The three pulmonary regions represent very substantial differences in the surfaces where particles can be deposited. Thus, even though the mass of 20 nm ultrafine particles deposited in the alveolar region represents over 50% of the total mass, the deposited dust concentration, expressed in lung surface units, will still be over 100 times greater in the nasal region and more than 10 times greater in the tracheobronchial region (Oberdörster, 2005a). The quantity of particles and the particle deposition site in the pulmonary system are also influenced significantly by the presence of a pre-existing lung disease (Maynard and Kuempel, 2005; US.
