Working hours as a risk factor for acute myocardial infarction in Japan: case-control study
BMJ 1998; 317 doi: https://doi.org/10.1136/bmj.317.7161.775 (Published 19 September 1998) Cite this as: BMJ 1998;317:775
- Shigeru Sokejima (sokejima{at}ms.toyama-mpu.ac.jp), senior lecturer,
- Sadanobu Kagamimori, professor
- Department of Welfare Promotion and Epidemiology, Faculty of Medicine, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama, 930-0194, Japan
- Correspondence to: Dr Sokejima
- Accepted 8 June 1998
Abstract
Objective: To clarify the extent to which working hours affect the risk of acute myocardial infarction, independent of established risk factors and occupational conditions.
Design: Case-control study.
Setting: University and general hospitals and routine medical examinations at workplaces in Japan.
Subjects: Cases were 195 men aged 30-69 years admitted to hospital with acute myocardial infarction during 1990-3. Controls were 331 men matched at group level for age and occupation who were judged to be free of coronary heart diseases at routine medical examinations in the workplace.
Main outcome measures: Odds ratios for myocardial infarction in relation to previous mean daily working hours in a month and changes in mean working hours during previous year.
Results: Compared with men with mean working hours of >7-9 hours, the odds ratio of acute myocardial infarction (adjusted for age and occupation) for men with working hours of >11 hours was 2.44 (95% confidence interval 1.26 to 4.73) and for men with working hours of 7 hours was 3.07 (1.77 to 5.32). Compared with men who experienced an increase of 1 hour in mean working hours, the adjusted odds ratio of myocardial infarction for men who experienced an increase of >3 hours was 2.53 (1.34 to 4.77). No appreciable change was observed when odds ratios were adjusted for established and psychosocial risk factors for myocardial infarction.
Conclusion: There was a U shaped relation between the mean working hours and the risk of acute myocardial infarction. There also seemed to be a trend for the risk of infarction to increase with greater increases in mean working hours.
Footnotes
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Contributors SS, SK, and Director Tsutomu Takata (Occupational Health Service Center, Japan Industrial Safety and Health Association, Tokyo) initiated and coordinated the formulation of the primary study hypothesis. SS and SK discussed core ideas and participated in protocol design, data collection, analysis, and writing the manuscript. Dr Kazuo Haze (Osaka City General Hospital, Osaka), Dr Akira Seki (Toranomon Hospital, Tokyo), Dr Shigeki Koda (Kochi Medical School, Kochi), Dr Masao Ishizaki (Kanazawa Medical University, Ishikawa), Dr Yoshihiro Nishimoto (Central Health Supervision Office, East Japan Railway Company, Tokyo), Dr Kazunori Kayaba (Jichi Medical School, Tochigi), and Professor Katsuo Kanmatsuse (School of Medicine, Nihon University, Tokyo) all participated in protocol design and data collection. SS and SK are guarantors for the paper.
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Funding The study was supported by grants from the Japanese Ministry of Labour.
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Conflict of interest None
- Accepted 8 June 1998