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Association between occupational exposure to chemical or physical factors and sleep disturbance: An analysis of the fifth Korean Working Conditions Survey

  • Seong-Sik Cho
    Affiliations
    Department of Occupational and Environmental Medicine, College of Medicine, Dong-A University, Pusan, Republic of Korea
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  • Mo-Yeol Kang
    Correspondence
    Corresponding author: Mo-Yeol Kang, MD, PhD, Department of Occupational and Environmental Medicine, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, 222, Banpo-daero, Seocho-gu, Seoul 06591, Republic of Korea.
    Affiliations
    Department of Occupational and Environmental Medicine, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea
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Open AccessPublished:August 01, 2022DOI:https://doi.org/10.1016/j.sleh.2022.06.004

      Abstract

      Objective

      A range of risk factors in occupational environments can negatively affect the sleep of workers. Although psychosocial factors have been emphasized in various studies, few have reported on the relationship between physical or chemical exposure in the workplace and sleep disturbances. Thus, this study aimed to investigate the relationship between occupational exposure to physical or chemical factors and sleep disturbances.

      Design

      Cross-sectional study.

      Setting

      Data from the fifth Korean Working Conditions Survey (KWCS).

      Participants

      The target population of the fifth KWCS was economically active individuals aged 15 years or older in all Korean households in 2017, resulting in a total study population of 50,176 participants. After excluding the unemployed, full-time students, homemakers, and the retired, 36,996 employees were included in the current study.

      Measurements

      Exposure to occupational physical or chemical risk factors was assessed by multiplying the exposure scales of physical or chemical risk factors and weekly working hours. Sleep disturbance was estimated using the Minimal Insomnia Symptom Scale.

      Results

      In the fully adjusted logistic regression model, exposure to the following risk factors was positively associated with sleep disturbance: vibration (odds ratio [OR], 1.74)); noise (OR, 2.28); high temperatures (OR, 2.43); low temperatures (OR, 2.51); smoke, fume, and dust (OR, 2.12); vapors of solvents or thinners (OR, 3.78); chemical substances (OR, 3.78); and environmental smoking (OR, 5.03).

      Conclusions

      The results of this study provide evidence of a relationship between occupational exposure to physical or chemical factors and sleep disturbances.

      Keywords

      Introduction

      Sleep is a basic human need and occupies one-third of a person's life.
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      The effect of short sleep duration on coronary heart disease risk is greatest among those with sleep disturbance: a prospective study from the Whitehall II cohort.
      Obtaining adequate, restorative, and healthy sleep, characterized by appropriate timing, quantity, and quality/satisfaction, is essential for human health, social relationships, efficiency, safety, and well-being.
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      Poor sleep health is reflected in sleep disturbances and inadequate, fragmentary, unrecovered, or delayed sleep, which may be inconsistent with the internal circadian rhythm or social obligations. Recent evidence suggests that the prevalence of sleep disorders has increased significantly in many societies over the past decade, and is now the second most frequent health problem after pain, affecting almost all individuals at some point in their life.
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      According to global estimates, about one-third of the general adult population reports sleep disturbances, and approximately 6%-10% meet the Diagnostic and Statistical Manual (DSM-IV) criteria for insomnia.
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      Sleep disturbance is a particularly serious health problem for workers in industrialized countries. It has been estimated that approximately 18% and 23% of workers in Europe
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      and the United States,
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      Insomnia and the performance of US workers: results from the America insomnia survey.
      respectively, suffer from sleep problems.
      To date, a large and growing body of evidence supports the hypothesis that sleep disturbance can cause physical and mental health problems, such as cardiovascular disease,
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      Insomnia and hypertension: a systematic review.
      obesity,
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      Obesity and sleep disturbance: the chicken or the egg?.
      syncope and presyncope,
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      Association of occupational distress and low sleep quality with syncope, presyncope, and falls in workers.
      depression, and suicidal ideation.
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      Persons who suffer from these disorders are much more likely to be absent from work.
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      This problem also incurs significant costs for employers and the community. Individuals with sleep disturbances have less self-confidence and lower job satisfaction in their workplace.
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      Organizational factors associated with work-related sleep problems in a nationally representative sample of Korean workers.
      In fact, there is a higher accident and job error rate, and lower productivity among this population.
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      Sleep problems and work injuries: a systematic review and meta-analysis.
      Therefore, understanding the risk factors of sleep disturbance is important and could be used to create strategies to promote a healthier, safer, and more productive workforce. A range of risk factors in occupational environments can negatively affect the sleep of workers.
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      The effect of the work environment on future sleep disturbances: a systematic review.
      These factors include job stress, organizational factors, work schedules, psychosocial factors, and exposure to various chemicals or physical hazards.
      Among these, psychosocial factors have been emphasized in various studies. For instance, Linton et al reviewed studies with prospective or randomized designs and suggested that the psychosocial work variables of social support at work, control, and organizational justice were related to fewer sleep disturbances, while high work demands, job strain, bullying, and effort-reward imbalance were related to more future sleep disturbances.
      • Linton SJ
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      • Franklin KA
      • et al.
      The effect of the work environment on future sleep disturbances: a systematic review.
      However, few studies have reported on the relationship between physical or chemical exposure in the workplace and sleep disturbances, even though a large number of workers may be exposed to an array of working conditions that include chemical and/or physical agents, and any one of which might influence sleep. This calls for a comprehensive study, using a representative sample, of general workers that elucidates the relationship between physical or chemical exposure in the workplace and sleep disturbances. Thus, this study aimed to investigate the relationship between occupational exposure to physical or chemical factors and sleep disturbances, using data that is representative of the general working population in Korea.

