An overview of the effects of e-cigarettes on human health

Little is known about the effects of e-cigarettes on the immune system. Interestingly, smoking both traditional cigarettes and e-cigarettes in non-smokers was found to have short-term effects on platelet function, increasing platelet activation (levels of soluble CD40 ligand and adhesion molecule P-selectin) and platelet aggregation, albeit to a smaller extent. The extent of e-cigarettes . As found in platelets, exposure of neutrophils to e-cigarette aerosols resulted in increased expression of CD11b and CD66b, both markers of neutrophil activation. Additionally, e-cigarettes have been reported in different human studies to cause increased oxidative stress, vascular endothelial damage, impaired endothelial function, and changes in vascular tone. In this setting, platelet and leukocyte activation and endothelial dysfunction are generally considered to be involved in atherosclerosis and cardiovascular morbidity. Based on these observations, the potential association between daily e-cigarette consumption and increased risk of myocardial infarction remains controversial, but switching from tobacco to long-term e-cigarette use may result in benefits in blood pressure regulation, endothelial function, and vascular stiffness. However, whether e- cigarettes have cardiovascular effects still requires further research.

Most recently, in August 2019, the U.S. Centers for Disease Control and Prevention (CDC) declared an outbreak of e-cigarette or e- cigarette product use-associated lung injury (EVALI), resulting in the deaths of several young people. In fact, computed tomography (CT scans) show that localized inflammation impairs the gas exchange caused by vaping carbureted oil. However, most reported cases of lung injury are associated with the consumption of tetrahydrocannabinol (THC) and vitamin E additives using e-cigarettes and are not necessarily attributable to other e-cigarette ingredients.

On the other hand, in a comparative study of mice exposed to laboratory air, e-cigarette aerosol, or cigarette smoke (CS) for 3 days (6 hours of exposure per day), the Mice exhibited significantly increased interleukin (IL)-6 but normal lung parenchyma with no evidence of apoptotic activity or elevated IL-1β or tumor necrosis factor-α (TNFα). In contrast, animals exposed to CS showed infiltration of inflammatory cells in the lungs and increased expression of inflammatory markers, such as IL-6, IL-1β, and TNFα. In addition to airway disease, exposure to e-liquid aerosols with or without nicotine has also been associated with neurotoxicity in early mouse models.

The results of the in vitro studies are generally consistent with the results of a limited number of in vivo studies. For example, in an analysis using primary human umbilical vein endothelial cells (HUVEC) exposed to 11 commercially available vapors, 5 were found to be acutely cytotoxic, only 3 of which contained nicotine. Furthermore, 5 of the 11 vapors tested (including 4 that were cytotoxic) reduced HUVEC proliferation, and one of them increased intracellular reactive oxygen species (ROS) production. Similar morphological changes were induced by the three most cytotoxic vapors, which had effects similar to traditional high-nicotine CS extracts. Endothelial cell migration is an important mechanism for vascular repair and may be disrupted in smokers due to endothelial dysfunction. In a comparative study of CS and e-cigarette aerosols, Taylor et al . It was found that exposure of HUVEC to e-cigarette water extract for 20 h did not affect migration in the scratch assay, whereas equivalent cells exposed to CS extract showed significant inhibition of migration in a concentration-dependent manner.

In cultured human airway epithelial cells, both e-cigarette aerosol and CS extract induced the release of IL-8/CXCL8 (neutrophil chemoattractant). In contrast, while CS extract reduced epithelial barrier integrity (determined by dextran translocation from the apical to the basolateral side of the cell layer), e-cigarette aerosol did not, suggesting that only CS extract Host defenses are negatively affected. Furthermore, Higham et al . E-cigarette aerosol was also found to cause the release of IL-8/CXCL8 and matrix metallopeptidase 9 (MMP-9), while enhancing the activity of neutrophil elastase, which may promote the migration of neutrophils to the inflamed site.

In a comparative study, repeated exposure of human gingival fibroblasts to CS condensates or nicotine-rich or nicotine-free electron vapor condensates resulted in morphological changes, proliferation inhibition , and induction of apoptosis in all three parameters of the cells. The changes are greater with exposure to CS condensate. Similarly, both e-cigarette aerosol and CS extract increased cell death in adenocarcinoma human alveolar basal epithelial cells (A549 cells), and the effect of CS extract was more damaging than e-cigarette aerosol (at 2 mg / mL Harmful effects were found at 64 mg/mL of CS extract versus 64 mg/mL of e-cigarette extract, which is consistent with another study examining battery output voltage and cytotoxicity.

