Background
In recent decades, environmental noise pollution has gained attention owing to its adverse impacts, such as cardiovascular and metabolic effects on human health [
1]. Among noise at different time periods during a day, nocturnal noise has been noted to have the strongest association with health problems [
2]. Cardiovascular and metabolic outcomes of noise, e.g., hypertension, ischemic heart diseases, stroke, diabetes, and obesity, have been reported to be associated with noise exposure in some studies, as summarized by Kempen et al. [
1]. For instance, an effect of noise (L
night, outside) on hypertension was proposed to start to occur when the noise level was above 50 dBA [
3]. Despite that there was no sufficient evidence of a threshold for the effect of noise on obesity, there might be a threshold around 45–50 dB (L
den) in the association between noise and waist circumference (WC) [
4].
Hypertension and obesity are risk factors for cardiovascular disease (CVD), which is the leading cause of death globally [
5‐
7]. At least 2.8 million and 17.9 million people die each year due to overweight or obesity and from CVD, respectively [
5,
8]. There is a clear link between obesity and hypertension [
9‐
11]. Specifically, increases in weight, body mass index (BMI), WC, waist-to-hip ratio (WHR), and waist-to-height ratio were associated with a higher incidence of hypertension [
9,
10].
Some other variables have also been identified to play important roles in the association between noise and non-auditory effects. One of them is noise sensitivity. It is suggested that people who are less tolerant of noise are more vulnerable to the negative effects of noise and experience more noise-related health problems [
12,
13], and it was noise sensitivity that predicts the non-auditory effects of noise instead of noise level [
14]. Moreover, the difference in the impact of noise on human beings has also been detected in specific subgroups, such as in different genders [
15]. Additionally, the interaction of noise with lifestyle, such as consumption of alcohol, has also been reported [
16].
Furthermore, a major effect of noise on physical health is assumed to be related to sleep [
2], which is important in regulating hormonal, glucose, and cardiovascular function [
17]. Long-time exposure to noise can cause annoyance and sleep disturbances [
2], with these two factors associated with increased activity in the hypothalamic-pituitary-adrenal (HPA) axis [
2]. This increased activity leads to the elevation of stress hormones such as cortisol and hence, an imbalance in stress regulation [
18,
19]. Elevated cortisol levels are associated with fat accumulation in visceral depots [
20], impaired glucose regulation [
21], and impact BMI, WC, and blood pressure [
22]. A previous study showed that general adiposity, measured by BMI, was associated with an increased death rate [
23]. Moreover, there is increasing evidence that both quantitative and qualitative sleep disturbances could result in obesity and hypertension [
24‐
26].
The evidence on the effect of noise on blood pressure is inconsistent, and relatively fewer studies analyzed the role of noise on obesity or BMI when compared with hypertension [
1]. The WHO Environmental Noise Guidelines have rated the evidence on the association of noise and hypertension as very low due to a response rate of less than 60% reported in many of the studies and the self-reported nature of hypertension measure [
1]. Despite that recent studies incorporating cohort studies with moderate quality showed that exposure to noise increased the risk of hypertension, most of the studies were conducted in Europe [
27,
28]; focused on aircraft, transportation, or occupational noise; and targeted participants residing in detached, semi-detached, or terraced houses [
29]. Moreover, most of the studies have not measured but have estimated or modeled noise levels, which are affected by the bedroom façade, opening of windows, and sound insulation of the house [
2]. However, the indoor noise level is determined by multiple factors and not just by outdoor noise, such as traffic noise [
30]. In high-density and modern cities such as Hong Kong, most people live in multi-story residential buildings. It was reported that people living in tall buildings experience reverberation noise from activities in adjacent flats in addition to traffic noise, which might be different from European neighborhood environment modeled noise [
31]. Moreover, they may also be exposed to sound sources such as public address system and domestic equipment [
31]. Recently, a study focused on multi-story residential buildings in Korea, but the noise levels were still estimated rather than measured [
29]. Lastly, potential moderators and mediators such as noise sensitivity and sleep were not well studied in a community setting.
Therefore, this study aimed to investigate the effect of nocturnal noise on BMI and blood pressure in Hong Kong Chinese adults residing in urbanized areas. The primary hypothesis was that nocturnal noise was positively associated with BMI and blood pressure. The secondary hypothesis was that noise sensitivity and some lifestyle factors would moderate the associations between noise and BMI as well as blood pressure, while objective sleep parameters and subjective sleep quality would mediate the associations. Moreover, the moderating effects of sociodemographic and lifestyle characteristics were also assessed.
Discussion
To the best of our knowledge, this is the first study to investigate the relationships of nocturnal noise level with BMI and with blood pressure in community-dwelling Chinese adults living in multi-story buildings. We demonstrated that an increase in nocturnal noise exposure measured over one week was associated with higher BMI and blood pressure in the daytime.
Previous studies showed that an increase in exposure to modeled noise levels was significantly associated with higher BMI and WC and a higher risk of obesity [
33,
48]. However, Pyko et al. [
49] did not find an association between road traffic noise and BMI, but the association with WC as well as central obesity was significant. The effect of noise on obesity was hypothesized through stress response with the activation of the HPA axis, which could lead to increased levels of stress hormones such as cortisol [
19,
50]. Alterations in cortisol levels are related to metabolic changes, including weight gain [
51]. The association between noise and BMI was only found in females in this study. In an earlier study, an association between noise and obesity was found in women who were highly sensitive to noise [
48]. Specifically, every 10 dB increase in road traffic noise was associated with 0.02 and 0.01 units increase in BMI and WC, respectively, and an odds ratio of 1.24 for WHR ≥ 0.85 in highly noise-sensitive women [
48]. We posited that females might be more annoyed by noise than males [
52]. Therefore, future studies incorporating noise annoyance or other reactions and emotions to noise are desirable to understand the underlying mechanism.