      Methods

      The fifth Korean Working Conditions Survey and study population

      Data from the fifth Korean Working Conditions Survey (KWCS) were used in this study. The fifth KWCS was conducted by the Korean Occupational Safety and Health Agency (KOSHA). Its target population was economically active individuals aged 15 years or older in all Korean households in 2017. The survey aimed to monitor comprehensive working conditions and the safety and health of the Korean working population. The survey questionnaire included demographic characteristics of participants, various physical, chemical, ergonomic, and psychosocial work exposures, and the health status of workers, including sleep disturbance. The KWCS is comparable to the European Working Conditions Survey and the British Labour Force Survey in its basic structure and content of raw data, the questionnaire, and the user guide that is available online.
      The sample design of the KWCS was a multi-stage probability proportion stratified cluster sample survey. First, census districts were selected using probability proportional to size systematic sampling to reflect the number of households in each census district. Second, in each selected census district, ten households were randomly chosen by systematic sampling. Finally, one eligible person in each household was randomly selected to be interviewed (eligible individuals were engaged in the labor market at the time of the survey). The survey was carried out from July to November 2017. Trained interviewers visited each household and administered the questionnaire through Computer Assisted Personal Interviewing. The survey weight of the fifth KWCS was estimated by sampling design-weight, adjustment weight for nonresponse rate, and adjustment weight for poststratification.
      The survey included the economically active population comprising employees, self-employed workers, and employers. However, the unemployed, full-time students, homemakers, and the retired were excluded from this study. The total study population of the fifth KWCS was 50,176 participants, while the eligible population (employees) in the current study was 36,996 (survey weighted).

      Physical and chemical risk factor exposure

      Exposure to occupational physical or chemical risk factors was assessed by multiplying the exposure scales of physical or chemical risk factors and weekly working hours. Participants were considered exposed to physical and chemical risk factors if their weekly exposure duration was 20 hours per week or more. Conversely, they were considered not exposed to physical and chemical risk factors if their weekly exposure duration was less than 20 hours per week. Exposure scales were classified as, all the time (1.0), almost all the time (0.95), around three-fourth of the time (0.75), around half the time (0.5), around one-fourth of the time (0.25), almost never (0.05), and never (0). Each exposure was scaled on the following descriptions: vibration “from hand tools, machinery, etc.,” noise “so loud that you would have to raise your voice to talk to people,” high temperatures that “make you perspire even when not working,” low temperatures “whether indoors or outdoors,” smoke, fume, powder, dust, or vapors from solvents and thinners that “you breathe in,” chemical products or substances “handled or been in direct contact with,” and environmental smoking or “tobacco smoke from other people.” For multiple exposures to physical and chemical risk factors, the number of factors whose exposure durations were the same or exceeding 20 hours per week was estimated.

