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"A significant proportion of the work force is employed in unusual work schedules. The combined effects of working hours and chemical agents at the workplace may depend on the duration and/or timing of exposure. However, occupational exposure limits (OELs) usually assume working day-time, an 8-hour workday, 5 days/week and a 40-hour work week. The aims of this document were to review the scientific support for a combined effect of unusual working hours (shift work or extended working hours) and chemical exposure and, to the extent possible provide recommendations for down-adjustment of the OEL to account for unusual working hours. Animal data from chronopharmacological studies suggest that the time of exposure (day-night) may affect the biotransformation and toxicity of chemicals. A few epidemiological studies, i.e. regarding effects of dust and endotoxin on lung function, effects of acetone on sleep quality and tiredness, effects of carbon disulphide on coronary artery disease and effects of chemicals on spontaneous abortion, suggest more pronounced effects during night shifts compared to day shift exposure. However, the reviewed data are considered insufficient to conclude on recommendations for OEL adjustment for shift work. The Quebec method is recommended to adjust for extended working hours. Each chemical is assigned in categories based on the toxic effect. No adjustment is applied for ceiling values, short-term exposure limits and limit values based on asphyxiation, irritation or malodour. For other substances producing effects following short- or long-term exposure, the method is based on Haber's rule and the type of adverse effect."