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

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 10^s particles! ml; sulfuric acid at 350 μg S0₄^2-/m³ layered on 10^s, 10⁷, or W⁶ carbon particles/ml; sulfuric acid at 50, 100, 200, and 300 μg SO₄^2-lm³ layered on 10⁸ carbon particles/ml. Alterations in phagocytic capacity (PC), intracellular pH (pH_i), 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, pH_t, and [Ca²⁺]_i. Furthermore, at a fixed mass concentration (350 μg SO₄^2-/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.