Numbers Game: Answer

After controlling for confounding variables, the HDL (protective cholesterol) levels of women who smoke a pack of cigarettes a day are, on average, 17 percent lower than those of their non-smoking counterparts.

Source: The National Heart, Lung, and Blood Institute Family Heart

"Controlling for confounding variables" means that, in this study, cigarette smoking was isolated as a singular factor.

Although it is common knowledge that smoking contributes to atherosclerosis (hardening of the arteries), its HDL cholesterol-lowering properties are not as widely publicized.

The best news? As the Harvard Medical School so effectively summarizes it, "every 1% increase in HDL is associated with a 1%–3% reduction in heart attack risk."