This study reveals that occupational noise exposure increases the risk of diabetes incidence. This relationship was significant in time-dependent Cox models of two hospital cohorts, as well as the pooled cohort. All models in each group showed statistical significance in terms of the relationship between occupational noise exposure and diabetes. The relationship was significant even after adjusting for potential confounding variables. Further, a Landmark analysis, a method to reduce immortal time bias, was performed as a sensitivity analysis, and the results showed a statistically significant relationship between occupational noise exposure and increased risk of diabetes.
Our time-dependent Cox and Landmark analysis models adjusted for several covariates that are established risk factors of diabetes. This includes demographic factors (age and sex), lifestyle factors (BMI, smoking, exercise, and drinking alcohol), and clinical history (hypertension)34–36. Furthermore, by adjusting for factors of cardiovascular-related exposure, in addition to the existing commonly known variables, it was possible to reduce the bias caused by exposure from various work environments.
There have been several studies that elucidate the relationship between lifestyle factors and improvement of diabetes symptoms. A strategy such as the Diabetes Prevention Program (DPP) in the workplace, for prevention of type 2 diabetes mellitus was promoted to manage costs and improve population health37. In 2002, the DPP research group demonstrated that a 7% body weight loss and 150 minutes of physical activity per week could reduce a three-year incidence of type 2 diabetes mellitus among people with prediabetes, by 58%37. The DPP lifestyle intervention has since been developed into a year-long, group-based program that helps people lose weight in a variety of clinical settings38,39. However, there is insufficient research on occupational environmental factors of diabetes, compared with lifestyle factors, and intervention research is especially lacking. Intervention studies on preventing occupational noise exposure have been focused on hearing loss40. Considering that a lot of workers are exposed to occupational noise16, policies for improving the health impacts of those workers are imperative. The strong relationship between diabetes and occupational noise exposure demonstrated in this study contributes to the recognition of occupational environmental factors as an important risk factor in diabetes incidence. Future studies focused on policies and protection guidelines should be implemented.
Several studies have elucidated the association between severe occupational noise exposure and diabetes; however, their results were mostly negative, and they had limitations. Dzhambov (2017) conducted a cross-sectional study with the 7th European Social Survey and showed a non-significant result of the relationship between occupational noise exposure and diabetes (Odds ratio (OR) 1.01 [95% CI 0.78–1.32])25. However, in this study, noise exposure was defined as being exposed to a very loud noise even once, which is less accurate for stratifying noise exposure. Dzhambov (2015) also conducted a meta-analysis of long-term noise exposure and the risk for diabetes. Occupational noise exposure studies yielded insignificant results of relative risk of diabetes (pooled effect 1.12 [95% CI 0.95–1.31]); however, those studies were cross-sectional or restricted to a specific sex23.
In contrast, Chang et al. (2020) conducted a retrospective cohort study of 905 industry-based workers. The study showed a significant relationship between occupational noise exposure and incident hyperglycemia (Relative Risk 1.80 [95% CI 1.04–3.10])27. However, this study focused on hyperglycemia—which includes impaired fasting glucose—and did not reflect co-exposure factors and time-varying lifestyle factors. The current study supplemented the limitations of previous studies by using time-dependent Cox models and reduced bias in various ways.
In terms of potential mechanism, noise exposure could be a risk factor for diabetes by significantly affecting stress or sleep. Noise increases catecholamine synthesis, resulting in insulin resistance and problems with glucose homeostasis, thereby increasing stress41,42. Furthermore, noise exposure could result in sleep disturbances, which cause irregular blood glucose and increase in adiposity43. According to these theories, just as environmental noise exposure is related to diabetes, severe occupational noise exposure is also considered to have some degree of correlation.
The current study has several strengths. First, various methods were applied to overcome the immortal time bias. A multivariable time-dependent Cox proportional hazard analysis was performed and reflected the variability of lifestyle factors and BMI. The Landmark analysis was also implemented for sensitivity analysis and indicated the same trend of results. Second, using the same statistical method analysis, two hospital cohorts in different regions were included, so that the data showed diversity and could minimize bias caused by specific regions or institutions. This distributed CDM method could result in a larger sample size by applying the same statistical syntax on different data while maintaining the security of data. Third, to reduce selection bias, the participants of companies with occupational noise exposure were enrolled and noise exposure and non-exposure groups in those companies were compared with each other. Finally, the models were adjusted with number of exposures related to cardiovascular risk, which was not considered in previous studies, along with well-known risk factors of diabetes (demographic characteristics and lifestyle factors).
However, there are some limitations due to the incompleteness of health examination data. First, the health worker effect could influence the outcome of the current study. This implies that healthier workers survive in the company so that they could be more exposed to harmful factors. The maximum follow-up period was set at three years, which is not exceedingly long, to reduce the health worker effect. Moreover, diabetes is less severe than, for example, cancer or CVD, which implies less probability of retirement due to the disease outcome.
Second, some factors such as hearing disease history, disease treated with high-dose steroids, and the presence of personal protective equipment, which could affect diabetes, were not included in our study. Moreover, information about participants’ previous occupational environment was unclear. Further well-designed studies that overcome this limitation should be implemented.
Third, the exact date participants were diagnosed with diabetes is unclear since the definition of diabetes outcome was based on the questionnaires and fasting blood glucose. Thus, the follow-up period may have a bias. Nonetheless, hypertension and diabetes are often found incidentally in health examinations, rather than diseases that are diagnosed by visiting a hospital due to severe symptoms. Moreover, although the time of diagnosis is not clear in the case of history of high blood pressure, diabetes, or taking medicine, the degree of duration will be spread randomly between noise exposure group and non-exposure group. Thus, this limitation can be overcome. Lastly, noise exposure was not quantified in this study. However, the occupational noise standard of 85 dBA lasting 8 hours or more suggested by the ACGIH was applied. Therefore, it is meaningful to evaluate the health effects of workers exposed to serious severe noise. It is also meaningful to check the dose-relationship through a quantitative noise exposure data cohort in the future.
In conclusion, there is a significant association between occupational noise exposure and increased risk of diabetes. Screening for diabetes, active management, and prevention are necessary to improve the health of numerous individuals exposed to occupational noise.