      Sleep disturbance

      Sleep disturbance was estimated using the Minimal Insomnia Symptom Scale (MISS),
      • Broman J-E
      • Smedje H
      • Mallon L
      • Hetta J.
      The minimal insomnia symptom scale (MISS).
      which consists of 3 items: “difficulty in falling asleep (difficulty initiating sleep),” “waking repeatedly during sleep (difficulty maintaining sleep),” and “waking up with a feeling of exhaustion and fatigue (nonrestorative sleep).” MISS was included in the fifth KWCS as part of the survey questionnaire. For each question, participants could describe the frequency of their sleep disturbance symptoms with a description that was assigned a numerical equivalent: “every day” (4 points), “sometimes a week” (3 points), “sometimes a month” (2 points), “rarely” (one point), “not at all” (no point), and “don't know” and “decline” (both of which were considered as non-responses). The range of the total MISS score was 0-12. In this study, a score of 0-5 suggested no sleep disturbance, while a score of 6 or more suggested the presence of sleep disturbance.
      • Broman J-E
      • Smedje H
      • Mallon L
      • Hetta J.
      The minimal insomnia symptom scale (MISS).

      Other covariates

      In this study, variables related to sleep disturbances were included in the analysis. Sex, age, and socioeconomic status (eg, educational level, income, occupation, employment status) were included. Working hours, shift work, and self-rated health status were considered covariates. Age was classified into 4 groups (15-39, 40-49, 50-59, and 60 or above years). Educational level was classified into 3 groups (middle school graduate or lower education, high school graduate, and college graduate or higher education). Income level was classified by quartile (lowest, lower-middle, upper-middle, and highest). Occupations were classified by the Korean Standard Classification of Occupations and divided into 4 groups: (1) the management/professional group (managers, professionals, and related practitioners), (2) the clerical worker group (office workers), (3) the service and sales worker group (service workers and salespersons), and (4) the manual worker group (blue-collar: skilled workers in agriculture, forestry and fishing, technicians and related craft workers, workers engaged in device/machine operation and assembly, and simple labor workers). Employment status classified according to the duration of employment contract was categorized into 3 groups: standard (at least one year or more), temporary (at least one month but less than one year), and daily (less than one month). Shift work was assessed by the question, “Do you work shifts?”, to which the participants responded as either “yes” or “no”, with the former also including night work. Weekly working hours were classified into 5 groups according to the number of hours worked per week (1-34, 35-40, 41-5, 53-60, and 61 hours or more). Job control was assessed by the question, “Can you influence decisions that are important for your work?”. Options “Always,” “Most of the time,” or “Sometimes” were considered to not indicate low job control, whereas options “Rarely” or “Never” were considered to indicate low job control. Effort-reward imbalance was estimated by the question, “Considering all my efforts and achievements in my job, do I feel I get paid appropriately?”. Options “Strongly agree,” “Tend to agree,” and “Neither agree nor disagree” were considered to indicate the absence of an effort-reward imbalance, whereas options “Tend to disagree” and “Strongly disagree” were considered to indicate the presence of an effort-reward imbalance.

      Statistical analysis

      Distribution of the general and occupational characteristics of participants grouped by the presence or absence of workplace exposure to physical and chemical factors is presented as numbers and percentages. Prevalence of sleep disturbance has been demonstrated across all participants with occupational exposure, and chi-square test was conducted using survey weighting. Survey-weighted logistic regression analysis was performed to explore the association between occupational exposure and sleep disturbance. Odds ratio (OR) and 95% confidence interval (CI) values were estimated for the unadjusted model; adjusted model 1 that included age, sex, and socioeconomic status; adjusted model 2 that included weekly working hours and shift work in addition to the above-mentioned variables; and fully adjusted model 3 that included low job control and effort-reward imbalance in addition to the previously mentioned variables. Furthermore, considering the multiple exposure conditions, additional analysis was conducted on the number of exposures and sleep disturbance for multiple physical or chemical exposures. Next, stratified analysis by shift work was conducted to explore the modifying effect of shift work on the relationship between chemical or physical exposure and sleep disturbances. All statistical analyses were carried out using Stata Version 16.1 (Stata Co, College Station, TX).

