Noise-Induced Hearing Loss and Hypertension: A Preliminary Analysis of Baseline Data of Occupational Noise-Exposed Workers

The present study aimed to determine the current levels of systolic blood pressure (SBP), diastolic blood pressure (DBP) and binaural high frequency (3, 4, and 6 kHz) threshold on average (BHFTA), to assess the prevalence of noise-induced hearing loss (NIHL) and hypertension and determine the factors inuencing the risk of both, and to evaluate the association between NIHL and hypertension among occupational noise-exposed workers. Questionnaire and occupational health checkup were performed to collect the personal information and physical examination data. Finally, baseline data from 42,588 occupational noise-exposed workers were analyzed. The mean levels of SBP, DBP, and BHFTA were 126.85 ± 15.94 mm Hg, 79.94 ± 11.61 mm Hg and 23.09 ± 11.32 dB, respectively. Of the 42,588 subjects, the prevalence of NIHL and hypertension were 24.38% (n = 10,383) and 25.40% (n = 10,816). The results suggested that higher risk of NIHL and hypertension were more likely to be the groups of male gender, aged > 35 years, exposure time to noise > 5 years, and smoking. The SBP, DBP and BHFTA levels increased with age and noise exposure time (P trend < 0.001). Besides, similar trends were also observed in the prevalence of NIHL and hypertension. Furthermore, there was a signicant association of NIHL with hypertension. We found that 32.25% (n = 3,348) workers with NIHL had hypertension. The mean levels of SBP and DBP in NIHL workers were signicantly higher than those with normal hearing (P < 0.001). In addition, NIHL workers had a signicantly higher risk of developing hypertension than normal hearing workers (adjusted OR = 1.08, 95%CI = 1.03–1.14). Our current ndings suggest that the prevalence of NIHL and hypertension are high in our study workers and occupational noise exposure is an important factor. Therefore, it is urgent to reduce noise exposure in the workplaces and to strengthen industrial noise monitoring. the ascending pure tones at the frequencies of 0.5, 1, 2, 3, 4, and 6 kHz. all hearing a ensure that the obtained results reect the true levels of hearing thresholds of In this study, workers whose binaural high frequency (3, 4, and 6 kHz) threshold on average (BHFTA) greater than 25 dB were dened as NIHL, while those less than 25 dB were dened as normal


Introduction
Noise is a widespread occupational hazardous factor affecting the health of workers in occupational health eld. To our knowledge, the most direct impact of noise is the damage to the auditory system, while the most serious health effect is irreversible noise-induced hearing loss (NIHL). NIHL is a progressive sensorineural hearing loss characterized by noise-induced auditory threshold shift at high-frequency. NIHL is a major occupational-related disease and is a severe occupational health problem in the workplace safety (Miao et al. 2019). It is worth noting that about ten percent of the world's population permanently are expose to high-intensity noise environment and are at risk of developing NIHL . Recently, with the rapid development of industry in China, noise is widespread in various industrial places, and a larger number of workers exposed to noise. Nowadays, NIHL has been the third largest occupational health hazard in China, the incidence of which has been increasing (Yu 2016). Recent studies have found that 16% of worldwide hearing loss in adults caused by occupational noise (Nelson et al. 2005). NIHL can cause communication disorder and result in social isolation, thus leading to self-abasement and cognitive decline and depression (Samelli et al. 2021). Therefore, it is an extreme important public health goal to prevent NIHL and reduce its adverse effects.
Except for adverse effects on auditory function, noise, as a psychosocial stressor, could induce biological effect on cardiovascular system and lead to hypertension (Ising &Kruppa 2004;Sbihi et al. 2008). Previous studies suggested that the mechanism by which noise causes hypertension may be through activation of the hypothalamus-pituitary-adrenal gland and sympathetic nervous system, thus resulting in elevated levels of norepinephrine, epinephrine, and cortisol ). Some studies have documented that workers exposed to occupational noise had a distinctly elevated blood pressure (Chang et al. 2003; Green et al. 1991) or had a higher risk of hypertension (Tomei et al. 2010;Zhao et al. 1991). It has reported that 9.4% of cardiovascular disease (CVD) deaths are caused by hypertension (Lim et al. 2012). Hence, it is necessary to take early intervention for the prevention of noise-induced hypertension and its adverse effects.
Occupational noise as a prevalent occupational health risk may be present in many workplaces affecting workers' health and putting the workers' health at a high risk. In addition, considering the harmful effects of occupational noise on human health, we conducted this study to investigate the status of NIHL and hypertension, determine the potential risk factors, and evaluate the association between NIHL and hypertension by analyzing the baseline data from occupational health checkups of occupational noise-exposed workers.

