Researchers from the University of Minnesota led efforts to quantify propylene glycol originating from the saliva of e-cigarette users. Their research, which uses gas chromatography–mass spectrometry (GC–MS), was published in the Journal of Chromatography B (1).
In 2021, it was estimated that there were approximately 82 million e-cigarette users across the world, based off available data from 48 countries (2). This compares to approximately 1.18 billion cigarette smokers worldwide in 2020. e-Cigarettes are believed to be less harmful than conventional cigarettes since they lack tobacco and combustion, therefore not generating multiple toxicants and carcinogens characteristic of cigarette smoking. However, e-cigarettes can generate harmful toxicants with use leading to potential nicotine addiction.
Clinical studies to characterize the patterns and consequences of e-cigarette use are needed to gain better understanding this form of nicotine. Reliable and practical biomarkers, which can easily distinguish different forms of nicotine product use, are vital for such clinical studies. One potentially promising biomarker of e-cigarette use is salivary propylene glycol. Propylene glycol is an organic alcohol that is commonly used as a skin conditioning agent; however, this substance is also a major component of e-liquid, and the oral cavity is the first site of exposure in e-cigarette users (3). However, there are relatively few studies examining salivary propylene glycol as a biomarker. One existing study used 1H nuclear magnetic resonance (NMR) spectroscopy to quantify metabolites in saliva, finding higher concentrations of propylene glycol in male compared to female saliva among non-smoker study participants; however, e-cigarette use was not investigated.
In this study, the researchers characterized and validated salivary propylene glycol as a potentially useful biomarker of e-cigarette use. Levels of the biomarker were determined in various clinical studies comparing e-cigarette users, cigarette smokers, and non-users of either product. Two separate protocols, termed programmed and non-programmed, were used. In the programmed protocol, e-cigarette users (n = 20) and cigarette smokers (n = 20) brushed their teeth, waited 15 min, then used their product every 30 s for 5 min and collected a 2 mL saliva sample immediately after the last puff. In the non-programmed protocol, cigarette smokers (n = 30) and e-cigarette users (n = 21) came to the clinic and directly provided a saliva sample for analysis. Non-users (n = 29) of any nicotine or tobacco product served as controls.
Propylene glycol in saliva was quantified by gas chromatography–mass spectrometry (GC–MS) as its heptafluorobutyrate ester, using propylene glycol-d8 as the internal standard. Accuracy and precision of the assay were established. Levels of salivary propylene glycol (mean ± S.D.) were 269 ± 319 and 8.04 ± 8.22 μg/mL saliva in e-cigarette users and cigarette smokers, respectively, in the programmed protocol, while the corresponding levels in the non-programmed protocol were 4.94 ± 8.68 and 1.91 ± 1.99 μg/mL. These amounts were all significantly greater than those found in non-users of any tobacco or nicotine product (0.39 ± 0.32 μg/mL). The straightforward validated assay featured in this study is expected to be useful in future clinical studies focusing on e-cigarette use.
(1) Tang, M-K.; Carmella, S. G.; Zhao, Y.; Maertens, L. Quantitative Analysis of Propylene Glycol in Saliva of E-Cigarette Users by Gas Chromatography-Mass Spectrometry. J. Chromatogr. B 2025, 1264, 124719. DOI:
(2) Jerzyński, T.; Stimson, G. V. Estimation of the Global Number of Vapers: 82 Million Worldwide in 2021. Drugs Habits Soc. Policy 2023, 24 (2), 91–103. DOI:
(3) Propylene Glycol. EWG’s Skin Deep 2025.
