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 NIHL and hypertension were 24.38% and 25.40%. We also found that the prevalence rates of NIHL and hypertension were significantly higher in subgroups of male gender, aged 35-44 and > 44 years, noise exposure time 5-10 and > 10 years, noise exposure level 86-91 and ≥ 92 dB (A), and smoking. Higher risks of NIHL and hypertension were also occurred in the above groups. In addition, there was a significant difference between NIHL and hypertension; the risk of hypertension in NIHL group was 1.07-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 . The WHO reported that mental, behavioral, and neurological diseases affected by noise account for 3% of global deaths and 10% of global burden of disease . Recent evidence showed that long-term exposure to environmental noise could cause some adverse health effects, such as sleep disturbance , annoyance , learning impairment , and hypertension .
Occupational noise, a common harmful factor that seriously affects health of workers in the field of occupational health. Noise-induced hearing loss (NIHL) is one of the worst adverse health effects induced by occupational noise exposure in workplaces . 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% to 21% of workers’ hearing loss, with the lowest incidence in industrialized countries and the highest in developing countries . 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 . Many epidemiological investigations showed that high frequency hearing loss is mainly caused by occupational noise exposure [31, 32]. In this study, the prevalence of NIHL was 24.38%, which was higher than the results from research performed by Lie A et al.  showing that approximately 7 to 21% of NIHL caused by occupational noise exposure. Therefore, we could conclude that NIHL has become an urgent occupational health problem that damages workers’ health in China. In this study, we observed that male workers had a significantly higher risk of NIHL than females. Nelson DI et al.  has reported that the harmful effects of occupational noise exposure on NIHL were larger among males than females. Previous studies also suggested that males may suffer more effects after occupational noise exposure than females . The gender difference might be due to the differences in auditory sensitivity. Besides, another possible explanation may be physiological differences induced by hormones between sexes. Relevant studies have found that estrogen and its signaling pathways may protect women from hearing loss . It has been shown that a key factor contributing to NIHL is oxidative stress damage to sensory hair cells . In the present study, a significantly higher risk of NIHL was observed in workers aged > 35 years, noise exposure time > 5 years, and noise exposure level > 85 dB (A), with adjusted ORs gradually increasing with increasing of age, noise exposure time and exposure level, which was consistent with from previous studies . A recent study has reported that age influenced hearing thresholds over time, and age groups of 30-44 years and 45-59 years workers were at higher risk of exposing to occupational noise and had higher hearing thresholds . Prolonged exposure to occupational noise has been shown to damage cochlear hair cells, thus leading to irreversible NIHL. In addition, the prevalence of hearing loss was also reported to increase with the increasing level of noise exposure [37-39]. 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 was the major product of oxidative stress .
Smoking was found to be associated with an increased risk for hearing loss [41, 42]. Wang D et al.  also found that smoking was an independent risk factor for NIHL, and there was a dose-response relationship between smoking and NIHL risk. In this study, we found that a similar result revealing smoking was significantly associated with the risk of NIHL. However, the specific pathogenic mechanisms underlying smoking increases the risk of hearing loss is not well understood. Available evidence has revealed that smoking may damage cochlea hair cells and cause hearing loss by increasing carboxyhemoglobin or reducing cochlear blood flow to the cochlea . Previous studies found a moderate intake of alcohol had a protective effect on hearing , but frequent alcohol consumption could increase the risk of hearing loss . In contrast, a previous study has found that no significant association between drinking and the risk of NIHL , which showed the same results as our study. Further studies in more detail are necessary to investigate the mechanism underlying this inconsistent.
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 [48, 49]. 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, noise exposure level > 85 dB (A) and smoking were more likely to develop hypertension, the results were in accordance with observed trends in previous studies [13, 40, 50, 51]. Males were observed with much higher blood pressure levels and higher risk of hypertension compared with females. The gender difference might be due to the differences in auditory sensitivity and pathogenesis of cardiovascular diseases. A cross-sectional study observed that a significant dose-response relationship between age, noise exposure time, noise exposure intensity and the risk of hypertension [22, 52]. Similarly, Chang et al.  performed a cohort study recruiting 578 workers and found a significant exposure-response between the noise exposure intensity and the risk of hypertension. Smoking has been identified as a risk factor for coronary heart disease, stroke, and hypertension by abundant evidence [53-55]. Our findings showed that workers with these characteristics were high risk individuals of developing hypertension in occupational noise exposure settings. According to recent reports showed that the consequence of hypertension induced by noise exposure might be due to stress response, thus resulting in the elevated level of stress hormones such as catecholamine [56, 57]. A study on the health effects of environmental noise exposure on cardiovascular disease found that catecholamine could raise myocardial contractibility and enhance the output of blood pressure and cardiac by regulating β-1 receptors . Yet other studies have not found any significant correlation [59-61]. There is still a lack of consensus on this issue. It is remarkable that the differences of results observed in different studies may be influenced by 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 significantly higher level of SBP and DBP compared to those in normal hearing group. Similarly, a study (n = 618) revealed that the mean levels of SBP and DBP in the NIHL group were significantly higher than those in the normal hearing group . Meanwhile, the findings 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 1.07 times that of normal hearing workers. Kuang D et al.  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 findings of another research involving 119 workers indicated that high frequency hearing loss was significantly associated with hypertension . In addition, a study enrolling 790 male aircraft-manufacturing individuals reported that workers with high frequency hearing loss (4 and 6 kHz) had higher risk of hypertension . Our current findings were consistent with the above-described findings regarding the association between NIHL and high levels of SBP and DBP, and hypertension risk. Therefore, our study provided evidence that NIHL may be 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 certified and experienced physician and audiologists, thus ensuring the accuracy and credibility of all data; findings 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 verified 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 important information was not collected, such as hearing protection measure, the usage of headphones, BMI, dietary habit, etc., which were crucial factors influencing 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 identified to have potential risk of developing NIHL and hypertension by screening should be transferred to new job post with less or no noise exposure.