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Introduction
Noise-induced hearing loss (NIHL) is an irreversible and gradually developing condition caused by exposure to excess noise. The disorder affects the sound-receiving department of the auditory analyzer (neuroepithelial structures of the inner ear) and is manifested as a chronic bilateral sensorineural hearing loss (Mohammadi, Mazhari, Mehrparvar, & Attarchi, 2009). The given health outcome is widespread in the United States. According to the statistical data provided by the National Institute on Deafness and Other Communication Disorders (2017), approximately 24 percent of the U.S. adults aged 20 to 69 years have symptoms of NIHL in one or both ears. A high rate may indicate the lack of awareness of the disease risk factors, symptoms, identification techniques, and prevention methods in the population. The research of NIHL is important and presents a significant interest as it may help to design strategies aimed to reduce the incidence of the given public health issue.
Occupational exposure to excess noise (over 85 dBA) is one of the major factors contributing to NIHL development. As Kitcher, Ocansey, Abaidoo, and Atule (2014) state, the number of hearing loss cases reported in different mill markets is three times greater if workers exposed to high levels of noise and do not undertake the preventive measures. Additionally, Wells et al. (2015) claim that over 7 percent of the deployed U.S. Army soldiers are diagnosed with NIHL, and the exposure to high intensity and impulse noise during the military operations is considered to be the major contributing factor.
In order to understand the relationships between the environmental factor and health outcome selected for the study, we will evaluate three articles in this paper. One of the articles is a cohort study, and two others are cross-sectional studies. All three papers are focused on the investigation of the effects of occupational noise on different population groups. The review findings will be evaluated based on Bradford Hill Criteria, and summarized in the table. The findings of the literature review will allow us to identify possible clinical, individual, and social implications associated with the health concern and the physical hazard existing in industrial and military working environments.
Methods
The selected articles were located via Washington State University online database by sing Smart Search option. Since the study is devoted to the research of the relations between the health outcome and one factor defining it, we used only two keywords including “NIHL” and “occupational noise.” In order to ensure a higher level of credibility and validity, we focused on research papers published in high-quality scholarly and professional journals such as Noise and Health and European Journal of Public Health. As a result, we chose one cohort study that addresses the problem of hearing impairments in the military environment, and two cross-sectional studies focusing on the same health problem in the industrial environment. Additionally, one of the cross-sectional papers analyzes how smoking may increase the chances of NIHL development.
Results
Study 1: Hearing Loss Associated with US Military Combat Deployment
In their article, Wells et al. (2015) employ data collected from the Millennium Cohort Study (MCS) – “a longitudinal cohort study designed to assess the effects of US military service on the health of participants over a follow-up period of at least 21 years” (p. 35). The researchers aim to identify the prevalence of risk for hearing loss among the U.S. soldiers and how it is correlated with their deployment experiences. Wells et al. (2015) evaluated the sample comprised of 48.540 participants. According to the MCS data, 7.5 percent of the assessed U.S. military members self-reported the onset of hearing impairment throughout the follow-up period. The initial analytical tools used by the researchers were Chi-square tests, percentages, and frequencies. The secondary analysis was conducted by using multivariable logistic regression which allowed identifying the odds of the progress in hearing loss development and their links to combat deployment, exposure to head traumas, and explosion blasts.
Noise is a significant physical hazard in the military service environment. The researchers suggest that the exposure to excess noise may be the major cause of suffering additional hearing damage in the target population, and especially in those military members who report a greater incidence of combat exposures. Along with the noise determinants to hearing impairments, the researchers investigated other contributing factors including combat exposures, smoking status, exposure to chemicals and pesticides, the use of protective devices, etc.
To substantiate their hypothesis, Wells et al. (2015) drew the statistical data retrieved from another study which indicates that hearing loss constitutes over 68 percent of post-deployment diagnoses in soldiers. The research findings are consistent with the evidence obtained from the previous studies related to a similar subject. Wells et al. (2015) observe that the deployed individuals with a higher level of combat experience tend to report the onset of hearing impairments in nearly 1.5 times more frequently comparing to the non-deployed individuals. Combat experiences are regarded as an essential determinant for the development of hearing impairment in soldiers. It is important to notice that combat is usually associated with the increased amount of impulse noise, i.e., “noise with a duration of <1 second and with peak levels 15 dB louder than background noise,” produced by various weapons and detonation of explosives (Wells et al., 2015, p. 38).
Along with this, the likelihood of hearing impairment proportionally increases if a person was exposed to combat-related head traumas or was insignificant proximity to an explosive blast. Soldiers who had head traumas reported hearing loss in six times for often, while those who were proximate to a blast – in two times more frequently. Wells et al. (2015) also claim that such factors as tobacco use, advancing age, non-black ethnicity, and exposures to other occupational hazards may contribute to the development of hearing loss in soldiers.