Study subjects
In this cross-sectional study, a total of 44,305 workers exposed to occupational noise who accepted occupational health checkups conducted from 2015 to 2017 by Yangzhou Center for Disease Control and Prevention. To avoid possible biases, a total of 1,717 workers who gave incomplete information of occupational noise exposure, did not accomplish the occupational health examinations, declined to participate, had a hereditary family history of deafness and a history of diagnosed hearing loss, otitis media, skull trauma, nervous tinnitus that could impair normal hearing, had a hereditary family history of cardiovascular diseases and hypertension, medical diagnosis history of hypertension, and previous or current use of drugs that could cause hearing loss and hypertension were excluded from this study. Finally, a total of 42,588 workers meeting the above inclusion criteria were recruited in this study. This study was reviewed and approved by the Ethics Committee of Zhongda Hospital, A liated Hospital of Southeast University before this study commenced. Informed consents were also obtained from each participant.

Questionnaire survey
A structured questionnaire was performed to collect information on demographic characteristics of all workers before occupational health checkups. The information included age, noise exposure time, hereditary history of deafness and hypertension, family history of cardiovascular diseases, smoking and drinking status, history of earrelated diseases (ear infections, head injury/trauma, and tinnitus), previous medical diagnosis of hypertension, and previous or current use of drugs was gathered. The information was used to screen participants for the current study. The Details of the de nitions of smoking and drinking have been proposed elsewhere (Miao et al. 2021). In brief, workers had one cigarette per day for the at least one year were de ned as smokers. Drinkers were de ned as workers consumed a bottle of beer or fty grams of wine per day for at least one year.
Pure-tone audiometric examination and NIHL de nition Pure-tone audiometric (PTA) examination was performed by a professional audiologist with a pure tone audiometer (Madsen Voyager 522, Taastrup, Denmark). All recruited workers underwent a PTA test after being away from the occupational noise exposure environment for at least fourteen hours or longer. According to the diagnostic criteria (GBZ 49-2014), hearing thresholds of left and right ears were assessed with the method of ascending pure tones at the frequencies of 0.5, 1, 2, 3, 4, and 6 kHz. Furthermore, all hearing examinations were conducted in a soundproof room to ensure that the obtained results re ect the true levels of hearing thresholds of workers. In this study, workers whose binaural high frequency (3,4, and 6 kHz) threshold on average (BHFTA) greater than 25 dB were de ned as NIHL, while those less than 25 dB were de ned as normal hearing.
Blood pressure measurement and hypertension Before blood collection and blood pressure measurement, all workers were asked to fast overnight. Blood pressure was evaluated by an experienced physician using an automated sphygmomanometer (TM-2655P, A&D Corp., Japan) after workers sat for more than 10 minutes rest. The average value of three repeated blood pressure measurements with 30 seconds interval was applied to re ect the true level of an individual's blood pressure. Workers who had mean resting systolic blood pressure (SBP) ≥ 140 mm Hg and/or mean resting diastolic blood pressure (DBP) ≥ 90 mm Hg were de ned as hypertensive.

Statistical analysis
Descriptive statistics were computed as mean, standard deviation (SD) and frequency percentages. Shapiro-Wilk test was used to determine the normality of continuous variables including SBP, DBP, and binaural high frequency hearing threshold. Student's t test and Kruskal-Wallis test were applied to determine the signi cance differences of continuous variables. Two-side χ 2 test was applied to examine statistical differences in frequency distribution of category variables. For the signi cantly different group, the Mann-Whitney test and χ 2 test were used to compare the statistical signi cance of variables among groups. In addition, multivariate logistic regression, calculated odds ratios (ORs) and 95% con dence intervals (CIs) to assess the strength of association between selected variables and NIHL and hypertension by controlling potential confounding factors, such as age, gender, noise exposure time, smoking and drinking status. The SPSS version 23.0 (SPSS, Chicago, Illinois, USA) for Mac was performed for all statistical analyses. The statistical signi cance was set as P values < 0.05.

