Effects and Fate of Inhaled Ultrafine Particles

Abstract

Concentration of inhaled particulate matter (PM) varies by orders of magnitude from ng/m 3 to mg/m 3 , depending on material, particle size, and proximity to sources. Particle number concentrations, expressed as particles/cm 3 , can also span several orders of magnitude, with ultrafine particles ( 2 μm, impaction) and smaller ultrafine (<5 nm, diffusion) particles; in the respiratory tract overall there is minimal deposition at 0.2 - 0.5 μm (settling and diffusional displacement minimal). While chemical composition does not influence deposition of inhaled particles-assuming no hygroscopicity-disposition of deposited particles is highly dependent on their chemical characteristics. Since PM is a chemically complex mixture, different mechanisms including dissolution, leaching, chemical binding and mechanical transport are involved in particle clearance. For solid poorly soluble particles, classical clearance mechanisms are size dependent and pathways include mucociliary escalator in nasal and tracheobronchial airways to the GI-tract, alveolar macrophage-mediated clearance in the alveolar region to the mucociliary escalator, interstitial translocation and lymphatic uptake to regional lymph nodes. More recently, two additional pathways to extrapulmonary organs have been described, specifically for poorly soluble ultrafine particles: translocation to blood circulation; and transport to ganglia and structures of the CNS along axons of sensory nerves located in nasal and tracheobronchial epithelia. This could provide a plausible mechanism for adverse cardiovascular and CNS health effects.