# Electronic Cigarettes' Toxicity: From Periodontal Disease to Oral Cancer
## 4 Chemical Composition of e-Liquids and Vapors and the Implications in Oral Carcinogenesis
### Tobacco-Specific Nitrosamines  
Tobacco-specific nitrosamines (TSNAs) are considered important risk agents in the induction of smoking-related cancers of the lungs, pancreas, esophagus, and oral cavity. NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone) and NNN (N\({}^{\prime}\)-nitrosonomicotine) are included in Group 1 of carcinogens to humans by the World Health Organization (WHO) International Agency for Research on Cancer (IARC) [69].  
Tobacco-specific nitrosamines (Figure 6) are procarcinogens and require metabolization and activation. A major metabolic pathway is TSNAs \(\alpha\)-hydroxylation, several CYP isoforms being involved. The malignant transformations caused by TSNAs are mainly due to the formation of adducts with DNA [70; 71]. Furthermore, TSNAs are also involved in the migration and invasion of cancer cells, promoting metastasis. A study performed on lung cancer cells revealed that NNK induced phosphorylation and activation of \(\mu\)- and m-calpain via ERK1/2. Calpains are matrix-degrading enzymes and have a key role in cancer cells dissemination [72; 73].  
NNK and NNN, similar to nicotine, have the ability to activate nicotinic acetylcholine receptors (nAChRs), which plays an important role in cancer initiation, but also in chemoresistance [60].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Flavors** &
\begin{tabular}{c} **Chemical Compounds** \\ **Found in Flavoring Agents** \\ \end{tabular} & **Toxic Potential** \\ \hline \hline \multirow{2}{*}{Mint} & \multirow{2}{*}{Menthol} & Cytotoxic, oxidative, inflammatory, loss \\  & & of epithelial barrier function \\ \hline \hline \multirow{2}{*}{Buttery} & \multirow{2}{*}{Diacetyl} & Oxidative, inflammatory, lung toxicity \\ \hline \hline \multirow{2}{*}{Chocolate} & Pyrazine derivatives & Cytotoxic \\ \hline \hline \multirow{2}{*}{Cherry} & \multirow{2}{*}{Benzaldehyde derivatives} & Oxidative, irritant, protein carbonylation \\  & & of extracellular matrix, DNA damage \\ \hline \hline \multirow{2}{*}{Cinnamon} & \multirow{2}{*}{Cinnamaldehyde} & Cytotoxic, oxidative, loss of epithelial \\  & & barrier function \\ \hline \hline \multirow{2}{*}{Vanilla} & \multirow{2}{*}{Vanillin} & Oxidative, cytotoxic, inflammatory, \\  & & irritant \\ \hline \hline \multirow{2}{*}{Caramel} & \multirow{2}{*}{Maltol} & Oxidative, inflammatory, loss of \\  & & epithelial barrier function \\ \hline \hline \end{tabular}
\end{table}
Table 1: The toxic effects of flavoring ingredients in e-liquids.  
[PERSON] et al. analyzed the tobacco nitrosamines in the vapors generated from 12 brands of e-cigarettes and determined the concentrations of NNK and NNN per e-cigarette (150 puffs). The values they found were between 0.8 ng and 4.3 ng for NNK and between 1.1 ng and 28.3 ng for NNN) [75].  
In another study, [PERSON] et al. analyzed 6 samples of e-liquids, and the concentration of TSNAs in liquid and aerosol samples was measured as below the detection limit [76].  
The conclusion of all these studies seems to be that TSNAs exist in a minimum amount in EC liquids, and the carcinogenic risk associated with the presence of TSNAs is low. However, their presence should not be completely ignored, because median levels of 0.47 pg/mL of NNAL (range: 0.125-3.2 pg/mL) were found in the urine of participants who were passively exposed to e-cigarettes. Although the levels are very low, it is difficult to quantify the risks, because there is no safe level of exposure. Additionally, the risks are closely related to the intensity and duration of exposure [77].