Results
Characteristics of study subjects Table 1 summarizes the demographic characteristics of this cross-sectional study comprised 42,588 subjects who exposed to occupational noise. Among the recruited subjects, 74.76% were male (n = 31,838), and 25.24% were female (n = 10,750). The mean age was 36.58 ± 9.56 years, with 44.30% of workers were at the age of 18-34 years, 31.38% were 35-44 years, and 24.33% were older than 45 years. The mean exposure time to noise was 9.20 ± 8.38 years, with 38.57% of workers were exposed to noise for less than 5 years, 30.03% were in the range of 5-10 years, and 31.40% had a noise exposure history of greater than 10 years. Furthermore, in all subjects, 33.44% and 33.52% were smokers and drinkers.
Strati ed analysis of the levels of SBP and DBP and prevalence of hypertension Table 2 shows the strati ed analysis results of SBP and DBP levels and prevalence of hypertension by study groups. There was a signi cant difference in the levels of SBP and DBP, and the prevalence of hypertension between different gender, age, noise exposure time, smoking and drinking groups (P < 0.001). The male, smoking and drinking workers had signi cantly higher levels of SBP and DBP, and prevalence rates of hypertension than the female, non-smoking and non-drinking workers. Besides, we found that a similar result revealing that males and smokers were at signi cantly higher risk for hypertension (adjusted OR = 2.19, 95%CI = 2.05-2.34; adjusted OR = 1.11, 95%CI = 1.05-1.17, respectively). There was a statistical signi cance in the SBP and DBP levels among the groups with different age and noise exposure time, and both SBP and DBP levels increased with the gradual increase of age and noise exposure time (P trend < 0.001). The multivariate logistic regression analysis showed that age and noise exposure time were signi cantly associated with hypertension even after adjusting for potential confounders. The subgroups of age 35-44 and > 44 years had a 2.33-fold and 3.82-fold greater OR for hypertension than age less than 35 years. A similar result in the subgroups of noise exposure time 5-10 and > 10 years was also found.

Analysis of association between NIHL and hypertension
To determine the association between NIHL and hypertension, the subjects were divided into normal hearing and NIHL groups. The results showed that the mean levels of SBP (130.03 ± 16.89 mm Hg) and DBP (82.16 ± 12.21 mm Hg) in 10,383 NIHL workers were signi cantly higher than those with normal hearing (125.82 ± 15.48 mm Hg for SBP and 79.23 ± 11.32 mm Hg for DBP) (P < 0.001). The prevalence of hypertension in normal hearing group was 23.19%, while 32.25% in NIHL group, and a statistically signi cant difference was found between the two groups (P < 0.001). Furthermore, compared with normal hearing workers, NIHL workers had 1.08 times higher risk of hypertension after correcting confounding factors (adjusted OR = 1.08, 95%CI = 1.03-1.14) ( Table 4).

