Electronic Cigarette Solvents, JUUL E-Liquids, and Biomarkers of Exposure: In Vivo Evidence for Acrolein and Glycidol in E-Cig-Derived Aerosols

Role of the Transient Receptor Potential Ankyrin-1 in the Pulmonary, Vascular, and Systemic Effects of Short-Term Acrolein Inhalation in Mice: Implications for the Toxicity of Electronic Nicotine Delivery Systems

The cardiovascular and pulmonary disease risks of the use of electronic nicotine delivery systems (ENDS) are uncertain. We recently showed that ENDS solvent-derived aerosol (propylene glycol and vegetable glycerin, PG:VG) exposure induced a transient receptor potential ankyrin-1 (TRPA1)-dependent endothelial dysfunction (ED) in healthy female mice. As thermal degradation of PG:VG generates aldehydes, we hypothesized that acrolein (AC), a constituent of ENDS-derived aerosol and a known TRPA1 agonist, was responsible, in part, for the observed TRPA1-dependent pulmonary and vascular effects of PG:VG. To test this, female wild-type (WT) and TRPA1 null mice were exposed by inhalation to either filtered air or AC alone, and biomarkers of exposure and of harm were measured. Compared with their genotype-matched air control group, JUUL Virginia Tobacco (VT), PG:VG, and AC alone exposures (6 h) significantly increased urinary levels of the AC metabolite, 3-hydroxypropyl mercapturic acid (3HPMA), in both female WT and TRPA1 null mice. AC exposures at 1 and 3 ppm led to the rapid onset and reversal (upon cessation) of 'respiratory braking' in female WT but not in TRPA1 null mice indicating a TRPA1 dependence. As AC stimulated TRPA1-dependent respiratory braking, we measured urinary monoamines and their metabolites after exposure as a proxy of nervous system activation. In WT mice, AC exposure suppressed levels of dopamine, metanephrine, serotonin (5HT), and 5HT metabolite (5HIAA), whereas in TRPA1 null mice only 5HT was equally suppressed by AC. To assess vascular effects, mice were exposed for 4 days to Air or AC (6 h/day, 1 ppm), and aortic function was measured ex vivo. Although endothelial-dependent relaxation was similar in air control and AC-exposed mice, aortic sensitivity to an NO donor was enhanced significantly and equally by AC in both WT and TRPA1 null mice reflective of a TRPA1-independent and compensatory effect. Collectively, AC exposure at a level present in ENDS aerosols stimulated both TRPA1-dependent and -independent pulmonary, vascular, and systemic effects. These data suggest that ENDS use may increase cardiovascular and pulmonary disease risk, in part, via AC present in ENDS-derived aerosols yet independent of either nicotine or flavorants. The level of AC present in ENDS aerosols should be lowered to an amount where it does not induce biomarkers of vascular, pulmonary, and systemic harm to mitigate potential long-term disease risk.

Advances in the mechanistic understanding, biological consequences, and measurement of DNA adducts induced by tobacco smoke and e-cigarette aerosol: A review

Components in tobacco smoke and electronic cigarette (e-cigarette) aerosol form adducts with DNA, which can cause DNA mutations and affect repair of DNA damage. Numerous studies have shown a strong association between inhaled smoke and lung cancer. The presence of DNA adducts can indicate chemical components of smoke. Therefore, DNA adducts are significant biomarkers of tobacco exposure that might predict lung disease status and serve as precursors to lung cancer, since they trigger DNA mutations and impair biological processes such as DNA replication and transcription. To date, no systematic review has compared tobacco smoke and e-cigarette aerosol in terms of the fate of DNA adducts. We reviewed recent studies comparing the formation of DNA adducts on exposure to components from conventional cigarette smoke versus e-cigarette aerosol. The aims of the review were threefold: (1) to summarize components of tobacco smoke and e-cigarette aerosol in relation to mechanisms for the formation of DNA adducts; (2) to highlight the biological consequences of exposure to tobacco smoke and e-cigarette aerosol; and (3) to summarize advances in understanding of the primary detection methods of DNA adducts in tobacco exposure studies. The findings of this review should benefit environmental toxicology studies of tobacco exposure.

Cardiovascular health effects of vaping e-cigarettes: a systematic review and meta-analysis

BACKGROUND

There is substantial interest in the cardiovascular effects of e-cigarette use, highlighting the need to update our knowledge on the subject. We conducted this review to analyse whether e-cigarette use increases cardiovascular health risks and how these risks vary among different populations.

