Recent studies have used as a reference group women who had no exposure to tobacco smoke - that is, they have never actively or passively smoked. These studies in most cases have reported increases in breast cancer risk for women who smoked or were passively exposed to tobacco smoke compared to reference women who were never exposed. Critics of this approach cite studies that indicate the reference women who have never been exposed to tobacco smoke are healthier, in general. They argue that the difference in risk is due to the better health of these women used as references for risk. Older studies used as a reference.
