New strong evidence that Juul is more toxic to lungs and hearts than earlier e-cigs

Two new papers strongly suggest that Juul (and, likely, other fourth generation e-cigarettes) are more dangerous than earlier tank systems.  One, Differential Toxicity of Electronic Cigarette Aerosols Generated from Different Generations of Devices In Vitro and In Vivo, finds that Juul has higher pulmonary toxicity than earlier tank systems and the second, Nicotine Formulation Influences the Autonomic and Arrhythmogenic Effects of Electronic Cigarettes, finds that breathing e-cigarette aerosols of nicotine salts interferes with nervous system control of the heart and causes potentially dangerous arrhythmias (irregular heart rhythm).

Some have assumed Juul and other fourth generation e-cigarettes are less dangerous than earlier third generation tank system e-cigarettes because Juul operates at a lower power which results in less production of some harmful compounds than tank systems. Juul also pioneered the use of nicotine salts (also called protonated nicotine) created by adding acid to the e-liquid, which allows more efficient delivery of nicotine and, so, lower puff volumes.  Juul also generates smaller aerosol particles that carry the nicotine deeper into the lungs, also increasing nicotine delivery.

The pulmonary study

Differential Toxicity of Electronic Cigarette Aerosols Generated from Different Generations of Devices In Vitro and In Vivo comprehensively compares Juul and a tank system by (1) measuring the physicochemical properties of the aerosol, (2) cytotoxicity, inflammation, and oxidative stress induced by the aerosols in isolated human lung cells, and in intact mice.  They also examined different nicotine levels and flavors.

Both Juul and the tank system had many adverse pulmonary effects, but using a variety of measures, Juul had more adverse pulmonary effects than the tank system. 

In particular, the investigators found that, despite operating at a lower power, Juul delivered more ultrafine particles and contained higher levels of reactive oxygen species and aldehydes than the tank system.  Juul aerosols induced higher toxicity and increased inflammation in the isolated human lung cell studies than the tank system. They also found that Juul induced more severe pulmonary inflammation and DNA than the tank system after normalizing for the amount of nicotine absorbed into blood (measured with the nicotine metabolite cotinine) in the mice.

As I was reading the paper, I expected that the authors would attribute their findings to the use of nicotine salts, but they also did an experiment in which they delivered nicotine salts with the same tank system (rather than the Juul device) and found that it did not produce the same adverse effects as delivering the same e-liquid with a Juul.  As a result, they concluded, “Our findings suggest that the device design plays a more important role in e-cig aerosol-induced toxicity than the composition of the e-liquid or voltage.”

The cardiac study

Nicotine Formulation Influences the Autonomic and Arrhythmogenic Effects of Electronic Cigarettes examines the effects of different forms of nicotine (freebase, used in older e-cigarettes, nicotine salt, used in Juul and other fourth generation e-cigarettes, and racemic nicotine, a mixture of two different nicotine forms present in synthetic nicotine) on heart rhythms.

There are two bottom lines:

1. Nicotine in e-cigarettes causes irregular heart beats (arrhythmias) in a dose-dependent manner by stimulating the very receptor that many heart medications (beta blockers) are designed to inhibit.  This suggests the nicotine is harmful to the heart, and counters popular claims that the nicotine itself is harmless.  
2. The nicotine type in newer pod-based e-cigarette devices like Juul (protonated nicotine, also called nicotine salts) more potently induces arrhythmia than the nicotine type found in older e-cigarette device types (freebase).

The investigators implanted small devices in mice that transmitted heart rhythms so they could measure how these rhythms changed when the mice were breathing clean air, an aerosol of propylene glycol and glycerin (the solvent in e-liquid) without nicotine, and with the different forms of nicotine for 30 minutes.  All experiments used the same tank system.

They measured the effects of the different nicotine forms on the heart rate and heart rate variability (small changes in the interbeat interval, HRV) in the mice. Lower heart rate variability predicts increased risk of death following a heart attack.

They also examined the mechanism of these changes on the autonomic nervous system that controls internal organs, including the heart. There are two main components of the autonomic nervous system: the sympathetic division is like the gas pedal that activates “fight or flight” reflexes while the parasympathetic division is like a break that activates to “feed and breed” and “rest-and-digest” The balance of these two divisions controls heart rate. Increases in heart rate with declines in HRV indicate sympathetic dominance and heart rate reductions with HRV elevations suggest parasympathetic dominance. Sympathetic dominance predicts cardiovascular disease risk and all-cause mortality, whereas parasympathetic dominance correlates with aerobic fitness.

Breathing freebase nicotine did not alter heart rate and increased heart rate variability, suggesting parasympathetic dominance.  In contrast, inhaling 5% protonated nicotine (the level in Juul) increased heart rate and decreased heart rate variability, suggesting sympathetic dominance.   They also found that inhaling protonated nicotine provoked subtle increases in spontaneous ventricular arrhythmias (irregular heartbeats that can be fatal or lead to pathological remodeling of the heart).

In terms of underlying mechanisms,. they found that nicotine in e-cigarettes causes arrhythmias in a dose-dependent manner by stimulating the very receptor that many heart medications (beta blockers) are designed to inhibit. 

The authors state that “The disproportionate sympathetic impacts of nicotine salt [protonated nicotine] may also stem from its greater palatability compared to free-base nicotine. Free-base nicotine is a respiratory irritant that, along with other e-liquid constituents and their degradation products, stimulates airway irritant reflexes when inhaled. Accordingly, we observed a nearly three-fold increase in nicotine intake with 5% salt [the concentration in Juul] compared to 5% free-base.”