      Results

      Table 1 shows the socio-demographic characteristics of participants grouped by the presence or absence of occupational exposure to physical or chemical factors. The proportion of exposure group was higher in males (47%) than females (26.1%). Higher proportions of workers were exposed to physical or chemical factors in older age groups. The proportion of exposure group was lowest (29.8%) among participants with a college degree or higher, and it was the highest (53.6%) among those with a middle school degree or less. Highest proportion of daily-contract workers (62.1%) were exposed to physical or chemical risks. Highest proportion of manual workers (65.4%) and lowest proportion of clerical workers (21.3%) were exposed to physical or chemical risks. Also, differences in exposure proportion were observed for weekly working hours, shift work, and effort-reward imbalance but not for low job control.
      Table 1Characteristics of the study population, grouped by presence or absence of occupational exposure to physical or chemical factors
      CharacteristicsTotalNo exposureExposure
      Exposure (+): Exposure to one or more physical or chemical factors in the workplace.
      n%n%n%
      Sex
      Male20,99556.711,12453.0987147.0
      Female16,00143.311,82173.9418026.1
      Age group
      15-3916,10443.510,84467.3526032.7
      40-49936125.3595063.6341136.4
      50-59742620.1401954.1340745.9
      ≥ 60410411.1213251.9197248.1
      Education
      Middle school or less31918.6148146.4171053.6
      High school11,52931.2581450.5571249.5
      College or more22,24560.215,62570.2662029.8
      Income
      Lowest726021.0485366.8240733.2
      Low middle812823.4527764.9285135.1
      High middle944427.2546357.8398142.2
      Highest987228.4596060.4391239.6
      Employment
      Regular29,96981.018,98763.410,98236.6
      Temporary510813.8323263.3187736.7
      Daily19195.272737.9119262.1
      Occupation
      Professional & managerial877523.8647173.8230326.2
      Clerical (office work)904124.5711778.7192421.3
      Sales & service745020.2528270.9216729.1
      Manual11,61331.5402034.6759365.4
      Weekly working hours
      1-34384810.4270070.2114929.8
      35-4018,13249.012,16967.1596332.9
      41-5210,13227.4577557.0435843.0
      53-6036429.8169646.6194653.4
      > 6012413.460648.863551.2
      Shift work
      No32,60788.120,61463.211,99236.8
      Yes438511.9233053.1205546.9
      Low job control
      No27,18873.516,97162.410,21737.6
      Yes979126.5596260.9382939.1
      Effort-reward imbalance
      No32,34588.220,37963.011,96637.0
      Yes431211.8231053.6200246.4
      a Exposure (+): Exposure to one or more physical or chemical factors in the workplace.
      Table 2 demonstrates the prevalence of sleep disturbance among participants, grouped by their exposure to physical and chemical factors. All forms of workplace exposures studied were associated with an increase in the prevalence of sleep disturbance. Workers who were not exposed to any physical or chemical factors had a sleep disturbance prevalence of 6%, while those who were exposed had a sleep disturbance prevalence that ranged from 10% to 26%. In general, the prevalence of sleep disturbance was higher in the exposure to chemical hazards subgroups than in the exposure to physical hazards subgroups. Notably, the prevalence of sleep disturbance was the highest (25.9%) in workers exposed to environmental smoking.
      Table 2Physical or chemical exposures in the workplace and prevalence of sleep disturbance
      TotalNo sleep disturbanceSleep disturbance
      n%n%n%p value
      Vibration<.001
      No30,64190.428,67893.619626.4
      Yes32559.6292889.932710.1
      Noise<.001
      No31,16593.329,28294.018836.0
      Yes22346.7195287.428212.6
      High temperatures<.001
      No31,46893.129,58594.018836.0
      Yes23396.9202786.731213.3
      Low temperatures<.001
      No32,34494.630,41494.019306.0
      Yes18615.4160486.225713.8
      Smoke, fume, dust, or powder<.001
      No32,23894.130,19593.720436.3
      Yes20385.9178987.824812.2
      Vapors of solvent or thinner<.001
      No34,72197.932,59193.921306.1
      Yes7422.158879.215420.8
      Chemical products or substances<.001
      No34,38597.732,26793.821186.2
      Yes7982.363879.916020.1
      Environmental smoking<.001
      No33,16398.331,14593.920176.1
      Yes5641.741874.114625.9
      p was calculated using survey weighted chi-squared test.
      Table 3 shows the prevalence of sleep disturbance according to the number of exposures to physical or chemical risk factors. The group not exposed to any physical nor chemical risk factors had the lowest prevalence of sleep disturbance (5.3%). As the number of exposures increased, the prevalence of sleep disturbance also increased, with the prevalence being highest among workers exposed to 8 risk factors.
      Table 3Number of physical or chemical exposures in the workplace and prevalence of sleep disturbance
      TotalNo sleep disturbanceSleep disturbance
      n%n%n%
      Number of exposures
      022,93662.021,72094.712165.3
      1522714.1483692.53917.5
      232128.7296792.42457.6
      321705.9198091.21908.8
      414013.8126290.21389.8
      59782.787789.710110.3