METHODS

We searched six databases and included peer-reviewed human, animal, cell/in vitro original studies but excluded qualitative studies, which were published between July 2021 and December 2023. Three types of e-cigarette exposure were examined: acute, short-to-medium term and long term. Different risk of bias tools were used for assessing the quality of the included human studies and we conducted meta-analysis when possible.

RESULTS

We included 63 studies in the main analysis, 12 studies in the meta-analysis and 32 studies in the sociodemographic factor-based subgroup analysis. Over half of the human studies had low risk of bias. Acute exposure to e-cigarette was associated with increased heart rate (HR) (mean difference (MD) 11.329, p<0.01) and blood pressure (BP) (MD 12.856, p<0.01 for systolic; MD 7.676, p<0.01 for diastolic) compared with non-use. While HR was lower after acute exposure to e-cigarettes compared with cigarettes (MD -5.415, p<0.01), no significant difference in systolic or diastolic BP was observed. Non-smoker current vapers had no significant differences in resting HR and BP compared with non-users but lower resting HR (MD -2.608, p<0.01) and diastolic BP (MD -3.226, p<0.01) compared with non-vaper current smokers. Despite some association between e-cigarette and endothelial dysfunction, short-to-medium-term transition from cigarettes to e-cigarettes may improve blood flow and BP, particularly among females and younger individuals. There is lack of evidence supporting any association of e-cigarette use with cardiovascular diseases and cardiac dysfunction or remodelling.

CONCLUSIONS

This review highlighted several important cardiovascular impacts of e-cigarette use compared with non-use and cigarette smoking. However, the evidence is still limited and requires future research.

PROSPERO REGISTRATION NUMBER

CRD42023385632.

Chemical Transformation of Vaping Emissions under Indoor Atmospheric Aging Processes

E-cigarette emissions, which contain a variety of hazardous compounds, contribute significantly to indoor air pollution and raise concerns about secondhand exposure to vaping byproducts. Compared to fresh vape emissions, our understanding of chemically aged products in indoor environments remains incomplete. Terpenes are commonly used as flavoring agents in e-liquids, which have the ability to react with the dominant indoor oxidant ozone (O3) to produce reactive oxygenated byproducts and result in new particle formation. In this study, mixtures of propylene glycol (PG), vegetable glycerin (VG), and terpenes as e-liquids were injected into a 2 m3 FEP chamber to simulate the indoor aging process. 100 ppbv O3 was introduced into the chamber and allowed to react with the fresh vape emissions for 1 h. Complementary online and offline analytical techniques were used to characterize the changes in the aerosol size distribution and chemical composition during the aging processes. We observed more ultrafine particles and a greater abundance of highly oxygenated species, such as carbonyls, in aged e-cigarette aerosols. Compared with their fresh counterparts, the aged emissions exhibited greater cytotoxic potential, which can be attributed to the formation of these highly oxygenated compounds that are not present in the fresh emissions. This work highlights the dynamic chemistry and toxicity of e-cigarette aerosols in the indoor environment as well as the indirect risks of secondhand exposure.

High-throughput UPLC-ESI/MSMS method for simultaneous measurement of the urinary metabolites of volatile organic compounds and tobacco alkaloids

Human exposure to volatile organic compounds (VOCs) poses significant health risks, contributing to cardiovascular disease, pulmonary disease, and cancer. Measurement of VOC metabolites (VOCm) in urine by liquid chromatography-mass spectrometry (LC-MS) is a preferred method for VOCm analysis; however, existing methods encounter challenges related to sensitivity, throughput, and analyte coverage. In addition to VOCm, the measurement of tobacco alkaloids (TAm) is critical to account for tobacco use in population-based studies. A method is needed that is highly sensitive, offers higher throughput, and can analyze VOCm and TAm in a single run. Herein, we present a robust dilute-and-shoot method aimed at overcoming these analytical challenges and expanding the targeted analysis to include 35 urinary VOCm and TAm and their metabolites. By leveraging high-speed polarity switching and optimized chromatographic parameters, our method achieved comprehensive analyte coverage and enhanced sensitivity, enabling reliable individual level VOC exposure assessment. Validation demonstrates robust linearity, sensitivity, accuracy, and precision, with minimal matrix effects. This high-throughput UPLC-MS/MS method significantly enhances VOC exposure assessment by enabling simultaneous measurement of 35 urinary VOC and TAm with high sensitivity and efficiency. Multiple metabolites from single parent xenobiotics are included in one run, expanding biomarker specificity. Our data indicate the method effectively accounts for tobacco consumption as a confounder in population-based studies, ensuring accurate VOC exposure assessment.

