Table 3.

The impact of e-cigarette liquid exposure on e-cigarette users (in vivo studies).

	Patients	Characteristic of E-Liquid	Action	Ref.
1.	Healthy never-smokers (before and after e-cigarette usage)	Base: PG:VG (70%/30%) Nicotine: nicotine-free Flavor: flavorless	Increased: oxidative stress, inflammation, circulatory burden of the serum	[177]
2.	Healthy never-smokers (before and after e-cigarettes usage)	Unknown	Increased: plasma endothelial microparticle levels Altered: transcriptomes of small airway epithelium and alveolar macrophages	[187]
3.	Healthy never-smokers (before and after e-cigarettes usage)	Base: PG:VG Nicotine:1.2% Flavor: tobacco	Decrease: high-frequency spectral component in electrocardiogram (0.15-0.4 Hz) Increase: low-frequency spectral component in electrocardiogram (0.04–0.15 Hz) and the low-frequency to high-frequency ratio Altered: in cardiac sympathovagal balance towards sympathetic predominance	[178]
4.	Healthy e-cigarette users (divided into groups either exposed or not exposed to e-cigarette vapor)	Unknown	Altered: gene expression taking part in Wnt/Ca+ pathway and Rho family GTPases signaling pathway in oral cells	[188]
5.	Healthy e-cigarettes users vs controls subjects (never smokers)	Unknown	Decreased: high-frequency spectral component in electrocardiogram Increased: low-frequency spectral component in electrocardiogram, low frequency to high frequency ratio, low-density lipoprotein oxidizability, oxidative stress	[179]
6.	Healthy e-cigarettes users (before and after e-cigarettes usage)	Base: PG:VG Nicotine: 18 mg/mL Flavor: tobacco	Decreased: peak expiratory flow Increased: particularly platelet microparticles	[189]
7.	Healthy e-cigarettes users vs controls subjects (never smokers)	Unknown	Decreased: lung function test parameters	[176]
8.	Healthy sporadic smokers (before and after e-cigarettes usage)	Base: PG:VG (49.4%/44.4%) Nicotine: 19 mg/mL Flavor: flavorless	Increased: heart rate and arterial stiffness, flow resistance, blood pressure	[190]
9.	Healthy sporadic smokers (divided into two groups either exposed or not exposed to e-cigarette vapor)	Base: PG:VG (49.4%/44.4%) Nicotine: 12 mg/mL Flavor: flavorless	Increased: endothelial progenitor cells level in blood, E-selectin positive microvesicles (endothelial origin)	[191]
10.	Healthy smokers vs controls subjects (never smokers)	Base: PD Nicotine: <10% Flavor: tobacco	Decreased: fraction of exhaled nitric oxide Increased: total respiratory impedance, flow respiratory resistance, overall peripheral airway resistance	[175]