Arguably the most significant change that came with the fourth-generation devices is the use of nicotine salts, such as benzoic acid, as opposed to free-base liquid nicotine. Freebase nicotine is alkaline, irritating, and harsh to inhale, which limits the concentration that can be inhaled. Addition of benzoic acid or other salts results in protonation of the tertiary amine on nicotine’s pyrrolidine ring, forming nicotine salts. The reduced pH of the e-liquid allows for inhalation of much higher nicotine concentrations with less of the acute respiratory irritation typical of alkaline e-liquids. Third and fourth generation devices achieve similar doses of blood nicotine as tobacco cigarettes and in some cases are being used to deliver substances other than nicotine, including cannabinoids.The Centers for Disease Control found that non-Hispanic white males are more likely to use e-cigarettes than Hispanic whites or African Americans.Lower-income smokers are also less likely to use e-cigarettes.There is concern that e-cigarettes could be a gateway to combustible cigarette use or encourage continued or enhanced nicotine addiction . Indeed, longitudinal studies have shown that young people who start using e-cigarettes are at risk of also using combustible cigarettes.However, we know little about how e-cigarette use affects other tobacco related behaviors,cannabis grow indoor and since the marketplace and demographics are changing, whether this observation holds true for fourth generation devices remains to be determined. Obtaining reliable data on e-cigarette use will depend on the development and adoption of standardized/validated self report measures of use behaviors, perceptions, and attitudes, which will allow researchers to compare across studies over time.

While there are measures currently used in surveillance studies such as Population Assessment of Tobacco and Health and CDC surveillance, the rapidly evolving landscape of e-cigarettes presents challenges including tracking the evolution of terminology and device design. Timeliness of data collection and availability for use are crucial given the rapidly evolving marketplace. The methods used for tobacco surveillance typically do not produce usable data until a year or more after data collection and rarely allow for more than annual data collection. While in-place surveillance can remain the backbone for e-cigarette investigation, novel innovative methods are required, including real-time sentinels, such as using informatics to analyze online and big data resources including Google Trends or Twitter. Thus, despite the heterogeneity in tobacco and nicotine use behaviors and trajectories, the skyrocketing popularity of e-cigarettes suggests an urgent need to better understand the implications of e-cigarette patterns of use and uptake.While cigarette smoke comes from combustion, e-cigarette vapor results from heating liquids to high temperatures, leading to a different type of exposure and dosimetry. Vaporized components of e-liquids cool and reach critical supersaturation conditions that result in a phase transition from vapor to aerosol,followed by condensational growth.Chemicals contained in the e-cigarette vapor-aerosol mixture are partitioned between the gas and particulate phases and this partitioning affects the deposition pattern in the human respiratory tract.E-cigarette aerosols have a wide particle size distribution that ranges from nanometers to micrometers, making the prediction of delivery/ deposition and dosimetry difficult.

The deposition of gas phase and nanoparticles is mostly driven by Brownian diffusion, whereas larger particles are deposited via inertial impaction, and sub-micron particles are deposited by both diffusional and gravitational forces.E-cigarette aerosols consist of hygroscopic and relatively volatile compounds, which may either quickly grow or evaporate, depending on ambient conditions. This obviously adds additional complexity to dosimetric predictions. The development of computer algorithms to predict deposition of ecigarette aerosol is an active, challenging area of investigation which includes models for multiple-path particle dosimetry, computational fluid-particle dynamics ,and thermodynamic interactions between the droplet and vapor phases.Vapers of high power devices typically use low nicotine e-liquids, while users of low power devices use high nicotine e-liquids. However, the serum cotinine levels of these groups are the same, suggesting that the users of high power devices may be exposed to more aerosol that is generated at much higher temperature, which is likely much more harmful.In addition to emission characteristics, vaping behavior is another critically important factor required to provide input parameters to the aerosol deposition models. Most studies have been conducted in the laboratory, and therefore reported puff profiles may differ from real-world scenarios.Interestingly, properties of the e-cigarette aerosol seem to affect puffing topography. For example, St Helen et al. recently demonstrated that e-liquid flavor influenced puff duration: subjects took longer puffs on a strawberry flavored e-liquid than on a tobacco flavored eliquid .The PG to VG ratio also affected puff topography. E-liquids containing more PG lead to shorter puff times, significantly greater nicotine delivery, but paradoxically, significantly less “pleasant” and/or “satisfying” feelings.Additionally, users of powerful sub-ohmic devices and nicotine salt-based e-cigarettes may display different topography profiles compared to the users of earlier generation e-cigarettes.

