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Background:PM2.5 (aerodynamic diameter ≤ 2.5 μtm) is a dominant and ubiquitous air pollutant that has become a global conc as PM2.5 exposure has been linked to many adverse health effects including cardiovascular and pulmonary diseases.Emerging evidence supports a correlation between increased air PM2.5 levels and skin disorders although reports on the underlying pathophysiological mechanisms are limited.Oxidative stress is the most common mechanism of PM2.5-induced adverse health effects.This study aimed to investigate PM2.5-induced oxidative damage and apoptosis in immortalized human keratinocyte (HaCaT) cells.Methods:HaCaT cells were exposed to 0,25,50,100,or 200 μtg/ml PM2.5 for 24 h.Reactive oxygen species (ROS) generation,lipid peroxidation products,antioxidant activity,DNA damage,apoptotic protein expression,and cell apoptosis were measured.Results:PM2.5 exposure (0-200 μtg/ml) for 24 h resulted in increased ROS levels (arbitrary unit:201.00 ± 19.28,264.50 ± 17.91,305.05 ± 19.57,427.95 + 18.32,and 436.70 ± 17.77) and malondialdehyde production (0.54 ± 0.05 nmol/mg prot,0.61 ± 0.06 nmol/mg prot,0.68 ± 0.05 nmol/mg prot,0.70 ± 0.05 nmol/mg prot,and 0.76 ± 0.05 nmol/mg prot),diminished superoxide dismutase activity (6.47 ± 0.28 NU/mg prot,5.97 ± 0.30 NU/mg prot,5.15 ± 0.42 NU/mg prot,4.08 ± 0.20 NU/mg prot,and 3.76 ± 0.37 NU/mg prot),and increased DNA damage and apoptosis in a dose-dependent manner in HaCaT cells.Moreover,cytochrome-c,caspase-3,and caspase-9 expression also increased proportionately with PM2.5 dosing.Conclusion:PM2.5 might elicit oxidative stress and mitochondria-dependent apoptosis that likely manifests as skin irritation and damage.