All this evidence suggests e-cigarettes may be less harmful than traditional cigarettes.

Consequences of Nicotine Content

Aside from flavor, one of the main issues in the e-liquid market is the range of nicotine content. Depending on the manufacturer, the concentration of this alkaloid may be expressed as low , medium , or high , or as mg/mL or percentage (% v/v). Concentrations range from 0 (0%, nicotine-free option) to 20 mg/mL (2.0%) - the maximum nicotine threshold in accordance with Directive 2014/40/EU of the European Parliament and of the Council of the European Union. However, despite this regulation, some commercial e-liquids have nicotine concentrations approaching 54 mg/ml, well above the limit set by the EU.

The issue of mislabeling of nicotine content in e-liquids has been previously addressed. For example, gas chromatography using a flame ionization detector (GC-FID) showed nicotine levels inconsistent with the manufacturer's claims (average of 22 ± 0.8 mg/mL vs. 18 mg/mL), which was equivalent to less than the product label The content indicated above is about 22% higher. It’s worth noting that some studies have detected nicotine in those e-liquids labeled as nicotine-free. One study detected nicotine in 5 of 23 nicotine-free labeled e-liquids via nuclear magnetic resonance spectroscopy (0.11–6.90 mg/mL), and another found 13.6% nicotine content (average 8.9 mg/mL) (17/125) of nicotine-free e-cigarette liquid was analyzed by high performance liquid chromatography (HPLC). Of the 17 samples tested in the latter study, 14 were identified as counterfeit or suspected counterfeit. A third study detected nicotine in 7 of 10 nicotine-free refills, albeit at lower concentrations than those identified in previous analyses (0.1–15 µg/mL). Not only is there evidence that the nicotine content in refills labeled as nicotine-free is mislabeled, but nicotine-containing e-liquids also appear to have a history of poor labeling accuracy.

Comparisons of nicotine serum levels in e-cigarette or conventional cigarette consumption were recently reported. Participants smoked an e-cigarette containing at least 12 mg/ml of nicotine or inhaled a traditional cigarette every 20 seconds for 10 minutes. Blood samples were collected at 1, 2, 4, 6, 8, 10, 12, and 15 minutes after the first puff, and nicotine serum concentrations were measured by liquid chromatography mass spectrometry (LC-MS). The results showed that the serum nicotine concentration in the traditional CS group was higher than that in the e-cigarette group (25.9 ± 16.7 ng/mL vs. 11.5 ± 9.8 ng/mL). However, based on delivering approximately 1 mg of nicotine every 5 minutes, an e-cigarette containing 20 mg/ml nicotine is more equivalent to a regular cigarette.

In this regard, one study compared the severe effects of e-cigarettes with e-cigarettes on healthy smokers and non-smokers with equivalent nicotine levels. Both increased oxidative stress markers and decreased nitric oxide bioavailability, blood flow-mediated dilation, and vitamin E levels, showing no significant differences between tobacco and e-cigarette exposure. Therefore, short-term e-cigarette use in healthy smokers can lead to significant impairment of endothelial function and increased arterial stiffness. Similar effects on endothelial dysfunction and arterial stiffness were found when animals were exposed to e-cigarette vapor for several days or for long periods of time. In contrast, other studies have found that smokers experience acute microvascular endothelial dysfunction, oxidative stress, and increased arterial stiffness after exposure to nicotine-containing e-cigarettes but not after exposure to nicotine-free e-cigarettes . situation. One study found that among female smokers, there was a significant difference in stiffness after smoking just one tobacco but not after using e- cigarettes .

