Study subjects
This study was conducted on 840 male workers, aged between 25 and 60 years, at a tire-manufacturing factory. They underwent specific health examinations from October 2014 to November 2014. We excluded 31 workers with a history of ear diseases or with abnormal lesions observed during otoscopy. Finally, we enrolled 809 workers in the current study and conducted the ISI, audiometric, clinical, and tests. We provided each subject for written informed consent for participation. Moreover, physicians who conducted clinical examination explained the method and purpose of this study to the participants. After obtaining informed consent from each subject, tests were performed.
Data collection
The participants were interviewed by trained physicians using a well-established questionnaire. We examined their age, working period, use of hearing protection devices, snoring, smoking, drinking, physical exercise, medical history, current medical problems, and drug administrations. Each subject was questioned about the use of hearing protection devices. Based on all the responses, we divided the subjects into “No or seldom” and “Always” groups. With respect to snoring, the subjects were asked, “Do you snore when you are sleeping or have you heard from family or friends that you snore when you are sleeping?”. The subjects who answered “Yes” were classified into snoring group, while the rest were classified into non-snoring group. With respect to the smoking status, subjects who were smoking currently were defined as current smokers, and those who had stopped smoking were defined as ex-smoker; the subjects who had never smoked were regarded as non-smokers. Further, subjects who consumed alcohol once or more per week were assigned to the drinking group. Subjects who exercised for 30 min or longer at least three times a week were classified into the physical activity group.
The height, weight, waist circumference, and blood pressure were measured. Height and weight were measured to the nearest 0.1 cm and 0.1 kg, respectively. Waist circumference was measured at the mid-point between the twelfth rib and anterior superior iliac spine. The body mass index (BMI) was calculated by dividing the weight (kg) by the squared height (meter). Blood pressure was measured in the right arm in the sitting position by using digital blood pressure monitor under stable conditions.
Blood samples (drawn from a peripheral vein) of all the participants were collected after 12-h fasting. Total cholesterol, high-density lipoprotein (HDL) cholesterol, triglyceride, and fasting glucose levels were measured. Hypertension was defined as systolic blood pressure ≥ 140 mmHg or diastolic blood pressure ≥ 90 mmHg or antihypertensive medication. Diabetes mellitus was defined as fasting glucose ≥126 g/dL or medication for diabetes.
We reviewed the data of working environment noise measured in 2014, which were provided by the company, and the mean noise level of each department to which the workers belonged was regarded as the noise level of the worker.
Insomnia assessment
For evaluating insomnia, we conducted the ISI test, which is regarded as a validating measurement worldwide [21, 22]. The Korean version of ISI has also proven to be a reliable and appropriate assessment of the severity of insomnia in Korean populations [23]. The ISI test helps to assess the severity, impact, and nature of insomnia. It consists of seven questions described below
(1) Difficulty in falling asleep; (2) Difficulty staying asleep; (3) Problems on waking up too early; (4) How satisfied/dissatisfied are you with your current sleep pattern?; (5) How noticeable to others do you think your sleep problem is in terms of impairing the quality of your life?; (6) How worried/distressed are you about your current sleep problem?; (7) To what extent do you consider your sleep problem to interfere with your daily functioning currently?
All the participants answered these questions in the form of ratings from 0 to 4 (for example, 0 = no problem, 4 = very severe problem, etc.). We added the scores for all seven questions and obtained a total score ranging from 0 to 28. The total score is interpreted as follows: absence of insomnia (0–7), sub-threshold insomnia (8–14), moderate insomnia (15–21), and severe insomnia (22–28) [21]. We evaluated the total score as a continuous variable. Further, the total score of ISI was dichotomized and was evaluated as a categorical variable as follows: absence and presence of insomnia (0–14 and 15–28, respectively).
Audiometric tests
Audiometry was performed for measuring the hearing thresholds of participants. Air conduction pure tone thresholds for the frequencies 1–4 kHz were obtained bilaterally by using an Itera II audiometer with TDH-39P headphones by experienced testers. An audiometric device was calibrated according to the guidelines for audiometry of the Korean Occupational Safety and Health Agency [24]. We considered workers to experience hearing impairment when the hearing threshold was greater than 25 dB hearing level in either ear at each frequency that we measured; this was consistent with a study about risk factors for hearing loss in United States adults based on the US National Health and Nutrition Examination Survey [25].
Statistical analyses
Age, working period, noise level, waist circumference, BMI, blood pressure, total cholesterol, triglyceride, HDL cholesterol, glucose, smoking, alcohol consumption, exercise, hypertension, diabetes mellitus, snoring, use of protection devices, and pure tone audiometry results were compared for those with and without insomnia. The Student t-test for continuous variables and chi-square test for categorical variables were performed. We compared hearing thresholds according to insomnia by using analysis of covariance (ANCOVA) with adjustment for covariates including age, working period, noise level, snoring, use of protection devices, hypertension, diabetes mellitus, smoking, alcohol consumption, regular exercise, waist circumference, and total cholesterol, triglyceride, HDL cholesterol levels.
Multiple logistic regression was applied to examine the relationship between insomnia and hearing impairments with four different models. The models were as follows: (1) adjusted for age; (2) adjusted for age, working period, noise level, use of protection devices, and snoring; (3) adjusted for age, working period, noise level, use of protection devices, snoring, diabetes mellitus, and hypertension; (4) adjusted for age, working period, noise level, snoring, use of protection devices, diabetes mellitus, hypertension, smoking, alcohol consumption, regular exercise, waist circumference, total cholesterol, triglyceride, and HDL cholesterol.
Additionally, we performed multiple logistic regression analysis to evaluate the relationship between the ISI test score and hearing impairments with same models as mentioned above. The ISI test score was analyzed as a continuous variable. Continuous variables were presented as means ± standard deviation (SD), and categorical variables were presented as numbers and percentages. Results were shown as odds ratio (OR) with 95% confidence intervals (CI). Statistical significance was set at p-value < 0.05. The SPSS version 23.0 software was utilized for all statistical tests (SPSS Inc., Chicago, IL, USA).