Thermal Degradation of Nicotine in E-Cigarettes: Increased Cotinine and Myosmine in Vaped E-Cigarette Aerosols and Investigation of Their Synergistic Toxicity

The dynamic thermal chemistry of nicotine ((S)-nicotine) remains under-characterized in e-cigarettes. E-cigarettes are often perceived as less harmful because of relatively lower operating temperatures; however, reduced heat can lead to alternative degradation pathways and increased aerosol toxicity. This study demonstrates that the tobacco alkaloids cotinine ((S)-cotinine) and myosmine increase during heating and aerosolization via distinct chemical pathways at temperatures relevant to e-cigarette use. Using flash vaporization and gas chromatography–mass spectrometry (GC–MS) analysis, we observed a mechanistic shift from radical-mediated hydrogen abstraction at lower temperatures (<375 °C) to pyrolytic unimolecular dissociation at higher temperatures (>475 °C). Analysis of e-cigarette aerosol at 300 °C showed a dramatic enrichment of cotinine and myosmine, by 3-fold and 1.5-fold, respectively, relative to the source e-liquid, effectively bypassing regulatory nicotine sample purity standards. Toxicological profiling of human bronchial epithelial cells (BEAS-2B) using concentration–response curves and isobologram analyses revealed that, while cotinine and nicotine exhibit additive toxicity, myosmine and nicotine mixtures exhibit synergistic toxicity. Each mixture scenario elicits greater cytotoxicity than its individual components, underscoring the need for further exposure mitigation for complex mixtures detected in e-cigarettes.