      65221.444585.17814.9
      73420.927680.66619.4
      81960.514674.45025.6
      Table 4 presents the association between workplace exposure and sleep disturbance by survey-weighted logistic regression analysis. Regardless of the specifics, all investigated physical and chemical factors resulted in an increased OR. In the fully adjusted model (model 3), the following exposures were positively associated with sleep disturbance: vibration (OR, 1.74; 95% CI, 1.41-2.14); noise (OR, 2.28; 95% CI, 1.83-2.85); high temperatures (OR, 2.43; 95% CI, 1.99-2.96); low temperatures (OR, 2.51; 95% CI, 2.04-3.09); smoke, fume, and dust (OR, 2.12; 95% CI, 1.71-2.63); vapors of solvents or thinners (OR, 3.78; 95% CI, 2.90-4.92); chemical substances (OR, 3.78; 95% CI, 2.96-4.84); and environmental smoking (OR, 5.03; 95% CI, 3.78-6.70). Among these exposures, environmental smoking had the highest OR. In addition, exposure to the vapors of solvents or thinners and exposure to chemical products or substances had relatively high ORs.
      Table 4Association between physical or chemical exposures and sleep disturbance by survey-weighted logistic regression analysis
      UnadjustedModel 1
      Model 1: Adjusted for age, sex, education, income, occupation, and employment status.
      Model 2
      Model 2: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, and shift work.
      Model 3
      Model 3: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, shift work, low job control, and effort-reward imbalance.
      ExposureOR95% CIOR95% CIOR95% CIOR95% CI
      Vibration1.631.381.931.731.402.131.721.392.121.741.412.14
      Noise2.251.862.712.291.842.862.281.832.852.281.832.85
      High temperature2.422.052.862.502.063.042.492.053.022.431.992.96
      Low temperature2.522.103.042.622.143.212.592.113.172.512.043.09
      Smoke, fume, dust, etc.2.051.712.462.101.692.592.101.692.602.121.712.63
      Vapors of solvent, etc.4.013.175.064.043.115.234.073.135.283.782.904.92
      Chemical products3.823.044.803.782.964.833.903.054.993.782.964.84
      Environmental smoking5.394.156.985.304.026.995.143.896.815.033.786.70
      a Model 1: Adjusted for age, sex, education, income, occupation, and employment status.
      b Model 2: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, and shift work.
      c Model 3: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, shift work, low job control, and effort-reward imbalance.
      Table 5 shows the association between the number of exposures to physical or chemical risk factors and sleep disturbance. As the number of exposures increased, the ORs of sleep disturbance also increased, with the OR being highest among workers exposed to 8 risk factors.
      Table 5Association between number of physical or chemical exposures and sleep disturbance by survey-weighted logistic regression analysis
      UnadjustedModel 1
      Model 1: Adjusted for age, sex, education, income, occupation, and employment status.
      Model 2
      Model 2: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, and shift work.
      Model 3
      Model 3: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, shift work, low job control, and effort-reward imbalance.
      Number of exposuresOR95% CIOR95% CIOR95% CIOR95% CI
      11.451.231.701.551.301.851.541.281.841.541.281.84
      21.471.221.781.641.342.021.621.322.001.601.301.98
      31.721.372.151.891.462.441.861.432.411.821.402.36
      41.951.512.522.221.662.972.201.642.942.161.612.89
      52.051.532.752.371.723.272.361.703.262.271.643.14
      63.122.224.384.012.795.784.022.805.783.822.645.54
      74.293.036.095.263.577.755.303.607.825.063.397.55
      86.164.129.207.554.9111.617.584.9211.687.234.7111.09
      a Model 1: Adjusted for age, sex, education, income, occupation, and employment status.
      b Model 2: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, and shift work.
      c Model 3: Adjusted for age, sex, education, income, occupation, employment status, weekly working hours, shift work, low job control, and effort-reward imbalance.
      Appendix Tables 1-3 show the associations between workplace exposure to the different physical or chemical factors and sleep-related symptoms, which constitute the MISS, difficulty in initiating sleep (Appendix Table 1), difficulty in maintaining sleep (Appendix Table 2), and nonrestorative sleep (Appendix Table 3). All investigated physical and chemical factors were associated with an increased risk of all sleep-related symptoms. Considering the amplitudes of the ORs, the effect of workplace exposure on sleep might be greater on difficulty in maintaining sleep and non-restorative sleep than on difficulty in initiating sleep. Appendix Table 4 shows the association between occupational physical or chemical exposures and sleep disturbance stratified by shift work. Regardless of shift work, all physical and chemical exposures increased the risk of sleep disturbance among shift workers. Appendix Table 5 presents the association between the number of occupational physical or chemical exposures and sleep disturbance stratified by shift work. As the number of exposures increased, the risk of sleep disturbance also gradually increased in both shift and nonshift work.