Study 2: Occupational Hearing Loss of Market Mill Workers in the City of Accra, Ghana
In their study, Kitcher et al. (2014) investigated the correlation between a continual exposure to excess noise (higher than 85 dBA) and hearing loss in a sample of Ghanian market mill workers (n = 101). The researchers measured the participants’ pure-tone hearing thresholds and then compared the data to the similar measures collected from the control group comprised of small-scale traders (n = 68). Additionally, Kitcher et al. (2014) aimed to evaluate the prevalence of NIHL in market mill workers, as well as the level of their awareness of the effects of noise on health.
To measure the occupational noise in the selected setting, the researchers used a standardized RS-232 sound level meter. Additionally, they administered the audiometric assessment of the study participants and the control group members in fourteen hours after their last exposure to occupational noise. The researchers carried out a standard eight-frequency pure tone audiometric behavioral threshold test at frequencies from 250 to 8000 Hz. They identified hearing damages by implementing the average frequencies of 500, 1000, 2000, and 4000 Hz; and then classified the collected data according to the criteria proposed by the World Health Organization. The hearing threshold level of lower than 25 dB implies normal hearing, 26-40 dB threshold level – mild hearing loss, 41-60 dB threshold – moderate hearing loss, and 61-80 dB threshold level – severe hearing loss (Kitcher et al., 2014). The initial data was evaluated using the Statistical Package for the Social Sciences (SPSS). A t-test was conducted to contrast the average threshold levels of all study participants for different pure tone frequencies.
The average level of noise in the market mill ranged from 85.9 to 110.8 dBA, while the controls were exposed merely to an average level of noise ranging from 61.5 to 69.4 dBA. It means that noise in the selected industrial setting exceeded the maximum permitted level (85 dBA). At the same time, the analysis results indicate the sensorineural hearing loss in one or both ears in nearly 40 percent of study participants, while only 3.9 percent of controls showed the symptoms of the disorder.
Another important finding in the study by Kitcher et al. (2014) is related to employees noise-protective behaviors. The researchers observe that 5 percent of study participants reported the regular use of hearing protection devices. At the same time, the largest part of the sample population (54.5 percent) had sufficient awareness of the negative impacts of exposure to noise. Thus, there is no correlation between knowledge about the effects of excess noise on employees behaviors.
Lastly, Kitcher et al. (2014) state that the data on self-reported hearing loss and the results of audiometric testing were not similar. It means that the majority of the assessed individuals do not know how to recognize the impairment.
Study 3: Cigarette Smoking and Occupational Noise-Induced Hearing Loss
The cross-sectional study by Mohammadi et al. (2009) is devoted to the investigation of the relationship between such factors contributing to NIHL as noise and cigarette smoking. It is suggested that tobacco use may increase the risk for the development of hearing loss and is regarded as one of the causes of cochlear damage (Mohammadi et al., 2009). The research of links between the two factors is of great interest because smoking is very common among workers.
The study setting selected by Mohammadi et al. (2009) was a large Iranian wagon manufacturing factory where the noise was the major and sole occupational hazard pertaining to hearing loss. The factory employees were regularly exposed to excess noise levels over 85 dBA. The total study sample was comprised of 252 smokers and 453 non-smokers. All study participants were males. The study was conducted as a part of the regular medical examinations. Along with clinical data, the researchers collected self-reported information by using questionnaires. The analysis was carried out by implementing the SPSS. A t-test was administered for quantitative variables, and a Chi-square test – for the qualitative ones. Additionally, the researchers applied the regression analysis. The estimated average age of the workers was 42.25 years, the average time of exposure to noise – 18.14 years, and the mean hearing loss rate at 4000 Hz – 25.46 dB.
The analysis results indicate a significant difference in predisposition to NIHL in smokers and non-smokers. Although both study populations were exposed to excess noise in a similar degree, Mohammadi et al. (2009) observe that the likelihood of hearing impairment significantly increases when the pack-years ratio related to smoking increases accordingly. The researchers claim that tobacco use modifies blood viscosity and oxygen saturation. In this way, it may contribute to the impairment of cochlear circulation which may consequently result in hearing loss.
Discussion
Based on the study results discussed in the previous section, it is possible to presume that hearing loss may be largely defined by the level of occupational noise and the overall period of employees’ exposure to it. NIHL is usually manifested as the sensorineural impairment in one or both ears and is characterized by the increase in the sound reception thresholds at the initial stages of disorder development due to the long-lasting contact with the excess noise. However, it is also possible to say that the likelihood of hearing ability deterioration does not depend on the level of noise alone can be defined but multiple environmental and individual behavioral and health factors as well. For instance, as Mohammadi et al. (2009) state, when combined with exposure to occupational hazards, such endogenic determinants as smoking-related hypertension and cardiovascular conditions increase the risks for NIHL progression. Wells et al. (2015) also note that such demographic factor as age may largely affect the course of the disorder development. Thus, when trying to identify the causes of hearing impairments in individuals, one should consider the changes in sound reception ability associated with aging and other etiologic factors.
