The main results of our study are that: 1) TiO2 NPs did not induce inflammation per se and did not aggravate elastase-induced pulmonary inflammation and emphysema, except for an increase in elastase-induced MMP-12 mRNA expression in the lung, and 2) CB NPs induced perivascular/peribronchial infiltration, increased HO-1 mRNA expression in the lung and increased MMP-12 mRNA and protein expression in alveolar macrophages without inducing emphysema. Here, MMP12 is linked to pulmonary emphysema.