Ambient mineral particles in the small airways of the normal human lung

Recent epidemiological data have indicated that the ambient concentration of the inhalable fraction of atmospheric particles (PM10) is associated with respiratory and cardiovascular morbidity and mortality. These effects may be related to particle induced inflammatory reactions in the airways, but little actual information is available about the deposition and retention of ambient particles in human airways, particularly in the very small airways. To examine this question, we isolated portions of large airways, large airway carinas, and small airways between the mainstem bronchus and the respiratory bronchioles (RB) from the autopsy lungs of seven never-smokers from the general population, and determined mineral content by analytical electron microscopy. Overall, RB at generations 11,12, and 13 showed consistently high particle concentrations, averaging 25 to 100 times the particle concentration in the mainstem bronchus, but roughly similar elevated concentrations were also seen in the carinas of airway generations 4 and 5. Mean aerodynamic particle sizes by site ranged between 0.30 and 0.60μoxm, with slightly larger particles in the RB. Crystalline silica was the predominant mineral species in all sites. Ultrafine particles were present in all sites but constituted less than 15% of the total number of particles in any location. Ultrafine particles were largely individual particles of metals and crystalline silica, and no aggregated carbonaceous or noncarbonaceous ultrafine particles were identified. More than 90% of the particles in every site had aerodynamic diameters less than 2.5μoxm. We conclude that the RB and the carinas of generations 4 and 5 in the normal human lung show very high and roughly equal concentrations of inhaled ambient particles, suggesting that these two anatomic compartments are potential sites of particle toxicity. All portions of the airways effectively retain PM2.5. Ultrafine particles are retained throughout the airways and also show concentration peaks in the large airway carinas and RB.