Discussion
Based on the data from occupational health checkups of 42,588 occupational noise-exposed workers, we investigated the levels of SBP, DBP and BHFTA, prevalence of NIHL and hypertension. Besides, we evaluated the association between NIHL and hypertension. Among the included 42,588 subjects, we found that the prevalence of hypertension and NIHL were 25.40% and 24.38%. We also found that the prevalence rates of NIHL and hypertension were signi cantly higher in subgroups of male gender, aged 35-44 and > 44 years, noise exposure time 5-10 and > 10 years, and smoking workers. Higher risks of NIHL and hypertension were also occurred in the above study groups. In addition, there was a signi cant difference between NIHL and hypertension; the risk of hypertension in NIHL group was 1.08-fold greater than in normal hearing group.
Generally, noise is considered as a set of sounds that could make people irritable or is loud enough to endanger everybody. In environment, there are diverse sources of noise, including transportation noise, occupational noise, and construction noise. Nowadays, noise pollution has been an important and neglected public health issue in the world. Adverse health effects of noise on human beings can be observed on psychologically and physiologically (Muzet 2007). The WHO reported that mental, behavioral, and neurological diseases affected by noise account for Occupational noise, a common harmful factor that seriously affects health of workers in the eld of occupational health. Noise-induced hearing loss (NIHL) is one of the worst adverse health effects induced by occupational noise exposure in workplaces (Masterson et al. 2016). The WHO reported that about 10% of the world's population is exposed to high level of noise and at risk of progressing to NIHL . A recent review revealed that occupational noise exposure resulted in 7-21% of workers' hearing loss, with the lowest incidence in industrialized countries and the highest in developing countries (Lie et al. 2016). According to previous study, NIHL is one of the leading occupational related disease in China, accounting for approximately one-sixth of the annual increase of occupational disease (Yu 2016). Many epidemiological investigations showed that high frequency hearing loss is mainly caused by occupational noise exposure (Alabdulwahhab et al. 2016;Maccà et al. 2015). In this study, the prevalence of NIHL was 24.38%, which was higher than the results from previous research conducted by Lie A et al. (2016) showing that approximately 7 to 21% of NIHL caused by occupational noise exposure. Thus, we could conclude that NIHL has become an urgent occupational health problem that damages workers' health in China. Nelson DI et al. (2005) reported that the harmful effects of occupational noise exposure on NIHL were larger among males than females, which was consistent with our current study. It has been shown that a key factor contributing to NIHL is oxidative stress damage to sensory hair cells (Yuan et al. 2015). In the present study, a signi cantly higher risk of NIHL was observed in workers aged > 35 years, and noise exposure time > 5 years, with adjusted ORs gradually increasing with increasing of age and noise exposure, which was corroborated from previous studies (Hormozi et al. 2017). Meanwhile, it should be worth noting that aging and duration with noise were the most common cause of NIHL, and both closely related to the formation of reactive oxygen species, which It has been reported that persons with chronic and repeated exposure to noise appeared a persistent increased of blood pressure or a higher risk of hypertension (Dzhambov &Dimitrova 2018; Liu et al. 2016). In this study, altogether, 25.40% of the workers included in our study had hypertension, suggesting a higher incidence of this disease. Workers particularly in the subgroups of male gender, aged > 35 years, exposure time with noise > 5 years, and smoking were more likely to develop hypertension, which in accordance with observed trends in previous studies (Evrard et  . We concluded that workers with the above characteristics were high risk individuals of developing hypertension in occupational noise settings. According to recent reports showed that the consequence of hypertension caused by noise exposure might be attributed to stress response, thereby resulting in the elevated level of stress hormones such as catecholamine (Ghotbi et al. 2013;van Kempen et al. 2002). Moreover, a study on the effects of environmental noise exposure on CVDs has revealed that catecholamine could raise myocardial contractibility and enhance the output of blood pressure and cardiac by regulating β-1 receptors (Münzel et al. 2014). Yet other related studies have not found any signi cant correlation (Gan &Mannino 2018; Stokholm et al. 2013;Zamanian et al. 2013). Hence, there is still a lack of consensus on this issue. It is remarkable that the differences of results observed in different studies may be in uenced by the population studied and auditory sensitivity. Therefore, further studies in more detail are needed to precisely determine the mechanism underlying this inconsistent.
To explore the association between NIHL and hypertension, we divided the workers into normal hearing group and NIHL group according to the level of BHFTA. The current study suggested that NIHL workers had a signi cantly higher level of SBP and DBP compared to those in normal hearing group. Similarly, a study (n = 618) performed by Ni CH et al. revealed that the mean levels of SBP and DBP in the NIHL group were signi cantly higher than those in the normal hearing group (Ni et al. 2007). Meanwhile, the ndings of our study suggested that the prevalence rate of hypertension in NIHL differed from that in normal hearing workers by 9.1%; the risk of developing hypertension in NIHL workers was 1.08 times that of normal hearing workers. Kuang D et al. (2019) investigated the association of high-frequency hearing loss and hypertension among occupational noise exposed workers and indicated that workers with mild high-frequency hearing loss (hearing level > 25 but < 40 dB) had a higher hypertension risk of 34%. Also, the ndings of another research involving 119 workers indicated that high frequency hearing loss was signi cantly associated with hypertension (Tarter &Robins 1990). Moreover, a cross-sectional study included 790 male aircraft-manufacturing individuals had reported that workers with high frequency hearing loss (4 and 6 kHz) had higher risk of hypertension (Chang et al. 2011). Our current ndings were consistent with the above-described ndings regarding NIHL association with higher levels of SBP and DBP, as well as hypertension risk. Therefore, our study provided evidence that NIHL was associated with the risk of hypertension and it may act a potential biomarker of noise-induced hypertension for occupational noise-exposed workers.

