Number Concentration and Mass Concentration as Determinants of Biological Response to Inhaled Irritant Particles

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Abstract
Particulate pollutants are mixtures of a variety of chemical species. Sulfuric acid aerosol is a highly irritating component of particulate matter less than 10 μm (PM-10) that can produce adverse health effects at current peak ambient concentrations in the United States. We hypothesized that, in addition to the mass concentration of sulfuric acid, the number of sulfuric acid droplets was also an important factor affecting lung injury. To test this hypothesis, guinea pigs were exposed for 3 h to either filtered air; inert carbon particles at 10s particles! ml; sulfuric acid at 350 μg S042-/m3 layered on 10s, 107, or W6 carbon particles/ml; sulfuric acid at 50, 100, 200, and 300 μg SO42-lm3 layered on 108 carbon particles/ml. Alterations in phagocytic capacity (PC), intracellular pH (pHi), and intracellular free calcium concentration ([Ca2+]i) of harvested macrophages were used as indices of irritant potency. At a fixed number concentration of particles (108 particles/ml), there was a sulfuric acid concentration-dependent decrease in PC, pHt, and [Ca2+]i. Furthermore, at a fixed mass concentration (350 μg SO42-/m3), sulfuric acid layered carbon particles at 108 particles/ml but not at other number concentrations decreased pH, of macrophages. The number concentration of sulfuric acid layered carbon particles did not affect PC or [Ca2+]i. These results suggest that there is a threshold for both number concentration and mass concentration for the aerosols to produce a biological response, and that epidemiologic studies should consider other aerosol characteristics in addition to mass when attempting to relate health endpoints to ambient pollutant exposures.