Increasing coil temperature of a third-generation e-cigarette device modulates C57BL/6 mouse lung immune cell composition and cytokine milieu independently of aerosol dose

Higher coil temperature in e-cigarette devices increases the formation of aerosols and toxicants, such as carbonyls. At present, the health implications of vaping at higher temperatures, including exacerbation of pulmonary inflammation, are largely unknown when aerosol dose is considered. To isolate the pulmonary effects of coil temperature, C57BL/6 mice were exposed to e-cigarette aerosols generated at lower (190°C) or higher (250°C) temperature for 3 days, while maintaining a similar chamber aerosol concentration. Increasing coil temperature did not markedly alter aerosol mass-normalized emissions of select carbonyls formed from thermal degradation pathways including formaldehyde, acetaldehyde, propionaldehyde, and acetone under the tested environment. Total bronchoalveolar cells, primarily macrophages, were significantly decreased in mice exposed to aerosols generated with higher coil temperatures compared to lower temperature exposures. The gene expression of IFNβ, IL-1β, TNFα, and IL-10 in mouse lung tissue was significantly reduced following e-cigarette exposure under both conditions, compared to filtered air exposure. Higher temperature exposures further exacerbated downregulation of IFNβ and IL-1β. Data suggest that higher temperature vaping might modulate acute pulmonary immune responses, potentially inducing immune suppression, even when normalized for aerosol dose exposure. Coil temperature thus appears to be an important parameter that needs to be regulated to ensure harm reduction for e-cigarette users.

Conventional and multi-omics assessments of subacute inhalation toxicity due to propylene glycol and vegetable glycerin aerosol produced by electronic cigarettes

Propylene glycol (PG) and vegetable glycerin (VG) are the most common solvents used in electronic cigarette liquids. No long-term inhalation toxicity assessments have been performed combining conventional and multi-omics approaches on the potential respiratory effects of the solvents in vivo. In this study, the systemic toxicity of aerosol generated from a ceramic heating coil-based e-cigarette was evaluated. First, the aerosol properties were characterized, including carbonyl emissions, the particle size distribution, and aerosol temperatures. To determine toxicological effects, rats were exposed, through their nose only, to filtered air or a propylene glycol (PG)/ glycerin (VG) (50:50, %W/W) aerosol mixture at the target concentration of 3 mg/L for six hours daily over a continuous 28-day period. Compared with the air group, female rats in the PG/VG group exhibited significantly lower body weights during both the exposure period and recovery period, and this was linked to a reduced food intake. Male rats in the PG/VG group also experienced a significant decline in body weight during the exposure period. Importantly, rats exposed to the PG/VG aerosol showed only minimal biological effects compared to those with only air exposure, with no signs of toxicity. Moreover, the transcriptomic, proteomic, and metabolomic analyses of the rat lung tissues following aerosol exposure revealed a series of candidate pathways linking aerosol inhalation to altered lung functions, especially the inflammatory response and disease. Dysregulated pathways of arachidonic acids, the neuroactive ligand-receptor interaction, and the hematopoietic cell lineage were revealed through integrated multi-omics analysis. Therefore, our integrated multi-omics approach offers novel systemic insights and early evidence of environmental-related health hazards associated with an e-cigarette aerosol using two carrier solvents in a rat model.