      Discussion

      This study aimed to evaluate the association between exposure to physical or chemical factors in the workplace and sleep disturbances in a nationwide representative sample of the Korean working population. The findings suggest that exposure to vibration, noise, high or low temperatures, dust or fumes, solvent vapors, chemicals substances, and secondhand smoke is associated with sleep disturbances, such as problems in initiating and maintaining sleep and fatigue at awakening. After adjusting for age, sex, education, income level, occupation, employment type, working hours, shift work, and job stressors, the physical and chemical risk factors retained a statistically significant relationship with sleep disturbance. Hence, it could be concluded that these workplace risk factors contributed to the workers’ sleep disturbance.
      It may be difficult to compare the results of this study with those of previous studies because the study population and methods are different; however, results similar to ours have been found. For example, Bertrais et al assessed the association between a wide range of occupational risk factors and sleep problems, and investigated the cumulative effects of these exposures using data from the 2016 French Working Conditions Survey.
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      Third, sleep problems could be caused by psychological factors. Thus, health concerns or anxiety can be influenced by direct exposure to handling hazardous substances, and increased stress by recognizing the poor working conditions associated with hazardous substance exposure can lead to sleep disturbances.
      • Heo Y-S
      • Chang S-J
      • Park S-G
      • et al.
      Association between workplace risk factor exposure and sleep disturbance: analysis of the 2nd Korean Working Conditions Survey.
      Fourth, exposure to a variety of risk factors can affect a worker's overall health status including chronic illness that may also lead to sleep disturbances.
      • Taylor DJ
      • Mallory LJ
      • Lichstein KL
      • Durrence HH
      • Riedel BW
      • Bush AJ.
      Comorbidity of chronic insomnia with medical problems.
      Another possible explanation is that poor sleep affects perceptions of the working environment, which is not plausible in the current cross-sectional context, given that our research utilized self-reporting measures. Individuals with poor sleep due to other factors (eg, poor health, loneliness, isolation, family environment, lack of sleep) may be more emotionally responsive, which can affect their subjective assessment of the working environment.
      • Magee C
      • Gopaldasani V
      • Bakand S
      • Coman R.
      The physical work environment and sleep: a latent class analysis.
      ,
      • Åkerstedt T
      • Garefelt J
      • Richter A
      • et al.
      Work and sleep—a prospective study of psychosocial work factors, physical work factors, and work scheduling.
      With respect to shift work, it is well demonstrated that shift workers experience high levels of sleep disturbance.
      • Åkerstedt T
      • Garefelt J
      • Richter A
      • et al.
      Work and sleep—a prospective study of psychosocial work factors, physical work factors, and work scheduling.
      As shown in Table 1, the proportion of exposure group was significantly higher in the shift workers. This may suggest that shift work can explain the higher likelihood of exposure and sleep disturbance. However, it is unclear to what degree shift work influences the relationship between occupational exposure to physical or chemical factors and sleep disturbances. Stratified analysis by shift work did not reveal any significant differences in relationship, depending on whether shift work is performed (Appendix Tables 4 and 5). Similarly, in a previous study of steel shift workers characterized by an identical physical, chemical, and mental working load during 3 different shifts, the working periods were constantly characterized by higher indexes of cardiac sympathetic modulation compared with the sleep time; no specific sleep disturbance was reported by the workers although the study did not aim to investigate sleep quality.
      • Furlan R
      • Barbic F
      • Piazza S
      • Tinelli M
      • Seghizzi P