Nicotine is known to be highly addictive and has a variety of harmful effects. Nicotine has significant biological activity and can adversely affect multiple physiological systems such as cardiovascular, respiratory, immune and reproductive systems, and can also damage lung and kidney function. Recently, wild-type (WT) animals or knockout animals were subjected to subchronic systemic exposure to e-cigarette liquids containing only PG or PG nicotine (25 mg/mL) (2 h/day, 5 days/week, 30 days ) [42] . Subchronic exposure to PG/nicotine promotes nAChRα7-dependent increases in the levels of different cytokines and chemokines in bronchoalveolar lavage fluid (BALF), such as IL-1α, IL-2, IL-9, interferon gamma (IFNγ) , granulocyte-macrophage colony-stimulating factor (GM-CSF), monocyte chemoattractant protein-1 (MCP-1/CCL2) and activation regulation, normal T cell expression and secretion (RANTES/CCL5), IL-1β , IL-5 and TNFα levels were enhanced independently of nAChRα7. In general, most cytokines detected in BALF were significantly increased in WT mice exposed to nicotine-containing PG compared with PG alone or air controls. The study found that some of these effects are achieved through nicotine-activated NF-κB signaling, which is present in women but not in men. Furthermore, nicotine-containing PG resulted in increased macrophage and CD4 ⁺ /CD8 ⁺ T lymphocyte counts in the BALF compared with air controls, but these effects were ameliorated when animals were subchronically exposed to PG alone.

Notably, another study showed that despite upregulation of RANTES/CCL5 and CCR1 mRNA in users of flavored/nicotine e-cigarettes , vaping flavored and non-nicotine e-cigarettes did not significantly dysregulate cytokines and inflammasomes activation.

In addition to its toxicological effects on fetal development, nicotine can disrupt brain development in adolescents and young adults. Some studies have also shown that nicotine is potentially carcinogenic, but more work is needed to prove that its carcinogenicity is independent of tobacco combustion products. Regarding the latter, there was no difference in the frequency of tumors in rats exposed to long-term (2 years) nicotine inhalation compared with rats in the control group. Despite the lack of evidence of carcinogenicity, there are reports that nicotine promotes tumor cell survival by reducing apoptosis and increasing proliferation, suggesting that it may function as a "tumor enhancer." In a recent study, chronic administration of nicotine (1 mg/kg every 3 days for 60 days) to mice enhanced brain metastasis by distorting the polarity of M2 microglia, thereby increasing metastatic tumor growth. . Assuming that a traditional cigarette contains 0.172-1.702 mg of nicotine, the daily dose of nicotine given to these animals is equivalent to a 70 kg adult smoking 40-400 cigarettes, which is the dose of an extreme smoker. We believe that further studies on long-term administration of low-dose nicotine are needed to clearly assess its impact on carcinogenicity.

In the above-mentioned study in which human gingival fibroblasts were exposed to CS condensates or nicotine-rich or nicotine-free electron vapor condensates, the deleterious effects in cells exposed to nicotine-rich condensates were greater than those exposed to nicotine-free condensates. The cells are larger, which means buying an e-cigarette case . It is also worth noting that among the 3 vapors evaluated, they were the most toxic to HUVEC cells . In the study, 2 did not contain nicotine, showing that nicotine is not the only harmful ingredient in e-cigarettes .

The lethal dose of nicotine for adults is estimated to be 30-60 mg. Given that nicotine easily diffuses from the dermis into the bloodstream, acute nicotine exposure due to e-liquid spillage (5 mL of 20 mg/mL nicotine-containing refill is equivalent to 100 mg of nicotine) can easily be toxic or even fatal. Therefore, devices with rechargeable refills are another concern for e-cigarettes , especially when the e-liquid is not sold in child-resistant containers, increasing the risk of spillage, swallowing, or breathing.

Overall, these data suggest that the harmful effects of nicotine should not be underestimated. Despite established regulations, there are still some inaccuracies in the nicotine content labeling of different brands of e-liquids . Therefore, the e-cigarette oil industry needs stricter supervision and higher quality control.

Moisturizers and their effects on heating-related products

In this particular aspect, there are also significant differences in the composition of e-liquids between different commercial brands . The most common and main ingredients in e-liquid are PG or 1,2-propanediol, and glycerin or glycerin (propane-1,2,3-triol). Both types of compounds are used as humectants to prevent e-liquids from drying out and are classified as "generally recognized as safe" by the U.S. Food and Drug Administration (FDA). In fact, they are widely used as food and medicine. In the analysis of 54 commercially available e-cigarette oils , PG and glycerin were detected in almost all samples, with concentrations ranging from 0.4% to 98% (average 57%) and 0.3% to 95% (average 37%), respectively.

