When applying different previously used exposure metrics
for night work, we found that estimates showed a slight
(10%–30%) but nonsignificant increase of risk for breast cancer in the highest exposure categories. By using consecutive
night shifts as the exposure parameter, we observed significantly elevated breast cancer risks in the range of 1.7–1.8 in
nurses who worked 5 or more years in jobs including 6
consecutive night shifts, compared with nurses who never