Comparison of urinary mercapturic acid excretions in users of various tobacco/nicotine products

Urinary mercapturic acids (MAs) are detoxification products for electrophiles occurring in the human body. They are suitable biomarkers of exposure to directly acting electrophilic chemicals or to chemicals which generate the electrophile during its metabolism. We determined the urinary excretion of 19 MAs in habitual users of combustible cigarettes (CCs), electronic cigarettes (ECs), heated tobacco products (HTPs), oral tobacco (OT), and nicotine replacement therapy (NRT) products, and nonusers (NUs) of any tobacco/nicotine products. The 19 MAs are assumed to be physiologically formed primarily from 15 toxicants with three of them belonging to IARC Group 1 (human carcinogen), seven to Group 2A (probable human carcinogen), four to Group 2B (possible human carcinogen), and one to Group 3 (not classifiable as carcinogen). Smoking (CC) was found to be associated with significantly elevated exposure to ethylene oxide (or ethylene), 1,3-butadiene, benzene, dimethylformamide, acrolein, acrylamide, styrene, propylene oxide, acrylonitrile, crotonaldehyde, and isoprene compared with the other user groups and NU. Users of HTPs revealed slight elevation in the MAs related to acrolein, acrylamide, and crotonaldehyde compared with the other non-CC groups. Vaping (EC) was not found to be associated with any of the MAs studied. In conclusion, the determination of urinary MAs is a useful tool for assessing the exposure to toxicants (mainly potential carcinogens) in users of various tobacco/nicotine products. Our data also give cause to clarify the role of vaping (EC) in urinary excretion of DHPMA (precursor: glycidol).

Electronic nicotine delivery systems (ENDS): A convenient means of smoking?

Electronic nicotine delivery systems (ENDS), which are becoming increasingly popular in many parts of the world, have recently become more sophisticated in terms of their more active content and better controlled vaporisation. This review begins by describing how cigarette smoking led to the development of ENDS as a means of combatting nicotine addiction. ENDS are usually categorised as belonging to one of only three main generations, but a fourth has been added in order to differentiate the latest, most powerful, most advanced and innovative that have improved heating efficiency. Descriptions of the principal substances contained in ENDS are followed by considerations concerning the risk of toxicity due to the presence of albeit low concentrations of such a variety of compounds inhaled over a long time, and the increasingly widespread use of ENDS as a means of smoking illicit drugs. We also review the most widely used pharmacotherapeutic approaches to smoking cessation, and recent epidemiological data showing that ENDS can help some people to stop smoking. However, in order to ensure their appropriate regulation, there is a need for higher-quality evidence concerning the health effects and safety of ENDS, and their effectiveness in discouraging tobacco smoking.

Beyond the label: current evidence and future directions for the interrelationship between electronic cigarettes and mental health

Electronic cigarette use has dramatically increased over the last decade. With this recent technological development and wide range of constituents in various products, putative adverse effects on the brain and body have been largely unexplored. Here, we review current evidence linking electronic nicotine cigarette use with potential health consequences and provide evidence supporting an association between drug use and depression in humans. We also examine the biological effects of individual constituents in electronic cigarette aerosols, which include labeled ingredients, such as propylene glycol, vegetable glycerin, nicotine, and flavorants, as well as unlabeled ingredients found in the aerosols, such as carbonyls and heavy metals. Lastly, we examine the effects of electronic cigarette use on endogenous metabolism via changes in cytochrome P450 enzymes, which can thereby impact therapeutic outcomes. While the current evidence offers insight into the potential effects of electronic cigarette use on biological processes, further studies are necessary to determine the long-term clinical relevance of aerosol inhalation.

Flavoured and nicotine-containing e-liquids impair homeostatic properties of an alveolar-capillary cell model

Most people consider that electronic cigarettes are safer than tobacco and are marketed as quit-smoking products. The e-liquid, which usually contains propylene glycol (PG) and vegetable glycerin (VG) in different ratios, nicotine and a wide variety of flavours, is heated by a coil and the aerosol droplets are primarily delivered to the alveolar area where nicotine and other molecules cross the alveolar-capillary barrier (ACB). However, e-cigarettes effects on the ACB are not yet established. In our study, a well-characterised in vitro model of the ACB was exposed to PG and VG and to five flavoured e-liquids with and without nicotine. The vehicles, due to their hypertonic properties, modulated the ACB integrity by modifying occludin expression. Below a 10% concentration, the vehicles did not trigger oxidative stress or cell death. Different results were observed between flavoured e-liquids: while red fruits and mint-eucalyptus disrupted ACB integrity, triggered oxidative stress and cell death, blond tobacco had no worse effect compared to the vehicles. However, the addition of nicotine in the latter e-liquid increased oxidative stress and cell death compared to the vehicles. Finally, mint-eucalyptus e-liquid increased some inflammation markers. Our results revealed that e-liquids alter ACB homeostasis, depending on flavour and nicotine presence.

