SPP1 and pulmonary fibrosis: Although a direct molecular mechanism linking CNTs to lung fibrosis remains unidentified, our previous studies using other nanoparticles (Ag [75] and CeO2 [76]) in nose-only inhalation experiments with Sprague–Dawley rats also showed significant upregulation of OPN in bronchoalveolar lavage fluid (BALF), similar to our current findings.