Co-existing ambient fine particulate matter exacerbated electronic cigarette toxicity on human respiratory cells.

Respiratory co-exposure to ambient PM_2.5 and electronic cigarettes (e-cigarettes) frequently occurs in public. However, the combined effects on human respiratory health have not been well documented. To discuss potential co-effects and possible biological mechanisms, A549/THP-1 co-cultures and BEAS-2B cells were exposed to unvapedtobacco or mint-flavored e-liquids (0-7.2% v/v), e-cigarette aerosol extract (ECE, 0-50% v/v), PM_2.5 (60 μg/mL), or PM_2.5 + ECE for 24 h. Cell viability assessments on e-liquids, ECE, PM_2.5 + ECE showed that the mint flavor exhibited higher cytotoxicity compared to the tobacco flavor in both A549/THP-1 and BEAS-2B. However, the influence of flavors on ROS levels and mRNA expression of inflammatory markers (IL-6, TNF-α, IL-8, IL-1β) after ECE exposure demonstrated inconsistency in the two cell models. PM_2.5 + ECE treatment notably elevated ROS production and inflammation responses compared to ECE alone exposure. Only co-exposure induced a significant increase in nuclear transcription factor-κB p65 (NF-κB p65) and NOD-like receptor 3 (NLRP3) protein expression regardless of flavors. Our results indicate that PM_2.5-treated cells exacerbate the adverse effects induced by ECE in both A549/THP-1 and BEAS-2B cells. Flavors in unvaped e-liquids affect cytotoxicity, oxidative stress and inflammation response, but these effects vary depending on the vaping process and the specific cell line.