Nowadays, the research about electronic cigarettes (e-cigarettes) has not achieved a strong consensus. On the one hand, most researchers and health professionals are concerned about its health consequences (McNeill et al., 2022, National Academies of Sciences, xxxx). In contrast, some researchers argue that e-cigarettes contribute to reducing the harms produced by tobacco smoking, and thus they can be a useful tool for smoking cessation (Chan, xxxx, Hartmann-Boyce et al., 2021 September). On the other hand, researchers are concern about the possibility that their use may be a gateway to starting smoking, especially among the younger population (Cao et al., 2020, Glantz, 2014). Some evidence seems to support the gateway hypothesis (National Academies of Sciences) while other does not (Beard et al., 2022 August 01, Shahab et al., 2022).

Due to the increasing prevalence of the e-cigarette use (Cullen et al., 2019, Kapan et al., 2020), and the controversy of e-cigarette use, there is the necessity of doing research on the possible e-cigarette’s harm. Studies comparing tobacco-specific biomarkers from e-cigarette and tobacco may be useful for that end. Regarding tobacco-specific biomarkers, one of the most prevalent carcinogens in unburned tobacco are N-nitrosamines (TSNAs), including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNK), its main metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and N′-nitrosonornicotine (NNN) (Hecht, 2017). In addition, cotinine, the main metabolite of nicotine, has been extensively used as marker of tobacco exposure in the 3 or 4 days prior to sample collection (Benowitz, 1996). Several studies have proved that TSNAs have strong organ-specific carcinogenicity, mutagenicity, and teratogenic action, among others (Hecht and Hoffmann, 1988, Magee, 1971). NNK and NNN have been shown to be the strongest carcinogens among the TSNAs (Hoffmann & Hecht, 1985). However, according to previous studies, lower levels of TSNAs are found in e-cigarettes compared to conventional tobacco, and it could be linked to a lower carcinogenic effect as well (Bustamante, Ma, & Yakovlev, 2018), however there is a lack of evidence regarding the health effect associated with TSNAs concentration, and therefore of the TSNAs dose-toxicity.

In this sense, studies analyzing cotinine and/or TSNAs in e-cigarette users compared with smokers (Carroll et al., 2018, Dai et al., 2022, Göney et al., 2016, McNeill et al., 2022, Shahab et al., 2017, Smith et al., 2020, Wagener et al., 2017) are necessary. Although previous studies show heterogenous results, most of them found lower levels of TSNAs (Carroll et al., 2018, Hoffmann and Hecht, 1985, McNeill et al., 2022, Shahab et al., 2017, Wagener et al., 2017) in e-cigarette users vs smokers, and no difference in cotinine (Carroll et al., 2018, Göney et al., 2016, Shahab et al., 2017, Wagener et al., 2017) between these groups. However, none of these studies included information about e-cigarettes users utilizing liquids without nicotine, and just a few of them included information about these biomarkers in non-smokers (Göney et al., 2016, Smith et al., 2020). Moreover, most studies were performed using urine samples and only NNAL was determined. While indeed NNAL is abundant in urine, saliva samples require the monitoring of NNN and NNK, since in this biological fluid, NNAL, metabolite of NNK, is less abundant. (Carroll et al., 2018, Dai et al., 2022, Fu et al., 2009, Shahab et al., 2017, Smith et al., 2020, Wagener et al., 2017).

Moreover, the main diluents found in the liquids needed to use electronic cigarette (e-liquids), glycerol and propanediol (PD also known as propylene glycol), are potentially hazardous (Clapp & Jaspers, 2017). Although both components are used in a wide variety of consumer products, and their exposure via oral and dermal appear to be innocuous, inhalation risks are still not well characterized. In this sense, some research (Bonner et al., 2021, Callahan-Lyon, 2014, Kaur et al., 2018) suggests that glycerol and PD are both airway irritants, and that PD causes eye, mouth, throat, mucous membrane, and respiratory irritation, dry cough, constriction of peripheral airways, and inflammatory responses through MAP kinases, JAK/STAT, and NF-kB pathways. Moreover, according to the International Agency for Research on Cancer (IARC) (IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Some Industrial Chemicals. Lyon (FR): International Agency for Research on Cancer; 1994 (FR): International Agency for Research on Cancer; 1994, 1994) there is sufficient evidence in experimental animals to conclude PD carcinogenicity. Moreover, to the best of our knowledge, no previous research analyzes these components comparing e-cigarette users with or without nicotine. However, according to The Committee of Toxicology (Committee on toxicity of chemicals in food, consumer products and the environment (COT), 2020), toxicity associated with PD is extremely low.

This study aims to address the existing gap in the literature, while adding evidence, by comparing tobacco biomarkers (cotinine and nitrosamines: NNK, NNAL, NNN), and e-cigarette biomarkers (glycerol, 1,2-PD, 1,3-PD) according to five groups: 1) non-smokers, 2) e-cigarette exclusive users without nicotine, 3) e-cigarette exclusive users with nicotine, 4) smokers, and 5) dual users (e-cigarette and tobacco).