This study was based on the 3rd Korean Working Conditions Survey performed by Korean Occupational Safety and Health Research Institute in 2011. The survey was conducted on a representative sample of workers (≥15 years). A worker was defined as “a person who during the reference week did any work for pay or profit”. A total of 50,032 subjects were included in the study, and in-person interviews were performed by professional interviewers from professional survey services after obtaining informed consent. This study focused on employed workers from the entire of 50,032 subjects. An employed worker was defined as “a person who signed either expressed or implied employment contract with individual, family, or business and is receiving salary or daily wage or spot goods in return for their labor”. Furthermore, since we aimed to investigate the effect of on-call work on the workers, only the employed workers who had been working for at least 1 year were included. Eventually, a total of 29,246 subjects were included in this study. Although the age limit for the Working Conditions Survey was 15 years, since the subjects included in this study had been working for at least 1 year, the subjects in this study were at least 16 years of age.
The subject was defined as working on-call if they responded “Yes” to the question “Do you work on-call (immediately providing work or service if contacted or called)?”
Health problems and injuries
For health problems and injuries, the subject was defined as having health problems and injuries if they answered “Yes” to the question “Over the last 12 months, did you suffer from any of the following health problems?”. The questionnaire contained sub-categories for questions regarding health problems and injuries including hearing problems, skin problems, back pain, muscular pains in upper limbs, muscular pains in lower limbs, headaches or eyestrain, abdominal pain, respiratory difficulties, cardiovascular diseases, injuries, depression or anxiety, fatigue, insomnia and general sleep difficulties. Any report of hearing problems, skin problems, backache, muscular pains in upper limbs, muscular pains in lower limbs, headaches or eyestrain, abdominal pain, respiratory difficulties, cardiovascular diseases, fatigue was defined as a physical health problem. Similarly, if the subject had depression or anxiety, insomnia, general sleep difficulties, the subject was defined as having a psychological health problem.
General and occupational characteristics
The general characteristics of the subjects investigated in this study included gender, age, education level, income level, alcohol consumption, smoking, obesity, and hypertension. For obesity and hypertension, the subject was defined as having a chronic condition if they answered “Yes” to the question “Were you ever diagnosed with chronic obesity or hypertension by your physician?”. However, for obesity, if the subject answered “Not obese” to the additional question “What is your current status in terms of obesity?”, the subject was not considered obese.
The occupational characteristics included job type, employment type, working hours, and shift work. Two methods were used to categorize the job types. First, the subjects were asked, “What is the job type that best describes your current occupation?”. Based on their responses, professional workers and senior managers were defined as “Professional”, general office workers as “Office”, sales workers and service providers as “Service”, and skilled/semi-skilled/unskilled occupations and agricultural/forestry/fishery workers as “Physical”. Second, based on the middle categorization of the Korean Employment Classification of Occupation (KECO), the subjects’ occupations were categorized into 24 job groups. Employment type was defined as regular or temporary, and working hours were categorized as ≤ 40 h, 41 – 60 h, or > 60 h per week.
Work environment and work-related stress
Work environment was assessed in terms of exposures to physical, chemical, and ergonomic factors. If the subject answered “All of the time”, “Almost all of the time”, “Around 3/4 of the time”, “Around half of the time” or “Around 1/4 of the time” to the question “Are you exposed at work to ...?”, the subject was considered to be exposed to the factor. However, if the subject answered “Almost never” or “Never” to the same question, the subject was not considered to be exposed to the factor. The physical factors included the vibrations, noise, high temperatures, and low temperatures. The chemical factors included breathing in smoke, fumes, powder or dust, vapors such as solvents and thinners, handling or being in skin contact with chemical products or substances, and tobacco smoke from other people. Lastly, the ergonomic factors included tiring or painful positions, lifting or moving people, carrying or moving heavy loads, standing, and repetitive hand or arm movements.
For work-related stress, if the subject answered “Always”, “Most of the time”, or “Sometimes” to the question “For each of the following statements, please select the response which best describes your work situation – You experience stress in your work.”, the subject was defined as having work-related stress. If the answer was “Rarely” or “Never”, the subject was defined as being free of work-related stress.
This study utilized IBM SPSS Statistics for Windows version 19.0 (IBM Corp: Armonk, NY, USA) for analysis of the data after applying weighting adjustments. Personal and occupational characteristics, work environment, and work-related stress were analysed using frequency analysis. Chi-square analysis was used to determine the associations between on-call work and characteristics of the subject and on-call work and health problems and injuries. In order to determine the risk of health problems and injuries due to on-call work, bivariate logistic regression analysis was performed to calculate the odds ratios. Model I was the analysis after adjusting for personal characteristics, while Model II was additionally adjusted for occupational characteristics including job type, employment type, working hours, and shift work. Model III further adjusted for physical, chemical, and ergonomic factors of the work environment, while Model IV had additional adjustment for work-related stress in addition to all controls from Model III.