EffectofExposuretoe-CigarettesWithSaltvsFree...

Original Investigation | Public Health

Effect of Exposure to e-Cigarettes With Salt vs Free-Base Nicotine on the Appealand Sensory Experience of VapingA Randomized Clinical TrialAdam M. Leventhal, PhD; Danielle R. Madden, PhD; Natalia Peraza, BS; Sara J. Schiff, BS; Lucas Lebovitz, BS; Lauren Whitted, MA; Jessica Barrington-Trimis, PhD;Tyler B. Mason, PhD; Marissa K. Anderson; Alayna P. Tackett, PhD

Abstract

IMPORTANCE Alkaline free-base nicotine is bitter and a respiratory irritant. High-nicotine electroniccigarette (e-cigarette) products contain acid additives that change nicotine from a free-base to aprotonated salt chemical form, which could improve the sensory experience of vaping, particularlyamong never smokers unaccustomed to inhaling free-base nicotine.

OBJECTIVE To determine whether exposure to e-cigarettes with salt vs free-base nicotineformulations improves the appeal and sensory experience of vaping e-cigarettes and whethernicotine formulation effects differ by e-cigarette flavor and ever combustible cigarettesmoking status.

DESIGN, SETTING, AND PARTICIPANTS Single-visit double-blind within-participant randomizedclinical trial was conducted in an academic medical center outpatient clinical research facility inSouthern California. Participants were 119 individuals with past 30-day e-cigarette or combustiblecigarette use aged 21 years or older recruited from November 2019 to March 2020.

INTERVENTIONS Participants self-administered standardized puffs of each 10 differently flavorede-cigarette solutions using a pod-style device. Each flavor was administered in salt (benzoic acidadded) and free-base (no benzoic acid) nicotine formulations with commensurate nicotineconcentrations (mean, 23.6 mg/mL). The 20 solutions were administered in randomly assignedsequences. Immediately after puffing each solution, participants rated appeal and sensory attributes.

MAIN OUTCOMES AND MEASURES Self-reported appeal (mean of like, dislike [reverse-scored],and willingness to use again ratings) and 4 sensory attributes (sweetness, smoothness, bitterness,and harshness; analyzed individually) on visual analog scales with not at all and extremely anchors(range, 0-100).

RESULTS Of the 119 participants; 39 (32.8%) were female. The mean (SD) age was 42.1 (14.4) years;105 (88.2%) were ever combustible cigarette smokers, and 66 (55.5%) were current e-cigaretteusers. Salt vs free-base nicotine formulations produced higher ratings of appeal (salt vs free-basemean difference effect estimate: b = 12.0; 95% CI, 9.9-14.1; P < .001), sweetness (b = 9.3; 95% CI,7.1-11.4; P < .001), and smoothness (b = 17.4; 95% CI, 15.2-19.6; P < .001) and lower ratings ofbitterness (b = −13.3; 95% CI, −15.4 to −11.2; P < .001) and harshness (b = −21.0; 95% CI, −23.2 to−18.7; P < .001). Nicotine formulation effects largely generalized across different flavors and thesmoothness-enhancing and harshness-reducing effects of nicotine salt were stronger in never vsever cigarette smokers.

(continued)

Key PointsQuestion Does exposure to electronic

cigarettes (e-cigarettes) in salt vs free-

base nicotine formulations improve the

appeal and sensory experience

of vaping?

Findings In this randomized clinical

trial, 119 adult nicotine or tobacco

product users rated puffs from

e-cigarettes in nicotine salt (benzoic acid

added) and nicotine free-base (no

benzoic acid) formulations. Salt vs free-

base nicotine formulations resulted in

statistically significant higher ratings of

appeal, sweetness, and smoothness,

and lower ratings of bitterness and

harshness.

Meaning In this study, acid additives in

e-cigarettes that change nicotine from

free base to salt appeared to enhance

the appeal and sensory experience of

vaping and merit consideration in

e-cigarette regulation.

+ Visual Abstract

+ Supplemental content

Author affiliations and article information arelisted at the end of this article.

Open Access. This is an open access article distributed under the terms of the CC-BY-NC-ND License.

JAMA Network Open. 2021;4(1):e2032757. doi:10.1001/jamanetworkopen.2020.32757 (Reprinted) January 12, 2021 1/13

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Abstract (continued)

CONCLUSIONS AND RELEVANCE In this randomized clinical trial of adult current nicotine ortobacco product users, controlled exposure to e-cigarette puffs with salt vs free-base nicotineformulations appeared to increase product appeal and improve the sensory experience of vaping,particularly among never smokers. Regulatory policies limiting acid additives in e-cigarettes mightreduce the appeal of high-nicotine e-cigarettes among populations deterred from vaping e-cigarettesthat emit harsh aerosol.

TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT04399031

JAMA Network Open. 2021;4(1):e2032757. doi:10.1001/jamanetworkopen.2020.32757

Introduction

Electronic cigarettes (e-cigarettes) have evolved over time. Between 2013 and 2018, there was a6-fold increase in the proportion of total US e-cigarette sales consisting of products with highnicotine concentrations,1 including the JUUL e-cigarette brand, which has been widely used byadolescents and young adults.2,3

Although nicotine has neuropharmacologically mediated reinforcing effects once absorbed intothe bloodstream, alkaline free-base nicotine is bitter and irritates the airways.4,5 Before the entry ofthe JUUL e-cigarette brand into the market, e-cigarettes contained nicotine in its alkaline free-basechemical form, and free-base nicotine products with higher nicotine concentrations producedaerosol that was perceived by users as harsh, bitter, and less appealing4 and were infrequently sold.1

JUUL and other manufacturers of high-nicotine e-cigarettes have begun adding organic acids to theirproducts, which changes nicotine from a free base to a protonated salt.6 It has been hypothesizedthat e-cigarettes with high nicotine concentrations in salt vs free-base nicotine formulations produceless aversive sensory effects, which might make e-cigarettes easier to inhale, more appealing, andmore addictive.7 To date, this hypothesis has gone untested. Evidence that nicotine salt formulationsenhance the appeal and sensory qualities of vaping might suggest that new regulations limiting salesof e-cigarettes with acid additives might benefit public health for populations who do not usee-cigarettes to quit smoking.

This trial tested the hypothesis that exposure to e-cigarettes with salt vs free-base nicotineformulations would increase user-reported appeal and improve the sensory attributes of vaping.Additional objectives were to determine the generalizability of the results across different e-cigaretteflavors and populations by examining whether nicotine formulation effects differed by flavor andever combustible cigarette smoking status.

Methods

ParticipantsIndividuals from the Los Angeles area were recruited to participate in a single-visit randomizedclinical trial conducted in an academic medical center outpatient research facility in southernCalifornia. Inclusion criteria required past 30-day nicotine or tobacco product use of eithere-cigarettes (e-cigarette use �3-day/week over the past 30 days; lifetime vaping duration �2months; used nicotine-containing e-cigarettes) or combustible cigarettes (cigarette use �3-day/week for �2 years; interest in trying e-cigarettes if not also using e-cigarettes). Current smokers whoalso used e-cigarettes were eligible. Exclusion criteria were planning to quit using nicotine or tobaccoproducts, currently or planning to become pregnant/breastfeeding, current daily use of tobaccoproducts other than combustible cigarettes or e-cigarettes, and positive results of breath alcohol testat study visit. Participants provided written informed consent and were enrolled from November

JAMA Network Open | Public Health Effect of Exposure to e-Cigarettes With Salt vs Free-Base Nicotine on the Appeal and Sensory Experience of Vaping



https://clinicaltrials.gov/ct2/show/NCT04399031


2019 to March 2020. Participant accrual was halted prematurely due to coronavirus disease 2019and trial registry information was updated on ClinicalTrials.gov in May 2020. The University ofSouthern California Institutional Review Board approved the study. The trial protocol is available inSupplement 1. This study followed the Consolidated Standards of Reporting Trials (CONSORT) reportingguideline for randomized clinical trials.8

Design and MaterialsA within-participant randomized double-blind design was used. The procedure used custome-cigarette solutions in 10 flavors (green apple, strawberry, chocolate, vanilla, menthol, cool menthol,peppermint, spearmint, subtle tobacco, and full-flavored tobacco) each in nicotine salt and free-base formulations with 50/50 propylene glycol/vegetable glycerin vehicle (Avail Vapors). For eachpair of solutions for a respective flavor, the constituents were equivalent with the exception that thenicotine salt formulation included benzoic acid at a 1:1 molar ratio to nicotine and the free-baseformulation did not. Nicotine concentration, density, propylene glycol/vegetable glycerin vehicle,and pH tests of each solution were conducted by the Roswell Park Comprehensive Cancer CenterNicotine and Tobacco Product Assessment Core (eMethods in Supplement 2) and used to calculatethe proportion of nicotine in protonated (salt) form. Each solution was administered via a pod-stylee-cigarette device (Suorin iShare Pod System Device; 7W; 2.0-Ω resistance; 130-milliamp hourbuilt-in battery), which resembles JUUL and includes pod cartridge inserts that can be filled withcustom solutions.

ProcedureAfter a telephone eligibility screen, participants were invited to a 3-hour study visit and instructed toabstain from using nicotine or tobacco products for 2 hours before arrival.9,10 During informedconsent, participants were instructed that the study investigated the effects of e-cigarettes withdifferent nicotine and flavorings. After informed consent, participants provided carbon monoxideand alcohol breath samples (BACtrack S80, BACtrack). Female participants provided urine samplesfor pregnancy tests.

