fourth type of presbycusis, cochlear conductive presbycusis, develops as the basilar membrane of the cochlea deteriorates and becomes rigid with ageing [25]. A study conducted to assess the prevalence of NIHL among industrial workers in Jordan found that the risk was high among workers aged above 35 years [10]. In addition, 196 workers were included in the study and their age ranged from 20 to 54 years. This is also in line with the reports on the high prevalence of hearing loss among middle-aged workers [18,22].

One of the findings is that the risk of developing hearing loss increases with an increase in working experience of workers. Moreover, it was found that workers with more work experience have a higher risk of developing NIHL than workers with less working experience. In this context, a working experience of 10 years or more was associated with the development of NIHL [10,18]. Furthermore, the same outcomes on hearing loss were discovered in other workplaces among workers who are dental professionals [28]. Equipment such as grinders, hammers and welding machines which produce high noise levels above 85 dBA are used by workers in the metal industry. Prolonged exposure for several years to such high noise levels produced by the equipment worsens the development of NIHL. In relation to the working experience, the study conducted in Ethiopia found that the risk of acquiring hearing loss was 3.5 times higher among workers with more than 10 years of working experience than those with 1 to 5 years of working experience [18]. In another study [10], the risk of developing hearing loss was high among industrial workers who had been exposed to noise for a duration of 10 years or more. A cross-sectional study in Thailand found that the risk of developing NIHL tends to increase with a working duration of more than 5 years among steel industry workers; however, the increase was not statistically significant [22]. Moreover, in Egypt, the study [13] found that the chance of developing NIHL was 3.8 times higher among workers in the steel industry with a working duration of 20 years or more. In addition, the study was conducted by comparing metal workers and welders who were exposed to noise for short, medium and long period with workers exposed to continuous noise at the cable factory in Finland [29]. The findings of the said study revealed that a high prevalence of hearing loss was associated with the longer duration of noise exposure.

According to the findings, cigarette smoking was associated with NIHL [18]. In addition to noise exposure, cigarette smoking enhances the effects of occupational factors in the occurrence of NIHL. Cigarettes contain nicotine, which is a vasoconstrictor that reduces blood flow and causes some damage to the blood vessels. The chances of damage to the cochlea by high noise level will increase due to reduction in blood flow caused by smoking, increasing the development of NIHL. The level of carboxyhemoglobin in blood increases with smoking [30]. Moreover, burning cigarettes is associated with the release of chemicals such as toluene, styrene and xylene which have a damaging effect on the ears [18]. In addition, carbon monoxide released by burning cigarettes reduces the oxygen blood level of the cochlear resulting in low oxygen supply to the hair cells, ultimately, leading to their degeneration [31]. In Iran, a cross-sectional study was conducted among 923 metal workers exposed to noise to investigate the effects that passive or active smoking have on hearing loss [32]. In the said study, it was found that the risk of developing NIHL was high among passive or active smokers who worked in areas with high noise levels. A cross-sectional study conducted to investigate the prevalence of hearing loss and its associated factors among metal workshop workers in Gondar city, Ethiopia found that the risk of developing hearing loss was higher among smokers than non-smokers [18]. In addition, another cross-sectional study was conducted in Brazil to assess the effect of smoking on hearing loss among smoking and non-smoking workers of a food-producing factory exposed to noise levels above 85 dBA [33]. In the said study, it was found that smoking increases the risk of hearing loss. In a study conducted at a steel factory in Japan, a statistically non-significant difference was found between smoking and non-smoking workers in relation to hearing loss [34].