Although the relationship between noise exposure and blood pressure has been extensively investigated, the results, to date, are still inconsistent. Concerning the association between transportation noise and the prevalence of hypertension, the relative risks (RRs) per 10 dBA of road traffic noise, aircraft noise, and rail traffic noise were all approximately 1.05 [
1]. However, some studies failed to find the significant associations between noise and blood pressure in specific population groups such as general women [
53] and pregnant women [
54]. The inconsistencies may result from the sample sizes, self-reported high blood pressure diagnosis, accurate exposure levels of noise, and other potential confounding factors. This study demonstrated the significant association between noise and SBP using measured noise levels and measured blood pressure after adjusting for various confounding factors. This result was consistent with some earlier studies [
55,
56]. Moreover, this study revealed that the association between noise and SBP was stronger in females than in males, which was in line with a previous cohort study of 701,174 residents [
57]. It also demonstrated no statistically significant association between noise and DBP, which was consistent with one previous study [
34]. The associations remained similar when excluding people with hypertension or who were taking medications for hypertension. The difference between the associations of noise with SBP and with DBP might be due to that SBP was more sensitive to noise level [
58]. Furthermore, there could be a threshold noise level for DBP [
59]. When individuals were exposed to a level lower than 80 dB, DBP did not show a significant increase [
60]. The mean nocturnal noise level in this study was around 51 dBA which might be relatively low to cause a significant increase in DBP. On the contrary, a significant association was also reported [
29]. Standardized acoustic stressors and time-dependent analysis might be vital for interpreting the association [
58]. Therefore, the inconsistent results call for more precise and professional research. Nevertheless, the association between SBP and the risk of CVD calls for the control of potential risk factors [
61].
Although the impact of noise on human beings has been found in some specific groups such as females, people who drink alcohol, and highly noise-sensitive people [
15,
16,
48], this study did not find any moderating effects of sociodemographic and lifestyle variables, and noise sensitivity, except for the moderating effect of aging on the association between noise and DBP. For the insignificant associations, it might be due to the difference between acute noise and chronic noise and the threshold of the noise level. Nevertheless, the impact of noise on DBP increases as people aging. The previous study indicated that the impact of noise could and should be differentiated between various groups such as children and the elderly [
62]. Age might moderate the vulnerability to noise on specific health outcomes given specific groups. It was reported that isolated diastolic hypertension was more common in middle-aged groups in the Chinese population [
63]. The mean age of the sample in this study was 39 years, and the middle-aged participants occupied around 50% of the sample. We proposed that age might also moderate the effect of noise on SBP, and a sample incorporating more elderly would be of interest due to the characteristics of systolic blood pressure. Therefore, attention needs to be paid to the effect of age and clarify the possible associations and mechanisms.
It has been hypothesized that noise may affect cardiovascular and metabolic functions through sleep disturbance, where sleep plays a significant role in modulating hormonal release [
49]. However, neither objective sleep parameter nor subjective sleep parameter was identified as the mediator in the association between noise and blood pressure or BMI. The effect of noise on hypertension has been hypothesized to be through the activation of the HPA axis [
50]. Sleep deprivation is associated with the activation of the HPA axis [
64]. Sleep restriction was shown to be associated with increased sympathetic activity and venous endothelial dysfunction [
65]. Hence, people who have sleep disturbance have higher risks of hypertension. In terms of BMI, previous studies have indicated that lack of sleep increases appetite and reduces energy expenditure by affecting serum levels of leptin and ghrelin, thus increasing the risk of being overweight and obese [
66]. The dysregulation of “hunger hormones” due to the anorexigenic hormone leptin levels resulting from sleep disturbances may induce increased food intake [
67]. Sleep deprivation may lead to reduced energy expenditure through altered thermoregulation and increased fatigue [
68]. Hence, the risk of being overweight and obese increases. Nonetheless, important factors such as diet were not investigated in this study. Therefore, future studies cover more sleep parameters and confounding factors are needed to verify how sleep affects the relationship between noise and health problems.
This study used a noise dosimeter to record nocturnal noise levels for a week, which was much more precise than the estimated noise levels used previously. Furthermore, both subjective sleep and objective sleep parameters were included. In addition, noise sensitivity, considered to affect vulnerability, was studied.
However, several limitations are worth noting. First, body weight and height measurements were self-reported by the participants, which might have led to imprecision. WC and laboratory indicators of obesity could also be good indices. Second, we did not include eating habits in this study, yet we were evaluating effects on BMI. Third, blood pressure was not measured at the same time in the participants, which might have affected the results. However, these might not be very feasible in a community setting. The time of blood pressure measurements needs to be recorded in the future, and any differences in neighborhood noise from other noise sources need to be noted. Fourth, future studies need to include the family history of hypertension, considering its predictive role in blood pressure and obesity [
69]. Fifth, this study only measured A-weighted sound levels. Future studies including more characteristics such as frequency and variations are desirable [
70,
71]. Lastly, the sample power might not be large enough to examine certain effects, such as the interaction, and studies with a larger sample size are of interest.
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