Seasonal Variation of Airborne Particle Deposition Efficiency in the Human Respiratory System

WALTER HAM (1), Chris Ruehl (1), Michael Kleeman (1)

(1) University of California, Davis

     Abstract Number: 514
     Last modified: May 13, 2010

Abstract
Airborne Particulate Matter (PM) causes adverse health effects in humans but the apparent toxicity of PM varies with location and season. Seasonal changes in the physical or chemical properties of PM may affect its overall toxicity; however, another possibility is that seasonal changes to particle size distributions affect deposition efficiencies which in turn affects toxicity. In the current study, size-resolved PM composition measurements were made in the largest city (Fresno) in California’s heavily polluted San Joaquin Valley (SJV) during the summer and winter months between 2006 and 2009 for 21 metals, organic carbon (OC), elemental carbon (EC), and water-soluble ions (Cl$^-, NO$_3$^-, SO$_4$^(2-), Na$^+, NH$_4$^+, K$^+, and Ca$^(2+)). The Multiple Path Particle Dosimetry model was applied to these measurements to determine if seasonal variations in size-resolved composition influences respiratory deposition patterns. Mg, Al, S, V, Mn, Fe, Ni, Ba, SO$_4$^(2-), Na$^+, and Ca$^(2+) had larger total deposition efficiencies (p<0.004) during the summer vs. the winter in all three regions of the respiratory tract. This trend results from increased relative concentrations of the target analytes per micro-gram per cubic meter ambient PM$_(1.8) concentration and would be detected with a routine PM$_(2.5) filter sample. V, Zn, Se, EC, NO$_3$^-, SO$_4$^(2-), NH$_4$^+, and K$^+ also experienced seasonal size distribution shifts that enhanced specific deposition efficiency in the tracheobronchial and pulmonary regions during the summer months (p<0.026). This enhanced deposition would not be detected by a routine filter sample because all of the size distribution changes occur at diameters smaller than 2.5 micro-meters. This study demonstrates that changes to the particle size distribution below 2.5 micro-meters can enhance respiratory deposition efficiencies for trace metals, OC/EC, and/or water-soluble ions and this may contribute to PM toxicity during the summer in the SJV.