As devices change, more studies will be needed to determine the input parameters required to accurately determine lung deposition models.Due to this reason, studies measuring health effects of aerosols have not always been consistent. In addition to studying the complete aerosol, there has been a parallel reductionist effort to understand the health effects of separate e-cigarette constituents. These studies have primarily focused on nicotine, flavors, PG, and VG, as well as thermal decomposition products generated during the course of e-liquid vaping. However, there is a growing appreciation that information is needed on the health effects of constituents not traditionally found in nicotine-based vaping products, including -trans-D9 -tetrahydro cannabinol and cutting agents such as Vitamin E acetate .Nicotine is a water-soluble nitrogenous organic molecule that is typically extracted from tobacco leaves. It is composed of an aromatic pyridine ring bound to a pyrrolidine ring. Nicotine binds to nicotinic acetylcholine receptors , which are ligandgated cation channels.Binding of endogenous acetylcholine, or exogenous nicotine, opens the channel pore,vertical farming supplies allowing conduction of Naþ, Kþ, and Ca2þ. The channel is subsequently closed and becomes temporarily unresponsive to further ligand stimuli.nAChRs are broadly expressed throughout multiple organ systems, and in addition to the psychotropic effects on the brain, nicotine can also affect immune cells and resident, specialized cells of the heart and lungs . Despite the long history of nicotine research, understanding the role of nicotine in cardiopulmonary disease is extraordinarily difficult due to factors such as sensitization and desensitization responses; complicated dose-response relationships; the importance of exposure route; and differences in response dependent on species, age, sex, or disease status.The known effects of nicotine on the cardiovascular system are numerous and complex. Nicotine affects angiogenesis, arrhythmogenesis, endothelial cells , hemodynamics, insulin resistance, and lipids.Nicotine increases blood pressure, heart rate, myocardial contractility, and myocardial work. Nicotine also constricts coronary arteries, reduces coronary blood flow reserve and constricts blood vessels in the skin. The majority of nicotinerelated clinical studies have involved combustible cigarettes, which expose smokers to nicotine as well as multiple combustion products. To isolate the specific effects of nicotine on human cardiovascular disease, researchers have studied populations who consume nicotine without combustion, including users of nicotine medications , and smokeless tobacco users . Smokeless tobacco has been used by non-smokers in Sweden lifelong, with no evidence of accelerated atherosclerosis or most other cardiovascular harms.Nicotine medication studies have provided evidence of tolerance to the cardiovascular effects of nicotine: Low doses of nicotine increase heart rate and systemic catecholamine release, while higher doses have little additional cardio-acceleratory and catecholamine effects. These factors are reassuring when considering whether there is a risk of treating smokers with nicotine replacement medications while they are still smoking.