Strengths And Limitations
Our present study is a cross-sectional study based on the data from occupational health checkups of 42,588 occupational noise-exposed workers. Strengths of the study are large sample size and high response rate among the subjects, as well as the occupational health checkups performed by certi ed and experienced physician and audiologists, thus ensuring the accuracy and credibility of all data; ndings obtained based on this large sample size is more convincing. Besides, subgroup analysis of variables was conducted to avoid potential confounding factors. However, several limitations are also present in the current study and need to be mentioned. First, this study was based on a cross-sectional analysis, the results should be veri ed by prospective cohort studies in the future. Second, the questionnaire survey based on self-report measures of participants, which may cause bias to the results of the study. Third, some data and information were not collected, including levels of noise exposure in the workplaces, hearing protection measure, the usage of headphones, BMI, which were crucial factors in uencing hearing loss and hypertension. Hence, in future studies, we will pay more attention to collecting the above missing data to make the results more convincing.

Prevention Recommendations On Noise Health Hazards
It can conclude that the prevalence rates of NIHL and hypertension are high in our study workers and occupational noise exposure is an important factor. We suggest that reducing noise exposure in the workplaces and improving preventive measure to prevent occupational noise-exposed workers from NIHL, hypertension, and other adverse health effects. In addition, occupational noise-exposed workers should strengthen own protection consciousness and wear earplugs properly while working.
Regular PTA examination and blood pressure monitoring should be performed by physicians among occupational noise-exposed workers. Also, safety standards for noise in the workplaces should also be monitored regularly to decide if compliance with the safety standards of industrial noise. Finally, if workers were identi ed to have potential risk of developing NIHL and hypertension by screening should be transferred to new job post with less or no noise exposure.

Conclusions
In this study, based on the data of occupational health checkups from 42,588 occupational noise-exposed workers, we can draw the following conclusions: (1) a large number of workers exposed to occupational noise, and the mean noise exposure time was > 9.20 years; (2) the prevalence rates of NIHL and hypertension were 24.38% and 25.40%, respectively; (3) the prevalence of NIHL and hypertension increased with age and noise exposure time; (4) signi cantly higher risk of NIHL and hypertension was observed especially in the group of male gender, aged > 35 years, noise exposure time > 5 years, and smoking; (5) nally, of all the study participants, 32.25% workers with NIHL had hypertension. NIHL workers were at higher risk of developing hypertension than that of normal hearing workers.
The ndings of this study suggest that occupational noise is an important occupational health risk affecting workers' health and putting the workers' health at a high risk. Hence, it is urgent to reduce noise exposure among workers in workplaces by performing regularly industrial noise monitoring and health risk evaluation, strengthening the implementation of safety standards of industrial noise, and raising workers' awareness of wearing earplugs properly, and carrying out high-risk population screening.

Declarations
This study was supported by the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University.

Data availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.
Compliance with ethical standards

Con ict of interest
The authors declare that they have no con icts of interest.

Ethical approval
The present study was approved by the Ethics Committee of Zhongda Hospital, A liated Hospital of Southeast University.

Consent to participate
Informed consent was obtained from all individual participants included in the study.

Consent to publish
The participant has consented to the submission of the case report to the journal.