Purpose: We used a multiple logistic regression model to calculate benchmark doses (BMDs) and their 95% lower confidence bound (BMDL) of working hours for subjective fatigue symptoms.
Methods: A self-administered questionnaire survey was conducted in 3069 male daytime workers between the ages of 18 and 60 years who worked in a Japanese steel company. Dependent variables were positive findings for eight properties by using the Cumulative Fatigue Symptoms Index (CFSI): decreased vitality, general fatigue, physical disorders, irritability, decreased willingness to work, anxiety, depressive feelings, and chronic tiredness. Independent variables were daily working hours, age, lifestyle factors, working conditions (type of work and number of holidays), marital status, and living arrangements. Using significant parameters for working hours and those for other covariates, BMD/BMDL of working hours was calculated for the corresponding property of the CFSI. Additionally, benchmark response (BMR) was set at 5% or 10%.
Results: The BMD/BMDL of working hours per day with a BMR of 5% were 11.8/10.8 (irritability), 11.4/10.2 (anxiety), and 11.2/10.6 (chronic tiredness), and those with a BMR of 10% were 14.2/12.6 (irritability), 14.2/12.1 (anxiety), and 13.1/12.2 (chronic tiredness).
Conclusions: These results strongly suggest that special attention should be given to workers whose working hours exceed these BMD/BMDL values.