Limitations of nicotine medication studies include the fact that the subjects are all former smokers, nicotine medication use is generally of short duration , and the delivery of nicotine by gum or patch is sustained and does not simulate the spike in nicotine levels seen after smoking combustible cigarettes or e-cigarettes. In contrast, regular e-cigarette exposure likely takes place over years to decades, with vascular nicotine eliciting chronic hemodynamic changes. Nicotine has complex dose-response patterns and may exert species- and cell/tissue-specific effects. Primary rat cardiomyocytes incubated with nicotine showed increased intracellular Ca2þ concentrations, which likely contributed to the observed hypertrophy.Rats and mice that either consumed or were injected with nicotine showed weakening of the aortic walls and destruction of aortic elastin and collagen, all of which were likely a result of the upregulation of matrix metalloproteases. These findings indicate a potential risk for abdominal aortic aneurysms as a result of chronic nicotine consumption.Nicotine also promotes trans-differentiation of vascular smooth muscle cells to a calcifying phenotype by inducing a proinflammatory state, impairing endothelial function, and causing oxidative stress.The growing body of evidence in animal models has illustrated that nicotine can directly affect cardiovascular function without exposure to the byproducts of combustion-based delivery mechanisms. The contribution of specific vapor constituents to the observed endothelial dysfunction has not been fully delineated. Some studies have suggested that nicotine may be partially responsible, as nicotine exposure can damage epithelial and ECs, induce epithelialmesenchymal transition markers such as a-SMA and fibronectin, and release inflammatory mediators such as transforming growth factor-beta, all of which suggests that nicotine may promote tissue remodeling and fibrosis.While some investigators have hypothesized that the adverse effects of e-cigarettes were largely due to nicotine delivery, others have proposed that e-cigarette aerosol without nicotine might also contribute to these effects. Of note, in the study by Schweitzer et al., adverse nicotine-independent effects of e-cigarette aerosol on endothelial barrier function were also observed, which the authors attributed to other toxic components of e-cigarettes such as acrolein.The effects of nicotine on the lung have been established from studies in humans, animal models, and cultures of human cells. The use of smokeless tobacco permits the study of people who were never smokers and have used nicotine for many years. For example, snus users have a lower mortality rate than tobacco smokers. As discussed before, nicotine levels are 50-fold lower in plasma after use of e-cigarettes, snus, or tobacco than in the lung after tobacco or e-cigarette inhalation . This suggests that vaping and tobacco smoke inhalation lead to higher levels of pulmonary nicotine than snus due to their delivery route. Because of this, snus may not have as much effect on the lung as vaping due to different nicotine pharmacokinetics.Incubating isolated alveolar epithelial cells with nicotine-induced oxidative stress via activation of NADPH oxidase 1, leading to cell apoptosis.This observation was confirmed in vivo with nicotine injections to Wistar rats, which resulted in oxidative stress signaling in lung tissue.In contrast to promoting oxidative stress, a study in mice found that nicotine can prevent inflammation by inhibiting the release of pro-inflammatory cytokines in the lungs.However, a more recent study has challenged this observation by demonstrating that nicotine binding to a7 nAChR, lead to increased type I collagen deposition in lung fibroblasts: Fibroblasts from wild type and a7 nAChR knockout mice were exposed to nicotine, and U937 monocytes were cocultured on matrices derived from these fibroblasts. Nicotine exposure in WT but not a7 nAChR knockout fibroblasts resulted in monocyte activation and release of IL-1ß.Finally, nicotine has been shown to cause mucociliary dysfunction in human airway epithelial cells and animal models by reducing ion channel function via TRP channels.However, the effects of nicotine may be both cell type- and concentration-specific, which may lead to the disparity of results .Nicotine readily crosses the placenta and produces higher nicotine concentrations in the fetal circulation than in the maternal circulation.Epidemiologic studies have shown that cigarette smoking during pregnancy is associated with an increased risk of cardiovascular disease in offspring, although this may or may not be due to direct effects of nicotine.Moreover, nicotine may be a link between maternal smoking and the risk for sudden infant death syndrome.It is also well-established that maternal smoking, mediated most likely through nicotine, results in impaired fetal lung development and function, and these effects, while small, persist into childhood and beyond.Recent animal studies further indicate that pre-conception and prenatal ecigarette exposure to nicotine and PG/VG impairs embryo implantation and offspring’s metabolic health later in life.This is of particular concern, given the current rate of e-cigarettes use among young adults of childbearing age.Regardless of its source, fetal nicotine exposure during pregnancy has become a public concern and the impact of vaping nicotine-containing eliquids on fetal development remains to be determined.