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.
Prevalence of NIHL and hypertension
As shown in Fig. 1, 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. Meanwhile, among the included 42,588 subjects, the prevalence of NIHL and hypertension were 24.38% (n = 10,383) and 25.40% (n = 10,816).
Stratified analysis of the levels of SBP and DBP and prevalence of hypertension
Table 2 shows the stratified analysis results of SBP and DBP levels and prevalence of hypertension by study groups. There was a significant 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 significantly 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 significantly 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 significance 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 (Ptrend< 0.001). The multivariate logistic regression analysis showed that age and noise exposure time were significantly 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.
Stratified analysis of binaural high frequency threshold on average and prevalence of NIHL
As shown in Table 3, we found that workers in the subgroups of male gender, smoking and drinking had a significantly higher BHFTA compared with female, non-smoking and non-drinking workers (P < 0.001). Totally, 8,162 males, 4,289 smokers and 3,715 drinkers out of all the recruited subjects were diagnosed as having NIHL, with the prevalence of 25.64%, 30.12% and 26.02%, significantly higher than female (20.66%), non-smoking (21.50%) and non-drinking (23.55%) Male and smoking workers had a significantly higher risk of NIHL (adjusted OR = 1.52, 95%CI = 1.42-1.62; adjusted OR = 1.09, 95%CI = 1.03-1.16). Besides, there was a significant difference in the level of BHFTA between the three subgroups of age and noise exposure time (P < 0.001), and significantly elevated with the cumulative of age and noise exposure time. Correspondingly, workers aged 35-44 (27.44%, P < 0.001) and > 44 (39.79%%, P < 0.001) with BHFTA of 23.86 ± 11.45 dB and 26.47 ± 14.72 dB had significantly higher prevalence rates of NIHL than workers younger than 35 years (13.75%). Workers who exposed to noise time 5-10 years (30.73%%, P < 0.001) and > 10 years (35.15%%, P < 0.001) with BHFTA of 24.30 ± 12.06 dB and 25.16 ± 13.51 dB had significantly higher prevalence rates of NIHL than noise exposure less than 5 years workers (10.67%). Similar results also could be observed in the risk of NIHL, which indicated that the risk of NIHL in the subgroups of age between 35 and 44 (adjusted OR = 1.93, 95%CI = 1.81-2.05) and > 44 years (adjusted OR = 3.16, 95%CI = 2.96-3.38), noise exposure time 5-10 years (adjusted OR = 3.35, 95%CI = 3.14-3.57) and > 10 years (adjusted OR = 2.72, 95%CI = 2.54-2.91) was significantly higher compared to aged < 35 years and noise exposure time < 5 years (P < 0.001).
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 significantly 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 significant 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).