Increased risk of ischaemic heart disease in shift workers.

504 papermill workers were followed up for 15 years and the incidence of ischaemic heart disease (IHD) in shift workers was compared with that in day workers. The relative risk (RR) of IHD rose with increasing duration of reported exposure to shift work. A significant risk of IHD was associated with an exposure of 11 - 15 years (RR = 2.2, p less than 0.04) and of 16 to 20 years (RR = 2.8, p less than 0.03. The association was independent of age and smoking history. The RR of IHD fell sharply after 20 years of shift work. This was ascribed to the pronounced positive selection that had taken place in this group.