Regarding toxicity, little is known about the effects of humectants when heated and inhaled over long periods of time. Studies have shown that PG can cause respiratory tract irritation and increase the likelihood of asthma. PG and glycerin in e-cigarettes may reach high enough concentrations to potentially irritate the respiratory tract. Indeed, the latter study showed that one puff of an e- cigarette resulted in PG exposure of 430–603 mg/m ³ , higher than the levels reported to cause airway irritation based on human studies (average 309 mg/m ³ 55 ). The same study also showed that one puff of an e- cigarette resulted in glycerol exposure of 348–495 mg/m ³ , close to the level reported to cause airway irritation in rats (662 mg/m ³ ).

Two randomized clinical trials in young smokers reported airway epithelial damage induced by acute inhalation of PG and glycerol aerosols (50:50 v/v) with or without nicotine. In vitro, aerosols from glycerol-only refills showed cytotoxicity to A549 and human embryonic stem cells, even at low cell output voltages. PG has also been found to affect early neurodevelopment in zebrafish models. Another important issue is that under heating conditions, PG can produce acetaldehyde or formaldehyde (average 119.2 or 143.7 ng/puff, respectively, at 20 W), while glycerol can also produce acrolein (average, respectively, at 20 W 53.0, 1000.0 or 5.9 ng/puff), on average), all carbonyl compounds have well-documented toxicity. Nonetheless, assuming 15 puffs per e-cigarette unit, the carbonyl compounds produced by heating of PG or glycerol would be lower than the maximum levels found in conventional cigarette combustion (Table 2). However, further research is needed to properly test the harmful effects of all these compounds at physiological doses similar to those to which individuals are exposed over long periods of time.

Although PG and glycerin are the main components of e-cigarette oil , other components have also been detected. When four commercially available e-cigarette liquids selected from the top 10 of "Best E-Cigarettes of 2014" were heated and subjected to aerosol analysis, a variety of compounds were detected, nearly half of which were detected. Some of them had not been identified before, thus showing that the heating process itself can produce new compounds with unknown consequences. Of note, the analysis identified formaldehyde, acetaldehyde, and acrolein, 3 carbonyl compounds known to be highly toxic. Although no information on formaldehyde and acetaldehyde concentrations was provided, the authors calculated that one puff could result in an acrolein exposure of 0.003–0.015 μg/mL. Assuming 15 puffs per 40 ml puff per e-cigarette unit (according to multiple manufacturers), each e-cigarette unit will produce approximately 1.8-9 micrograms of acrolein, which is less than the acrolein emitted by traditional e-cigarette units content. Tobacco cigarettes (18.3–98.2 μg). However, given that e-cigarette devices are not yet perfect, users may puff intermittently throughout the day. Therefore, assuming 400 to 500 puffs per cartridge, users may be exposed to up to 300 μg of acrolein.

In a similar study, acrolein was found in 11 of 12 aerosols tested in a similar range of amounts (approximately 0.07–4.19 μg per e-cigarette unit). In the same study, formaldehyde and acetaldehyde were detected in all aerosols tested, at levels of 0.2-5.61 μg and 0.11-1.36 μg per unit of e-cigarette, respectively [68]. It should be noted that the content of these toxic products in e-cigarette aerosol is significantly lower than that in CS: formaldehyde is 9 times lower, acetaldehyde is 450 times lower, and acrolein is 15 times lower.

Other compounds detected in aerosols include acetamide, a potential human carcinogen, and some aldehydes, although these are present in extremely low amounts. Interestingly, the presence of harmful concentrations of diethylene glycol (a known cytotoxic agent) in e-liquid aerosols is controversial, with some studies detecting its presence and others finding low subtoxic concentrations. Similar observations were made for ethylene glycol content. In this regard, either it is detected at concentrations that do not exceed the authorized limits, or it is not present in the aerosol generated. Only one study has shown it to be present in high concentrations in very small amounts of samples. However, the FDA does not allow it to exceed 1 mg/g. It seems likely that future studies should analyze the possible toxic effects of moisturizers and related products at concentrations similar to those to which e-cigarette users are exposed in order to draw conclusive results.

