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Abstract: A study was conducted to investigate the in vitro toxicities of dust storm particulate matter with an aerodynamic diameter⩽2.5μm (PM2.5) on rat alveolar macrophages (AM). This was based on the ambient PM2.5 collected in March 2004 from Baotou city, Inner Mongolia Autonomous Region, China. The particles were classified as normal (from sunshiny and clean days) and dust storm samples according to the dust storm classification, and the local weather and air quality monitoring data. The cell viability, levels of cellular thiobarbituric acid-reactive species (TBARS), glutathione (GSH), and cytosolic free calcium ions (Ca2+), and the plasma membrane ATPase activities and membrane lipid fluidity were determined 4h following the in vitro treatment of AM with differing dosages of the collected samples. Results revealed that dust storm PM2.5 and their water-soluble fractions at high dosages generated oxidant stress on AM, induced leakage of lactate dehydrogenase (LDH), significantly decreased activities of plasma membrane Na+K+-ATPase, increased intracellular Ca2+ levels, and led to significant alterations in membrane lipid fluidity, finally resulting in cytotoxicity. A two-way ANOVA showed that there was no significant difference in the above indices measured between the normal and dust storm PM2.5, suggesting the deleterious effects of ambient PM2.5 from Baotou city were based on the dose used rather than the type of particles. But due to a higher concentration of airborne PM2.5 mass concentration in dust storm days than that in normal days, it concluded that during dust storm episodes, a much more serious effect on AM was imminent. [Copyright &y& Elsevier]