Proximal airway epithelial damage (swelling and vacuolization) was evident at all ages following 10-ppm NA exposure, but was most severe for the juvenile female group. A, Neonatal male, B, juvenile male, C, adult male, D, neonatal female, E, juvenile female, and F, adult female. These representative images show sex and age differences in cellular vacuolization and swelling for the proximal airways following 10-ppm NA exposure (n = 5). Black arrowheads show damaged Club cells.