Samples of PM2.5 were gravimetrically collected during the winter 2005/2006 in the urban area of Milan (North Italy). Samples were chemically characterized and the particles were detached from filters to determine their cytotoxic effects on the A549 cell line. Based on the potential toxicological relevance of its components, Milan winter PM2.5 contained high concentrations of pro-oxidant transition metals and PAHs, while re-suspended particles showed a relatively high frequency of dimensional classes ranging from 40 nm to 300 nm. A549 cells exposed to particle suspensions showed a concentration-dependent decrease in viability, starting from 10 μg/cm2. Phagocytosis of particles by A549 cells and particle aggregates were morphologically characterized and seemed to depend on both particle concentration and exposure time, with the majority of particles being engulfed in membrane-bound vacuoles after 24 h of exposure. The ability of ultrafine particles to penetrate and spread throughout the cells was also verified. Cell membrane lysis and mitochondrial ultrastructural disruption appeared to be the main modifications induced by PM2.5 on A549 cells. Concomitantly to the adverse effects observed in terms of cell mortality and ultrastructural lesions, a significant intracellular production of reactive oxygen species (ROS) was observed, suggesting that the cytotoxicity, exerted by the winter PM2.5 in Milan, derived also from its oxidative potential, probably associated with particle-adsorbed metals and PAHs. © 2009 Elsevier Ireland Ltd. All rights reserved.

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