The authors conclude, “Collectively, our findings suggest that exposure to e-cig aerosols containing commercially relevant concentrations of nicotine salt [protonated nicotine] induces cardiac dysfunction by increasing sympathetic influence and spontaneous ventricular arrhythmias.”

(This blog post focuses on the comparison of freebase and protonated nicotine. For the results showing the adverse effects of racemic nicotine [similar to the synthetic nicotine used in “tobacco free” nicotine products], see the paper.)

The bottom line

These two very thorough and carefully controlled studies strongly suggest that the Juul device and likely other similar fourth generation e-cigarettes are more toxic to the lungs and heart than earlier tank systems.  The scientific community, FDA and other policymakers should stop assuming that Juul is less toxic than earlier generation e-cigarettes.

Here are the full citations and abstracts for the two papers:

Tiancong Ma, Haoxuan Chen, Yu-Pei Liao, Jiulong Li, Xiang Wang, Liqiao Li, Jing Li, Yifang Zhu*, and Tian Xia*. Environ. Health 2023, 1, 5, 315–323 Publication Date:September 6, 2023. https://doi.org/10.1021/envhealth.3c00099

Electronic cigarettes (e-cigs) have become increasingly popular, especially among youth, raising concerns about their potential health risks. JUUL and Tank devices are two common types of e-cigs that deliver aerosols with varying nicotine levels and flavors. However, the differences in the aerosols generated from different devices and their corresponding cytotoxicity and pulmonary injury effects remain poorly understood. This study addresses these knowledge gaps by characterizing the aerosols of JUUL and Tank e-cig devices and testing their toxic effects on THP-1 and BEAS-2B human cell lines as well as the C57BL/6J mouse model. In our study, the lower-voltage device, the 3.7 V JUUL generates 2.72 mg/puff aerosols by using e-liquid containing 3% nicotine salt (i.e., nicotine benzoate), which is less than the 11.06 mg/puff aerosols generated by the 7.5 V Tank using e-liquid containing 2.4% freebase nicotine. Yet, the cytotoxicity results reveal that JUUL aerosols induced higher toxicity and increased production of pro-inflammation cytokines compared to Tank aerosols per puff. Additionally, we observed that JUUL induced more severe pulmonary inflammation and DNA damage compared to Tank after normalizing for cotinine, a nicotine metabolite, in vivo. Our findings suggest that the device design plays a more important role in e-cig aerosol-induced toxicity than the composition of the e-liquid or voltage. These results provide valuable insights into the health risks associated with various electronic-cig devices and offer an approach for evaluating them.

Cory Kucera, Anand Ramalingam, Shweta Srivastava, Aruni Bhatnagar, Alex P Carll, Nicotine Formulation Influences the Autonomic and Arrhythmogenic Effects of Electronic Cigarettes, Nicotine & Tobacco Research, 2023;, ntad237, https://doi.org/10.1093/ntr/ntad237

https://pubmed.ncbi.nlm.nih.gov/38011908/

Introduction. Evidence is mounting that electronic cigarette (e-cig) use induces cardiac sympathetic dominance and electrical dysfunction conducive to arrhythmias and dependent upon nicotine. A variety of nicotine types and concentrations are available in e-cigs, but their relative cardiovascular effects remain unclear. Here we examine how different nicotine forms (racemic, free-base, and salt) and concentrations influence e-cig-evoked cardiac dysfunction and arrhythmogenesis and provide a mechanism for nicotine-salt-induced autonomic imbalance.

Methods. ECG-telemetered C57BL/6J mice were exposed to filtered air (FA) or e-cig aerosols from propylene glycol and vegetable glycerin solvents either without nicotine (vehicle) or with increasing nicotine concentrations (1%, 2.5%, and 5%) for three 9-min puff sessions per concentration. Spontaneous ventricular premature beat (VPB) incidence rates, heart rate, and heart rate variability (HRV) were compared between treatments. Subsequently, to test the role of β₁-adrenergic activation in e-cig-induced cardiac effects, mice were pretreated with atenolol and exposed to either FA or 2.5% nicotine salt.

Results. During puffing and washout phases, ≥ 2.5% racemic nicotine reduced heart rate and increased HRV relative to FA and vehicle controls, indicating parasympathetic dominance. Relative to both controls, 5% nicotine salt elevated heart rate and decreased HRV during washout, suggesting sympathetic dominance, and also increased VPB frequency. Atenolol abolished e-cig-induced elevations in heart rate and declines in HRV during washout, indicating e-cig-evoked sympathetic dominance is mediated by β₁-adrenergic stimulation.

Conclusions. Our findings suggest that inhalation of e-cig aerosols from nicotine salt-containing e-liquids could increase the cardiovascular risks of vaping by inducing sympathetic dominance and cardiac arrhythmias.

Implications. Exposure to e-cig aerosols containing commercially relevant concentrations of nicotine salts may increase nicotine delivery and impair cardiac function by eliciting β₁-adrenoceptor-mediated sympathoexcitation and provoking ventricular arrhythmias. If confirmed in humans, our work suggests that regulatory targeting of nicotine salts through minimum pH standards or limits on acid additives in e-liquids may mitigate the public health risks of vaping.