Study participants
The Korea National Health and Nutrition Examination Survey (KNHANES) data from 2013 to 2018 were used in this study. KNHANES is a nationwide cross-sectional survey conducted in the Republic of Korea to assess the health and nutritional status of Koreans. Representatives are selected using multistage cluster sampling. Annually, 20 households throughout 192 regions are included as a new sample, and about 10,000 individuals aged one year old and older are targeted. This survey provides participants’ information based on a health examination, a health interview, and a nutrition survey, conducted by trained staff members 16.
The total number of KNHANES participants from 2013 to 2018 was 47,217. We only included participants aged 40 to 79 years old, as the noise exposure survey was performed at 40 years of age or older and the survey recorded all patients of 80 years or older as 80 years old. To minimize the effect of kidney disease intervention, participants with a previous diagnosis of renal failure were excluded. Missing or “participant refusal” values of the variables used in this study—such as hearing discomfort status, occupational noise exposure, environmental noise exposure, serum creatinine, educational level, HTN, DM, dyslipidemia, BMI, smoking status, high-risk alcohol consumption, and aerobic physical activity—were also excluded. The final sample size for analysis was 17,202 (Figure 2).
Renal function
Renal function was evaluated using the serum creatinine level. This was measured with a Hitachi Automatic Analyzer 7600-210 (Hitachi/JAPAN) and CREA reagent (Roche/Germany) using the Jaffe rate-blanked and compensated method. eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration equation 17. An eGFR lower than 60 mL/min/1.73 m2 was classified as CKD.
Noise exposure
The KNHANES survey used self-reported questionnaires to assess noise exposure. For occupational noise, the question was “Have you ever worked in place with loud noise such as machines or generators for more than three months? A loud noise means that you have to raise your voices for a conversation.” Those who answered yes were asked about their total working period, in months. The question addressing environmental noise was “Have you ever been exposed to loud noise for more than five hours a week, except for occupational noise? A loud noise means that you have to raise your voices for a conversation, such as cars, trucks, motorcycles, machines, or loud music (ex. singing room, concert hall).” If answered “yes,” the average minutes of exposure per day were asked. Long-term exposure to noise was defined as the third quartile of noise exposure period, ≥ 240 months for occupational noise, and ≥ 300 minutes for environmental noise. As noise exposure is closely linked to auditory problems, we conducted an additional analysis of the association between hearing discomfort and CKD or eGFR. For hearing discomfort, this question was asked: “Among the following, choose the most appropriate sentence to describe your hearing ability (without wearing a hearing aid).” The options were “comfortable,” “a little uncomfortable,” “very uncomfortable,” and “cannot hear at all.” We excluded participants who responded “cannot hear at all” and combined “a little uncomfortable” and “very uncomfortable” as just “uncomfortable.”
Other covariates
Our socioeconomic variables were age, sex, and educational status. Education level was classified as elementary school, middle school, high school, college, or higher as the highest level of completed education.
For chronic disease variables, we included HTN, DM, dyslipidemia, and BMI. HTN was defined as participants who satisfied at least one of the following criteria: (1) Systolic blood pressure ≥ 140 mmHg, (2) diastolic blood pressure ≥ 90 mmHg, or (3) diagnosed with hypertension before or with drugs for blood pressure control. Participants with any of the following conditions were considered to have DM: (1) fasting glucose level ≥ 126 mg/dL, (2) diagnosed with DM before, or (3) use oral hypoglycemic medications or insulin injections. Pregnant women were excluded because gestational diabetes was a transient state. Dyslipidemia was defined according to the 2018 Korean Dyslipidemia Management Guidelines 18 and participants who were diagnosed before or used oral drugs were included. The guidelines describe dyslipidemia as (1) total cholesterol ≥ 240 mg/dL, (2) triglyceride ≥ 200 mg/dL, (3) LDL-C ≥ 160 mg/dL, or (4) HDL-C < 40 mg/dL. As LDL cholesterol levels were not measured, they were calculated using the Friedewald equation 19. BMI was grouped into two categories: (1) < 25 kg/m2 and (2) ≥ 25 kg/m2.
The health behavioral variables were smoking status, high-risk alcohol consumption, and aerobic physical activity. Smoking status was classified according to their current smoking status: never smoked, past smoker, or current smoker. High-risk alcohol consumption is described as averaging ≥ 7 drinks at a time and drinking at least twice a week for males, and an average of ≥ 5 drinks at a time and drinking at least twice a week for females. Aerobic physical activity refers to doing moderate-intensity physical activity (≥ 2.5 h) or high-intensity physical activity (≥ 1.25 h) or mixing moderate- and high-intensity physical activity (1 min of high-intensity physical activity is equivalent to 2 minutes of moderate-intensity) per week.
Statistical analysis
We conducted a χ2 test to examine the differences in CKD prevalence. A student’s t-test and one-way ANOVA were used to show the variances in eGFR by sociodemographic characteristics. We assessed the association between CKD and noise exposure using a logistic regression adjusted for age, sex, educational status, HTN, DM, dyslipidemia, BMI, smoking status, high-risk alcohol consumption, and aerobic physical activity. A linear regression analysis was performed to examine the association between eGFR and noise exposure time. Participants who were not exposed to noise were excluded from the analysis. The adjusted covariates were the same as described above; however, age was not included to avoid overadjustment. Statistical analyses were performed using SAS version 9.4 (SAS Institute, Cary, NC, USA) and R software (version 4.1.0) was used to draw the plots in Figure 1. Two-tailed p-values less than 0.05 were considered to be statistically significant.
Ethics approval and consent to participate
The study was performed in accordance with the ethical standards of the Declaration of Helsinki (1964) and its subsequent amendment. KNHANES data were anonymized prior to its release to the authors. All participants in the survey provided written informed consent. The Institutional Review Board of the Gil Medical Center, Gachon University, approved the current study (IRB number: GCIRB2020-147).