The study design was cross-sectional using data from the fifth KNHANES conducted between 2010 and 2012. Of the 25,534 subjects who had completed pure tone audiometry, laboratory, and physical examinations during their participation in the fifth KNHANES, we excluded 11,873 who were less than 40 years old. We further excluded participants with any of the following criteria: missing required data for MetS diagnosis (waist circumference, blood pressure, fasting blood sugar, triglyceride, high-density lipoprotein cholesterol [HDL-C]), diagnosed with or receiving treatment for otitis media, using hearing aids or cochlear implants, abnormality found in ear inspection, external ear canal stenosis or eardrum abnormality found during 0° endoscope examination, or if they reported a restriction in daily life and social activities due to hearing problems. Thus, 10,356 subjects were included in the final analysis (Supplementary Table 1).

Subject characteristics were collected using a standardized questionnaire, including age, household income, education level, smoking, alcohol consumption, physical activity, history of noise exposure, and use of ear protection. Subjects were categorized into several groups according to the distribution of collected variables. Household income was categorized into five groups: upper, upper-middle, middle, lower-middle, or lower class. Highest achieved public education level was categorized into four groups: university, high school, middle school, or elementary school. Smoking consumption was categorized into three groups: nonsmokers for subjects who were not smoking at the present and whose lifetime smoking amount was less than 5 packs (100 cigarettes), ex-smokers for subjects who had smoked more than 5 packs during their lifetime but had quit smoking, and current smokers for subjects who smoked currently regardless of the amount of smoking. Alcohol consumption was categorized into four groups depending on frequency of drinking during the recent year: non-drinkers for those who did not drink alcohol during their lifetime or who drank less than once a month for the last 1 year, drinking once a week, drinking 2–3 times a week, or drinking more than 4 times a week. Regular physical activity group was defined as subjects who satisfied any one of the following conditions: (1) vigorous physical activity for more than 20 minutes, more than 3 days per week, (2) moderate physical activity for more than 30 minutes, more than 5 days per week, or (3) walking for more than 30 minutes, more than 5 days a week.

Study subjects were further categorized into two groups based on noise exposure experience: If a person reported exposure to workplace noise, other place (extra-work) noise, or sudden loud noise, then she/he was categorized into the noise exposure group. Workplace noise exposure was defined as an affirmative answer to the question regarding whether they had worked for more than 3 months in a place with high noise such as a mechanical or generator sound. Other place noise exposure was defined as having experienced exposure to loud noises for more than 5 hours a week. Sudden loud noise exposure was defined as exposure to noise such as a gunshot or explosions.

Height was measured with a SECA 225 (SECA, Hamburg, Germany), and weight was measured with a GL-6000-20 (G-Tech International, Uijungbu, Korea) at intervals of 0.1 kg. Body mass index was calculated by dividing weight (kg) by squared height (m²). Waist circumference was measured with a SECA 200 (SECA) at the midpoint between the bottom of the last rib and the top of the iliac crest, after expiratory breathing.

Four nurses from the Specialized Survey Team of the Disease Control Headquarters who oversaw and performed blood pressure measurements. Blood pressure was measured 3 times at an interval of 30 seconds, and the mean of the second and third blood pressures was used.

After participants fasted for at least 12 hours, a laboratory test for low-density lipoprotein cholesterol, HDL-C, triglyceride, total cholesterol, and fasting plasma glucose was performed.

Skilled otorhinolaryngology residents conducted participant ear examinations. Pure-tone audiometry was performed using an automated diagnostic audiometer (SA 203; Entomed, Malmo, Sweden) at 6 frequencies (0.5, 1, 2, 3, 4, and 6 kilohertz [kHz]) in a double-walled audiovisual booth where noises were interrupted.

In this study, we defined low-frequency HL as pure tone averages at three low frequencies (0.5, 1, and 2 kHz) exceeding 25 decibels (dB) unilaterally and or bilaterally. High-frequency HL were defined as pure tone averages at three high frequencies (3, 4, and 6 kHz) exceeding 25 dB unilaterally and/or bilaterally [4,13].

We applied two MetS criteria recommended by the 2005 revised National Cholesterol Education Program’s III (NCEP III) and by the International Diabetes Federation (IDF). NCEP III criteria for Asia was published by the National Heart, Lung and Blood Institute of the National Institute of Health and it requires the presence of at least three of the following components: (1) abdominal obesity (waist circumference ≥90 cm for male, ≥80 cm for female), (2) taking antihypertensive medication or systolic blood pressure (SBP) ≥130 mm Hg or diastolic blood pressure (DBP) ≥85 mm Hg, (3) fasting plasma glucose (FPG) ≥100 mg/dL or hemoglobin A1c (HbA1c) ≥6.5% or receiving drug treatment for elevated FPG, (4) triglyceride ≥150 mg/dL, or (5) HDL cholesterol ≤40 mg/dL for male, ≤50 mg/dL for female.

The IDF criteria includes abdominal obesity (waist circumference ≥90 cm for male, ≥80 cm for female) as a prerequisite for the identification of MetS. In addition to abdominal obesity, two or more of the following four factors should exist: (1) SBP ≥130 mm Hg or DBP ≥85 mm Hg or taking antihypertensive medication; (2) FPG ≥100 mg/dL or HbA1c ≥6.5% or drug treatment for elevated FPG; (3) triglyceride ≥150 mg/dL; or (4) HDL cholesterol ≤40 mg/dL for male, ≤50 mg/dL for female.

Descriptive statistics were used to present the general characteristics of the study subjects. Chi-square tests were performed to evaluate univariate associations of risk factors and MetS with type of HL. Multivariable-adjusted association of MetS and metabolic components with each type (low-frequency, high-frequency) of hearing impairment were evaluated using multiple logistic regression analysis taking into consideration the sampling weights for the Korean standard population. We repeated the multiple logistic regression analyses according to the presence of noise exposure history. Statistical analyses were performed using SAS software ver. 9.4 (SAS Institute Inc., Cary, NC, USA). Significance was set a prior at P<0.05, two-tailed.

The study was approved by the institutional review board of Samsung Medical Center (IRB approval no., SMC 2019-06-102). The informed consent requirement was waived because subjects’ consent had been given for the KNHANES. The dataset is in the public domain and does not include individually identifiable information.