Another finding is that tinnitus was associated with hearing loss among workers in the steel industry [13]. Tinnitus is a warning sign that occurs before the actual hearing loss and is characterized by a ringing sound in one or both ears. When the hair cells of the cochlea are damaged due to exposure to high noise levels, which is common in the metal industry, tinnitus will occur [35,36]. The inconsistent use of hearing protective devices and exposure to high noise levels may enhance the development of tinnitus in one or both ears. The study conducted at the two steel factories in the United Arab Emirates found that 45% of workers were not using hearing protective devices [12]. The use of hearing protective devices is an important control measure to prevent hearing loss. The risk of hearing loss is higher among workers who are not using hearing protective devices than workers who use them consistently [18]. The effectiveness of hearing protective devices in preventing hearing loss was found in previous studies [37,38]. A cross-sectional study was conducted among 14,945 participants in Norway to evaluate the link between noise exposure in the workplace and the risk of tinnitus [39]. The results of the said study showed a tinnitus prevalence of 6.5%. Furthermore, the results showed no link between noise exposure at the workplace and the risk of tinnitus in relation to job exposure matrix. This could be due to the consistent use of hearing protective devices which had a protective effect on the participants. Furthermore, an association of self-reported noise exposure with the risk of tinnitus was found among participants with increased hearing thresholds in the said study. Another study by Rubak et al. discovered a link between noise exposure and the risk of tinnitus among participants with hearing impairment [40]. A link was also found between noise exposure and the risk of tinnitus among participants with normal hearing and those with hearing impairment [41].

The prevalence of NIHL is influenced by noise levels in working areas. In a cross-sectional study conducted in Tanzania to assess the prevalence of hearing loss among iron and steel factory workers, it was reported that the exposed workers were exposed to average noise level of 92 dBA, whereas the control group workers were exposed to the average noise level of 79.7 dBA [17]. Moreover, the prevalence of NIHL was high among exposed workers than the control group workers [17]. The findings of the study conducted in Thailand to assess the prevalence of hearing loss among steel industry workers showed that the measured noise levels ranged from 91.79 to 96.07 dBA [22]. Moreover, in the said study, most workers (32.1%) had a working duration ranging from 20 to 29 years in the steel industry. These findings suggest that the workers have an increased risk of developing NIHL. In Zimbabwe, the results of a cross-sectional study conducted to evaluate the prevalence of hearing loss among workers in the mining industry showed that the working area was associated with the prevalence of hearing loss [42]. Furthermore, the noise levels varied according to the mining areas with the average noise levels of 94, 102 and 103 dBA at the plant processing, underground mining area and underground workshop, respectively. In addition, a study conducted in New Java, Indonesia at a navy hospital and steel factory found that the noise exposure levels in the administration room and steel production room with a machine were 60.4 dBA and 102 dBA, respectively. The noise exposure levels in a steel production room were higher than 85 dBA, suggesting an elevated risk of hearing loss among affected workers [43].

The strength of this review is based on the fact that most of the studies included large sample sizes, and the response rates were high. Moreover, the control groups from other workplaces with low noise exposure level were used in most of the included studies to compare with the exposed workers in the metal industry. Furthermore, the audiometric examinations were conducted in most of the studies to confirm the results on hearing loss. Most of the studies included in this review included the area and personal measurements for noise levels. The risk of selection bias for the studies was reduced by searching for studies from different databases.

The limitation of the review is that most of the included studies used cross-sectional study design and data were collected at one point in time without any follow up with participants. In majority of the studies, data was collected from many participants using questionnaires to evaluate the prevalence of hearing loss and determine the associated factors. Cross-sectional study design was used in most of the studies as it allows collection of data from many participants at one point in time. Another limitation is that most of the participants in the included studies were males and there were few female participants. Moreover, the literature search was limited to databases that include ScienceDirect, Google Scholar, Pub Med, ResearchGate and African Journals Online.

5. Conclusions

The review showed a varying prevalence of NIHL among workers in the metal industry. The results of this review emphasize the importance of developing and implementing hearing conservation programmes in the metal industry to protect workers from hearing loss. Future research should evaluate the impact hearing conservation programmes have on the prevention of hearing loss in the metal industry. Moreover, factors such as working experience, age, cigarette smoking, tinnitus and working in an area with high noise levels were found to increase the risk of developing NIHL. When implementing preventative measures in the workplace, it is important to consider these factors. To reduce exposure to high noise levels and occurrence of NIHL in the workplace, it is essential to implement appropriate control measures such as engineering controls, administrative controls and the use of hearing protective devices.