Objectives The joint effect of shift work and certain adverse life-style factors on coronary heart disease (CHD) was studied.

Methods Base-line measurements were obtained for a 6-year follow-up of an industrially employed cohort (N=1806), whose shiftwork status was recorded from a questionnaire filled out by a sample of the cohort. The CHD end points (codes 410-414 of the 9th revision of the International Classification of Diseases) were obtained from official Finnish registers. In order that the joint effects of shift work and life-style factors on the risk of CHD could be studied, dichotomized variables and their combinations as a dummy variable system in Cox's proportional hazards models were used.

Results The relative risks were 1, 1.6 [95% confidence interval (95% CI) 1.1-2.5], 1.3 (95% CI 0.9-2.1), and 2.7 (95% CI 1.8-4.1) for the following combinations of shift work (SW) and smoking (SM): SW-&SM-, SW-&SM+, SW+&SM-, and SW+&SM+, respectively; and the corresponding figures for shift work and obesity (BMI ³28 kg/m²) were 1, 1.2 (95% CI 0.8-1.9), 1.3 (95% CI 0.9-1.9), and 2.3 (95% CI 1.5-3.6), respectively. In both cases the effect was at least multiplicative. For the shift workers the relative risk for CHD rose gradually with increasing numbers of adverse life-style factors, but for the day workers there was no clear dose-response pattern.

Conclusion Shift work seems to trigger the effect of other, lifestyle-related risk factors of CHD and therefore calls for active prevention among shift workers.

Key terms circadian rhythm; interaction; obesity; physical activity; smoking; thrombosis