Effect of Flavoring Compounds

The wide variety of e-liquid flavors available to consumers to appeal to current smokers and new e-cigarette users is a growing public health issue. In fact, more than 5 million middle school students were current users of e-cigarettes in 2019, and 81% of young users believe that attractive flavors are the main reason for consuming e-cigarettes . The FDA has regulated flavors used in the e-cigarette market since 2016 and recently issued enforcement policies targeting unauthorized flavors, including fruit and mint flavors that are more appealing to younger users. However, the long-term effects of all the fragrance chemicals used by the industry (more than 15,000) remain unknown, and they are often not included on product labels. Additionally, because they may contain potentially toxic or irritating properties, there is no guarantee of safety.

For the many available fragrances, some have been shown to be cytotoxic. The toxicity of 36 different e-cigarette oils and 29 different flavors on human embryonic stem cells, mouse neural stem cells, and human lung fibroblasts was evaluated using metabolic activity assays. In general, those bubblegum, butterscotch, and caramel-flavored e-liquids did not exhibit any significant cytotoxicity, even at the highest doses tested. In comparison, those containing the Freedom Smoke Menthol Arctic and Global Smoke Caramel flavors had significant cytotoxic effects on lung fibroblasts, while those containing the Cinnamon Ceylon flavor had significant cytotoxic effects on lung fibroblasts in all cell lines. The most cytotoxic. Further research by the same group showed that high cytotoxicity is a recurring feature of cinnamon-flavored e-cigarette oils. In this regard, the results of GC-MS and HPLC analyzes showed that cinnamic aldehyde (CAD) and 2-methoxycinnamaldehyde, rather than dipropylene glycol or vanillin, were the main contributors to the high cytotoxicity of cinnamon-flavored e-liquids. Of the 51 flavored e-liquid aerosols tested, 47 other flavoring-related compounds, such as diethyl, 2,3-pentanedione or acetoin , were found to be associated with respiratory complications . Each cartridge was calculated to contain an average of 239 μg of diethyl. Again assuming 400 puffs per pod, 40 mL per puff, can one estimate that each puff contains an average of 0.015 ppm of diacetyl, which may impair normal lung function in the long term.

The cytotoxic and pro-inflammatory effects of different e-cigarette flavoring chemicals were also tested on two human monocyte cell lines, mono mac 6 (MM6) and U937. Among the flavoring chemicals tested, CAD was found to be the most toxic, with O-vanillin and pentylenedione also showing significant cytotoxicity; in comparison, acetoin, diacetyl, maltol, and coumarin were found to be the most toxic. It did not show any toxicity at the concentrations measured (10-1000 µM). Interestingly, the toxicity was significantly higher when different flavor combinations or mixing equal proportions of 10 different flavors of e-liquid were tested, suggesting that vaping a single flavor is less toxic than inhaling a mixture of flavors. Furthermore, in cell-free ROS production assays, all tested flavors produced significant ROS. Finally, diethyl, pentylenedione, O-vanillin, maltol, coumarin, and CAD induced significant IL-8 secretion by MM6 and U937 monocytes. However, it should be remembered that the concentrations measured are in the supraphysiological range and these concentrations may not be reached in the airway space once inhaled. In fact, one of the limitations of the study is that the human cells themselves were not exposed to the e-liquid , but rather to lower concentrations of the aerosol. At this line, the maximum concentration tested (1000 µM) corresponds to approximately 80 to 150 ppm, which is much higher than the levels found in aerosols of some of these compounds. Additionally, e-cigarette users' lungs are not exposed to these concentrations of chemicals for 24 hours on a daily basis. Similar limitations were found when five of seven seasonings were found to be cytotoxic to human bronchial epithelial cells.

Recently, a common commercial crème brûlée- flavored aerosol was found to contain high concentrations of benzoic acid (86.9 micrograms per puff), a recognized respiratory irritant. When human lung epithelial cells (BEAS-2B and H292) were exposed to this aerosol for 1 hour, significant cytotoxicity was observed in BEAS-2B cells but not in H292 cells after 24 hours. However, ROS production increased in H292 cells.

Therefore, to fully understand the effects of these compounds, the cell cultures used to perform these assays were selected and their effects on human health were elucidated using in vivo models that simulate real-life situations in chronic e-cigarette smokers.

electronic cigarette device

While most research related to the human health effects of e-cigarette use has focused on the components of e-cigarette liquids and the aerosols they produce when heated, some studies have examined the materials of electronic devices and their potential consequences, specifically, those derived from Metals such as copper, nickel or silver particles may be present in filaments and wires, as well as in atomizer e-liquids and aerosols.