      • Malliani A.
      Modifications of cardiac autonomic profile associated with a shift schedule of work.
      The combining or modifying effect of shift work on the relationship between physical or chemical exposure and sleep is rife for further research.
      The strengths of the current study include the use of a nationwide representative survey of the working population, assessment of sleep disturbance using a validated tool, and the examination of various occupational exposures that have seldom been explored in the literature. However, the following limitations were recognized in our study. First, as the study design was cross-sectional, a causal relationship could not be verified. Furthermore, given the healthy worker effect, we could not exclude the possibility that workers with sleep problems may have changed jobs or left the labor market because of their occupational exposures, which may lead to an underestimation of the true associations. Second, information on symptoms and exposures was obtained using self-administered questionnaires; this method relies on the accuracy of the responders’ memory and may be subject to recall bias. Third, there is a lack of detailed workplace risk factor evaluation. We were not able to confirm the exposure root, quantity, duration, or whether protective equipment was being worn; although, it is expected that these factors may have distinct health effects. Fourth, this study evaluated sleep disturbances with a particular focus on occupational exposure without considering key confounding factors such as sleep medication use, history of mental illness (eg, major depressive disorder, anxiety disorder), and other health behaviors (eg, smoking, alcohol drinking, exercise), because the information provided in this survey did not include these variables.
      In conclusion, the results of the current study provide evidence of a relationship between occupational exposure to physical or chemical factors and sleep disturbances. In future studies, improving upon the limitations of our study, a causal relationship should be confirmed by using a longitudinal study design and an objective assessment method, and by gathering detailed information about exposure and confounders.
      From a public health viewpoint, it is important to tackle workers’ sleep problems as they negatively impact their health, quality of life, and work performance. Based on the current findings, interventional programs to improve sleep and prevent sleep disturbances need to be tailored for workers exposed to various kinds of chemical and physical exposures. Improving the workplace environment by, for example, lowering the intensity and duration of workers’ exposure to physical and chemical hazards would be a useful preventive strategy. It also requires proper training in self-protection. In addition, identifying and managing other risk factors that may affect the sleep health of workers are necessary to improve workers’ sleep health.

      Declaration of conflict of interest

      All authors declare no competing interests.

      Funding

      This research is supported by the National Research Foundation of Korea ( NRF-2021R1C1C1007796 ).

      Ethics statement

      The fifth Korean Working Conditions Survey contains anonymous data and is open to the public. Written informed consent and review of the study protocol were waived by the Dong-A Institutional Review Board (Approval No: 2-1040709-AB-N-01-202104-HR-020-02).

      Data sharing

      The KWCS data and the user's guide are available at https://oshri.kosha.or.kr/oshri/researchField/downWorkingEnvironmentSurvey.do

      Author contribution

      SSC conceived and designed the study. SSC analyzed the data. SSC and MYK were involved in interpreting the results and discussion. SSC and MYK drafted the manuscript. All authors reviewed, approved, and agreed to submit the final version of the manuscript for publication.

      Acknowledgment

      This research is supported by the National Research Foundation of Korea (NRF-2021R1C1C1007796).
      The authors appreciate the Occupational Safety and Health Research Institute (OSHRI) for providing the raw data of the fifth Korean Working Conditions Survey.

      Appendix. Supplementary materials

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