The effects of exposure to 15-nm, 30 nm or micro-sized SiO
particles onon cellular apoptosis of HaCaT cells. A shows the representative images of apoptosis of HaCaT cells after 24-h exposure to 15-nm, 30-nm or micro-sized SiO2 particles at the dosages of 2.5 μg/mL, 5 μg/mL, 10 μg/mL or 15 μg/mL. B shows the quantitative results. Quadrant a, b, c and d denote the ratio of necrotic cells, late-stage apoptotic cells, normal cells, and early-stage apoptotic cells, respectively. Values were mean ± SD from three independent experiments. **p < 0.01 vs control cells; ##p < 0.01 vs cells exposed to 30-nm SiO2 particles; &p < 0.05, &&p < 0.01 vs cells exposed to micro-sized SiO2 particles.