Diet as a Source of Acrolein: Molecular Basis of Aldehyde Biological Activity in Diabetes and Digestive System Diseases

Acrolein, a highly reactive α,β-unsaturated aldehyde, is a compound involved in the pathogenesis of many diseases, including neurodegenerative diseases, cardiovascular and respiratory diseases, diabetes mellitus, and the development of cancers of various origins. In addition to environmental pollution (e.g., from car exhaust fumes) and tobacco smoke, a serious source of acrolein is our daily diet and improper thermal processing of animal and vegetable fats, carbohydrates, and amino acids. Dietary intake is one of the main routes of human exposure to acrolein, which is a major public health concern. This review focuses on the molecular mechanisms of acrolein activity in the context of its involvement in the pathogenesis of diseases related to the digestive system, including diabetes, alcoholic liver disease, and intestinal cancer.

Exposure, Retention, Exhalation, Symptoms, and Environmental Accumulation of Chemicals During JUUL Vaping

Little is known about the chemical exposures that electronic cigarette (EC) users receive and emit during JUUL vaping and if exposures produce symptoms dose dependently. This study examined chemical exposure (dose), retention, symptoms during vaping, and the environmental accumulation of exhaled propylene glycol (PG), glycerol (G), nicotine, and menthol in a cohort of human participants who vaped JUUL "Menthol" ECs. We refer to this environmental accumulation as "EC exhaled aerosol residue" (ECEAR). Chemicals were quantified using gas chromatography/mass spectrometry in JUUL pods before and after use, lab-generated aerosols, human exhaled aerosols, and in ECEAR. Unvaped JUUL "Menthol" pods contained ∼621.3 mg/mL of G, ∼264.9 mg/mL of PG, ∼59.3 mg/mL of nicotine, ∼13.3 mg/mL of menthol, and ∼0.1 mg/mL of the coolant WS-23. Eleven experienced male EC users (aged 21-26) provided exhaled aerosol and residue samples before and after vaping JUUL pods. Participants vaped ad libitum for 20 min, while their average puff count (22 ± 6.4) and puff duration (4.4 ± 2.0) were recorded. The transfer efficiency of nicotine, menthol, and WS-23 from the pod fluid into the aerosol varied with each chemical and was generally similar across flow rates (9-47 mL/s). At 21 mL/s, the average mass of each chemical retained by the participants who vaped 20 min was 53.2 ± 40.3 mg for G, 18.9 ± 14.3 mg for PG, 3.3 ± 2.7 mg for nicotine, and 0.5 ± 0.4 mg for menthol, with retention deduced to be ∼90-100% for each chemical. There was a significant positive relationship between the number of symptoms during vaping and total chemical mass retained. ECEAR accumulated on enclosed surfaces where it could contribute to passive exposure. These data will be valuable to researchers studying human exposure to EC aerosols and agencies that regulate EC products.

The Tobacco Smoke Component, Acrolein, as a Major Culprit in Lung Diseases and Respiratory Cancers: Molecular Mechanisms of Acrolein Cytotoxic Activity

Acrolein, a highly reactive unsaturated aldehyde, is a ubiquitous environmental pollutant that seriously threatens human health and life. Due to its high reactivity, cytotoxicity and genotoxicity, acrolein is involved in the development of several diseases, including multiple sclerosis, neurodegenerative diseases such as Alzheimer’s disease, cardiovascular and respiratory diseases, diabetes mellitus and even the development of cancer. Traditional tobacco smokers and e-cigarette users are particularly exposed to the harmful effects of acrolein. High concentrations of acrolein have been found in both mainstream and side-stream tobacco smoke. Acrolein is considered one of cigarette smoke’s most toxic and harmful components. Chronic exposure to acrolein through cigarette smoke has been linked to the development of asthma, acute lung injury, chronic obstructive pulmonary disease (COPD) and even respiratory cancers. This review addresses the current state of knowledge on the pathological molecular mechanisms of acrolein in the induction, course and development of lung diseases and cancers in smokers.