Other important components in aerosols include silicate particles derived from fiberglass cores or silicone resins. Many of these products are known to cause respiratory abnormalities and respiratory disease, but more in-depth research is needed. Interestingly, battery output voltage also appears to have an impact on aerosol vapor cytotoxicity, with e-liquids from higher battery output voltages exhibiting greater toxicity to the A549 battery .

A recent study compared the acute effects of e-cigarette vapor (containing PG/vegetable glycerin plus tobacco flavoring but no nicotine) generated from stainless steel atomizer (SS) heating elements or nickel-chromium alloy (NC). Some rats received a single e-cigarette exposure for 2 hours from an NC heating element (60 or 70 W); other rats received a similar exposure to e-cigarette vapor for the same amount of time (60 or 70 W) using an SS heating element , and the final group Animals were exposed to air for 2 hours. Neither air-exposed rats nor rats exposed to e-cigarette vapor using SS heating elements developed respiratory distress. In comparison, 80% of rats exposed to e-cigarette vapor using the NC heating device developed clinical acute respiratory distress when the 70 W power setting was used. Therefore, it is recommended that operating the device at higher than recommended settings may have adverse effects. Nonetheless, there is no doubt that the harmful effects of battery output voltage are not comparable to those produced by CS extracts.

E-cigarettes as a smoking cessation tool

CS contains a large number of substances—about 7,000 different ingredients in total, ranging in size from atoms to particulate matter, hundreds of which may be responsible for the harmful effects of the habit. Given that tobacco is largely being replaced by e-cigarettes, which have different chemical compositions, manufacturers claim that e- cigarettes do not cause lung diseases such as lung cancer, chronic obstructive pulmonary disease, or cardiovascular disease commonly associated with traditional cigarette consumption. However, the World Health Organization believes that e-cigarettes cannot be considered a viable method of smoking cessation due to a lack of evidence. In fact, research findings on the use of e-cigarettes as a smoking cessation tool remain controversial. Additionally, both the FDA and CDC are actively investigating the incidence of severe respiratory symptoms associated with the use of e-cigarette products. Since many e-liquids contain nicotine, which is known for its powerful addictive properties, e-cigarette users can easily switch to traditional cigarettes to avoid quitting smoking. Still, the possibility of vaping nicotine-free e-cigarettes has led to these devices being labeled as smoking cessation tools.

A recently published randomized trial of 886 subjects willing to quit smoking found that the cessation rate was twice as high in the e-cigarette group as in the nicotine replacement group. Notably, the cessation rate in the nicotine replacement group was lower than usual for this therapy. Anticipated abstinence rate. Nonetheless, the incidence of throat and oral irritation was higher in the e-cigarette group than in the nicotine replacement group (65.3% and 51.2%, respectively). Additionally, participants in the e-cigarette group (80%) were significantly more adherent to treatment one year after quitting than those in the nicotine replacement product group (9%).

On the other hand, it is estimated that COPD may become the third leading cause of death by 2030. Since COPD is often associated with the smoking habit (about 15% to 20% of smokers have COPD), smokers with COPD must quit smoking. Published data show that smokers with COPD who switch to e-cigarettes significantly reduce their consumption of traditional cigarettes. In fact, a significant reduction in exacerbations was observed in COPD smokers compared to those who did not smoke, and as a result, the ability to perform physical activities was improved. However, longer follow-up of these COPD patients is needed to determine whether they quit traditional smoking or even e-cigarettes, as the ultimate goal in this setting is to quit both habits.

Based on the current literature, it appears that several factors contribute to the success of e-cigarettes as a smoking cessation tool. First, some e-cigarette flavors have a positive impact on smokers’ quitting outcomes. Second, e-cigarettes have been described to increase quit rates only among highly dependent smokers but not among traditional smokers, suggesting that individual dependence on nicotine plays an important role in this process. Third, there is widespread agreement about their relative harmfulness to consumer health compared with traditional combustible tobacco. Finally, e-cigarette point-of-sale marketing has also been identified as affecting smoking cessation success.

in conclusion