Mitotic process analysis. (A) Cyclin B1 expression was measured by flow cytometry in BEAS-2B cells exposed for 3, 6, 10 and 24 h to 7.5 μg/cm2 of winter PM2.5. The results are reported as fluorescence arbitrary units (mean ± SEM of 3 independent experiments). *Statistically significant difference from untreated cells (control), P < 0.05. (B) Nuclear morphology of BEAS-2B cells exposed for 24 h to 7.5 μg/cm2 of winter PM2.5: fluorescence microscopy illustrating micronucleated (arrows, 18.8% in treated samples vs. 3.2% in controls; statistically significant difference, P < 0.05) and binucleated (asterisk) cells. The results are representative of 3 independent experiments; in each experiment 300 cells were scored.