ABSTRACT
Deposition of inhaled nanoparticles (NPs) is governed by their diffusivity, leading to a rather homogeneous deposition density on the epithelium of the respiratory tract. This results in a major deposited fraction in the alveolar region of the lungs, with a maximum of 30%–40% of the inhaled aerosol with a size of 20 nm (International Commission on Radiological Protection 1994; MPPD software). Below that size, increasing fractions deposit in the airways of the head and thorax according to their increasing diffusivity with decreasing size, such that fewer NP reach the distal alveolar region. Once deposited in the peripheral lungs, NP are not only subject to phagocytosis by alveolar macrophages but also to endocytosis by epithelial cells. It appears plausible that macrophages take up all NP deposited in their immediate vicinity, while distant NP are not recognized (Kreyling et al. 2013). As a result, a major fraction (>80%) of the NP in the peripheral rodent lung enter the epithelium and penetrate into interstitial spaces (Semmler-Behnke et al. 2007a).