Biomarkers of Electronic Nicotine Delivery Systems (ENDS) Use

This perspective summarizes available evidence on biomarkers of exposure in electronic nicotine delivery system (ENDS) users to aid the overall assessment of the health consequences of using ENDS. Identification of novel biomarkers of exposure specific to ENDS use remains challenging because chemicals emitted from ENDS devices have many familiar sources. The biomarker levels of many tobacco-related toxicants measured in biological samples collected from ENDS users did not differ significantly from non-users, except for nicotine metabolites and a small number of biomarkers of exposure to volatile organic compounds and tobacco-specific tobacco nitrosamines. Several studies have shown that while exposed to nicotine, long-term exclusive ENDS users showed significantly lower levels of toxicant biomarkers than cigarette smokers. Studies have also shown that concurrent users of ENDS and combustible cigarettes ('dual users') are not reducing overall exposure to harmful toxicants compared to exclusive cigarette smokers. Because of an absence of validated ENDS-specific biomarkers, we recommend combining several biomarkers to differentiate tobacco product user groups in population-based studies and monitor ENDS compliance in randomized controlled trials. Using a panel of biomarkers would provide a better understanding of health effects related to ENDS use.

Pod-based e-liquids impair human vascular endothelial cell function

Pod-based electronic (e-) cigarettes more efficiently deliver nicotine using a protonated formulation. The cardiovascular effects associated with these devices are poorly understood. We evaluated whether pod-based e-liquids and their individual components impair endothelial cell function. We isolated endothelial cells from people who are pod users (n = 10), tobacco never users (n = 7), and combustible cigarette users (n = 6). After a structured use, pod users had lower acetylcholine-mediated endothelial nitric oxide synthase (eNOS) activation compared with never users and was similar to levels from combustible cigarette users (overall P = 0.008, P = 0.01 pod vs never; P = 0.96 pod vs combustible cigarette). The effects of pod-based e-cigarettes and their constituents on vascular cell function were further studied in commercially available human aortic endothelial cells (HAECs) incubated with flavored JUUL e-liquids or propylene glycol (PG):vegetable glycerol (VG) at 30:70 ratio with or without 60 mg/mL nicotine salt for 90 min. A progressive increase in cell death with JUUL e-liquid exposure was observed across 0.0001-1% dilutions; PG:VG vehicle with and without nicotine salt induced cell death. A23187-stimulated nitric oxide production was decreased with all JUUL e-liquid flavors, PG:VG and nicotine salt exposures. Aerosols generated by JUUL e-liquid heating similarly decreased stimulated nitric oxide production. Only mint flavored e-liquids increased inflammation and menthol flavored e-liquids enhanced oxidative stress in HAECs. In conclusion, pod e-liquids and their individual components appear to impair endothelial cell function. These findings indicate the potential harm of pod-based devices on endothelial cell function and thus may be relevant to cardiovascular injury in pod type e-cigarette users.

Increased Levels of the Acrolein Metabolite 3-Hydroxypropyl Mercapturic Acid in the Urine of e-Cigarette Users

Carcinogen and toxicant uptake by e-cigarette users have not been fully evaluated. In the study reported here, we recruited 30 e-cigarette users, 63 nonsmokers, and 33 cigarette smokers who gave monthly urine samples over a period of 4-6 months. Their product use status was confirmed by measurements of exhaled CO, urinary total nicotine equivalents, cyanoethyl mercapturic acid (CEMA), and total 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol. Urinary biomarkers of exposure to the carcinogens acrolein (3-hydroxypropyl mercapturic acid, 3-HPMA), benzene (S-phenyl mercapturic acid, SPMA), acrylonitrile (CEMA), and a combination of crotonaldehyde, methyl vinyl ketone, and methacrolein (3-hydroxy-1-methylpropyl mercapturic acid, HMPMA) were quantified at each visit. Data from subject visits with CEMA > 27 pmol/mL were excluded from the statistical analysis of the results because of possible unreported exposures to volatile combustion products such as secondhand cigarette smoke or marijuana smoke exposure; this left 22 e-cigarette users with 4 or more monthly visits and all 63 nonsmokers. Geometric mean levels of 3-HPMA (1249 versus 679.3 pmol/mL urine) were significantly higher (P = 0.003) in e-cigarette users than in nonsmokers, whereas levels of SPMA, CEMA, and HMPMA did not differ between these two groups. All analytes were significantly higher in cigarette smokers than in either e-cigarette users or nonsmokers. The results of this unique multimonth longitudinal study demonstrate consistent significantly higher uptake of the carcinogen acrolein in e-cigarette users versus nonsmokers, presenting a warning signal regarding e-cigarette use.