Instillation of six different ultrafine carbon particles indicates a surface area threshold dose for acute lung inflammation in mice

Abstract

Increased levels of particulate air pollution are associated with increased respiratory and cardiovascular mortality and morbidity. Some epidemiologic and toxicologic research suggests ultrafine particles (UFPs) (< 100 nm) to be more harmful per unit mass than larger particles. Our study was aimed at a quantitative comparison of acute adverse effects of different types of carbonaceous UFPs at a dose range that causes a moderate inflammatory response in lungs. We used six different particle types (primary particle size 10-50 nm, specific surface area 30-800 m2/g, and organic content 1-20%): PrintexG, Printex90, flame soot particles with different organic content (SootL, SootH), spark-generated ultrafine carbon particles (ufCP), and the reference diesel exhaust particles (DEP) SRM1650a. Mice were instilled with 5, 20, and 50 microg of each particle type, and bronchoalveolar lavage was analyzed 24 hr after instillation for inflammatory cells and the level of proinflammatory cytokines. At respective mass-doses, particle-caused detrimental effects ranked in the following order: ufCP > SootL > or = SootH > Printex90 > PrintexG > DEP. Relating the inflammatory effects to the particle characteristics--organic content, primary particle size, or specific surface area--demonstrates the most obvious dose response for particle surface area. Our study suggests that the surface area measurement developed by Brunauer, Emmett, and Teller is a valuable reference unit for the assessment of causative health effects for carbonaceous UFPs. Additionally, we demonstrated the existence of a threshold for the particle surface area at an instilled dose of approximately 20 cm2, below which no acute proinflammatory responses could be detected in mice.

Figure 1

Particle-dependent elevation of PMNs (A), IL-1β (B), and MIP2 (C) in BAL from noninstilled (Co) and water-instilled (H₂O) control mice and mice instilled with water, DEP, PrintexG (PtxG), Printex90 (Ptx90), SootH, SootL, and spark-generated ufCP of increasing inflammatory potency. Results are expressed as a function of gravimetric dose (5, 10, and 50 μg). Bars represent mean ± SE of eight animals. *Significantly elevated compared with Co and H₂O controls (p < 0.05).

Figure 2

PMN cell numbers and IL-1β and MIP2 concentrations are plotted as a function of different characteristics of the instilled UFPs: (A) primary particle diameter (for an exemplary dose of 20 μg); (B) OC mass; and (C) particle surface area (BET). Each symbol represents mean values of eight animals. A regression analysis revealed a strong logarithmic relation for the surface area (r² = 0.65) and a weaker relation for the organic contribution (r² = 0.38).

Figure 3

Threshold for inflammatory response for ufCP shown as the correlation between particle surface area and PMN cell numbers (A) or IL-1β (B) or MIP2 (C) concentrations. Two additional instilled doses of 0.5 and 2 μg, together with 5, 20, and 50 μg ufCP represent BET surface areas of 0.4, 16, 40, 161, and 404 cm², respectively. The dashed line indicates baseline levels of control animals; each circle represents the mean of eight animals.