Particle Toxicology. Donaldson K, Borm P, eds., Boca Raton, FL: CRC Press, 2006 Dec; :351-386
Inhaled airborne particles may be deposited in the respiratory tract with a probability that depends on the physical properties of the particles, the velocity of the air, and the structure of the airways. Once deposited, particles may be retained at the site of deposition, translocated elsewhere in the body, or cleared by the biological processes specific to each region of the respiratory tract. The major regions of the human respiratory tract include the extrathoracic (nasopharynx or head airways), thoracic (tracheobronchial airways), and alveolar (pulmonary or gas-exchange) (ICRP 1994). These regions differ in structure and function (Miller 1999; McClellan 2000), 'The functions of the extrathoracic and thoracic regions include air conditioning and conducting, while the main function of the alveolar region is the gas exchange. Clearance of particles depositing in the alveolar region occurs primarily by alveolar macrophage (AV)-mediated clearance to the thoracic region, where they are cleared via the "mucociliary escalator" and then expectorated or swallowed. All regions of the respiratory tract include lymphatic tissue. The extrathoracic region drains to the extrathoracic lymph nodes, and the thoracic and alveolar regions drain to the thoracic (also called hilar) lymph nodes. Particles that are not cleared from the lungs may enter the lung interstitium and translocate to the lymph or blood circulation. Several terms have been adopted to describe particles based on their size and probability of deposition within the respiratory tract. Inhalable particles are those capable of depositing anywhere in the respiratory tract. Thoracic particles are those capable of depositing in the lung airways. Respirable particles are those capable of depositing in the gas exchange region of the lungs (ACGIH 2005). The respirable particle size distribution includes the ultrafine or nanoparticles (primary particle diameter < 0.1 microm), fine particles (< 2.5 microm), and coarse particles with diameters < 10 microm.