The main instrument used by the researchers in the selected studies was the pure-tone threshold audiometry – a standardized tool which helps to identify the symptoms and the type of NIHL. However, the fact that audiometry does not provide any data on etiology and pathogenesis of hearing loss may be considered the major weakness associated with the assessment instrument. Therefore, 2e should consider the non-specificity of clinical testing measures used in the studies to indicate NIHL and the polietiologic nature of the health problem.
Nevertheless, despite the potential limitations of the assessment tool, the studies provided sufficient information supporting the direct links between the exposure to high level of noise and hearing loss. For example, the findings obtained by Wells et al. (2015) demonstrate that the incidence of hearing impairment and hearing loss among military veterans is greater than in members of the civilian population. Therefore, the continual military service characterized by the exposure to multiple physical hazards and high-impulse noise, in particular, is correlated with hearing loss. Additionally, the evidence provided by Kitcher et al. (2014) makes it clear that market mill workers who are regularly exposed to noise over 85 dBA are predisposed to the development of the hearing impairment in a greater extent than traders who work in less hazardous environments. The researchers observe that the pattern of hearing impairment in workers, “coupled with the significant presence of 4 kHz audiometric notch” implies that the hearing loss among the study participants was likely associated with NIHL (Kitcher et al., 2014, p. 186). First of all, the differences in hearing impairment rate in two population groups (continually exposed to excess noise and non-exposed) signify that there are the direct relations between the hazardous physical factor and the evaluated health outcome.
Overall, it is possible to say that excess noise is the primary cause of hearing loss that may be developed in unfavorable working environments. There is a significant dependence between the health outcome and the level, and duration of noise exposure, as well as various socio-demographic determinants. These secondary factors define the dynamic nature of the relationship between the noise factor and individual health outcomes. It is possible to conclude that the combination of those factors may increase the risk for hearing loss development.
Conclusion
According to the researchers, about 600 million employees around the globe are exposed to occupational noise (Mohammadi et al., 2009). Since the findings of the literature review support the assumption that the exposure to noise exceeding the permissible levels is detrimental to human health and can be regarded as the major risk factor for NIHL, it is essential to design prevention and intervention strategies aimed to reduce the incidence of hearing loss within the target population. Occupational hearing loss is a topical public health concern. It leads to adverse consequences at both individual and social levels affecting the functionality of people diagnosed with NIHL and leading them to social isolation. It is apparent that the condition deteriorates the quality of life and, therefore, the effective methodology should be designed to reduce the current rate of occupational hearing loss.
As Mohammadi et al. (2009) state, “an important point is that NIHL is permanent and irreversible, but it is completely preventable” (p. 452). The given statement emphasizes the importance of policies regulating employees’ behaviors, working environments, health monitoring criteria, and training on health risks and prevention methodologies.
The review of the literature helped to understand that individual employees’ noise-protective behaviors and the level of awareness are correlated with the progress of hearing impairment development. Overall, the irregular use of protection devices, along with the inability to identify the hearing loss without the professional help identified by Kitcher et al. (2014), prompt the areas which should be addressed in the prevention strategy. First of all, the government and organizational control of occupational hearing protection should be increased because it seems that the absence of efficient policies contributes to the high prevalence of the adverse condition among workers in the industrial sectors associated with the exposure to excess noise. Moreover, it can be recommended to make regular audiometric testing and assessment of the working environment the integral parts of a complete NIHL prevention program.
Due to the multifactorial nature of NIHL, the policies and prevention programs should target as many areas of concern as possible. We should consider that if preventive measures implemented independently of each other, their positive effect may be reduced. Therefore, it is important to develop an integrated approach to improving the situation by targeting many factors contributing to NIHL simultaneously. Therefore, a population-based prevention program is needed. When developing the policy, we should focus on both qualitative and quantitative aspects of prevention efforts. The policy should stimulate the involvement of as many stakeholders, individuals, and organizations as possible. Moreover, it should address causal and, at the same time, modifiable risk factors such as employees’ behavior (e.g., promotion of healthier lifestyles, smoking cessation, etc.). In this way, it will be possible to reduce NIHL incidence and achieve sustainable positive outcomes.
Appendix 1: Summary of Studies
Appendix 2: Bradford Hill Criteria
References
Kitcher, E., Ocansey, G., Abaidoo, B., & Atule, A. (2014). Occupational hearing loss of market mill workers in the city of Accra, Ghana. Noise and Health, 16(70), 183. Web.
Mohammadi, S., Mazhari, M., Mehrparvar, A., & Attarchi, M. (2009). Cigarette smoking and occupational noise-induced hearing loss. European Journal of Public Health, 20 (4), 452–455.
National Institute on Deafness and Other Communication Disorders. (2017). U.S. adults aged 20 to 69 years show signs of noise-induced hearing loss. Web.
Wells, T., Seelig, A., Ryan, M. K., Jones, J., Hooper, T., Jacobson, I., & Boyko, E. (2015). Hearing loss associated with US military combat deployment. Noise and Health, 17(74), 34. Web.
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