MMP12 and pulmonary emphysema: Our study is unique in emphasizing the association between in utero SHS exposures and aggravation of adult emphysema, and is the first to show that Mmp12 is a key element in the in utero SHS exposure-induced predisposition to emphysema, following adult exposure to a second toxicant, even though, in our study, there was no direct exposure of the fetal lung to CS or SHS.