Third-Hand Smoke Exacerbates H₂O₂-Driven Airway Responses in A549 Cells

Third-hand smoke (THS) is residual smoke after extinguishing a cigarette and adhering to surfaces. Re-emission into the air also makes THS a health concern for those who suffer from respiratory diseases. The present study aimed to elucidate the mechanistic pathways involved in THS-induced respiratory toxicity and the accelerative potential of THS in an H₂O₂-induced oxidative stress model of human airway epithelia in vitro.

THS extracted from terrycloth exposed to 3R4F cigarettes was assessed via 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay to identify cytotoxicity. The reactive oxygen species (ROS) level was determined via 2,7-dichlorofluorescein diacetate (DCFDA) fluorescence intensity in a flow cytometer, and glutathione (GSH), malondialdehyde (MDA), and catalase (CAT) activity were assessed spectrophotometrically. Interleukin-6 (IL-6) level was measured via enzyme-linked immunosorbent assay.

THS 50% (v/v) with significant cytotoxicity in A549 cells upregulated intracellular ROS levels via a right-shifted fluorescence intensity of DCFDA compared with the control (p < 0.05), which was also amplified with H₂O₂ co-treatment. MDA levels remarkably increased with THS (p < 0.05). Both THS and THS + H₂O₂ led to notable GSH depletion, increased CAT activity, and increased IL-6 levels, which were attenuated by the negative control (N-acetylcysteine, 1 mM) (p < 0.05).

The induction of oxidative stress may be an important event in THS-induced airway toxicity that may contribute to the progression of respiratory diseases.