Participants then completed the standardized e-cigarette appeal and sensory rating procedure,which has shown sensitivity to nicotine, device power, and flavor manipulations,9-12 in a ventilatedtesting room. The 20 trials (10 flavors presented in 2 nicotine formulations [free-base and salt])followed a practice trial with flavorless nicotine-free solution. Each participant received a randomizedordering of the 20 e-cigarette solutions developed using a random sequence generator by staff whodid not interact with participants and prepared products in the randomized order sequence beforethe visits. Participants and data collection staff were blinded to the solutions administered at eachtrial. During each administration, participants viewed a tutorial video with instructions directing themthrough the controlled guided puffing procedure, which involved a 1-puff cycle (10-secondpreparation, 4-second inhalation, 1-second hold, and 2-second exhale interval) for each productimmediately followed by appeal and sensory attribute ratings. After rating each product, participantswere given water before the next trial to minimize sensory carryover. The procedure was separatedinto four 5-trial blocks spaced at least 10 minutes apart. Participant characteristics questionnaireswere completed between blocks.

MeasuresStudy OutcomesAfter each single-puff trial, participants rated the product they just vaped on visual analog scales(range, 0-100) with answers to the following questions: (1) “How much did you like the e-cigarette?”;(2) “How much did you dislike the e-cigarette?”; (3) “Would you use this e-cigarette again?”; (4) “Howsweet was the e-cigarette?”; (5) “How smooth was the e-cigarette?”; (6) “How bitter was thee-cigarette?”; and (7) “How harsh was the e-cigarette?” Rating anchors were not at all and extremelyfor each measure, except use again (not at all and definitely). The ratings liking, disliking





https://www.equator-network.org/reporting-guidelines/consort/


(reverse-scored), and willingness to use again represent related but nonredundant measures ofappeal (eg, individuals may feel ambivalent, expressing both liking and disliking of a product or theycan dislike a product but be willing to try it again).9,13 Thus, we calculated an appeal composite scorebased on the mean of these 3 ratings for each trial (Cronbach α = .93) per prior work.9-12 Sensoryratings were analyzed separately per previous factor analyses indicating that appeal items shared acommon factor, whereas sensory attributes loaded onto distinct item-specific factors.9

Participant CharacteristicsQuestionnaires assessing demographic and tobacco product use history characteristics included aquestion assessing ever combustible cigarette smoking status, defined as lifetime smoking 100 ormore cigarettes.14-16 The question responses are shown in the Table. The Penn State ElectronicCigarette Dependence Index,17 an e-cigarette dependence severity measure (range, 0-20), and theFagerström Test for Cigarette Dependence,18 a cigarette dependence severity questionnaire (range,0-10), were also administered. Test strips (NicAlert; Jant Pharmacal Corporation) that provide asemiquantitative index of salivary or urine cotinine and exhaled carbon monoxide (Vitalograph) werecollected to provide descriptive data on nicotine and combustible tobacco exposure, respectively.

Statistical AnalysisAfter descriptive analyses of the study sample and materials, the primary analysis used multilevelmodels (MLMs). MLMs modeled rating outcomes from each trial as separate data points (20 totalobservations per participant; 10 per nicotine formulation condition) with participant-level randomeffects. The MLMs generate results that can be interpreted as mean effects collapsed across all trialswithin each condition. Primary models tested the fixed-effects nicotine formulation (salt vsfree-base) on each outcome with and without adjusting for trial order (range, 1-20). Secondarymodels examined the generalizability of nicotine formulation effects across flavors and smokingstatus by testing the within-by-within interaction for nicotine formulation and flavor (10-levelcategorical variable) and the between-by-within interaction for never vs ever smoking status andnicotine formulation on each outcome, respectively. The MLMs yielded unstandardized effectestimates with standard errors (B [SE]), which reflect the difference in mean ratings betweenconditions. Of the analytic sample (N = 119), there were 5 participants with trial-level missing data(range, 4-16 trials); MLMs used all available data for participants with 1 or more observations.Smoothness ratings were introduced after the first 8 participants, resulting in an analytic sample of111 for this outcome. P values were 2-tailed with .05 significance levels. Benjamini-Hochberg multipletest corrections were used to maintain a .05 studywise false discovery rate.19 Analyses wereconducted using Stata version 16 (StataCorp LLC).20 Additional sensitivity analyses are detailed inthe Results section.

Results

Descriptive ResultsOf the 119 participants; 39 (32.8%) were female. The mean (SD) age was 42.1 (14.4) years; 105(88.2%) (22 former and 83 current smokers) were ever combustible cigarette smokers, and 66(55.5%) were current e-cigarette users (Table; Figure 1). Ever combustible cigarette smokersreported on average to have been moderate smokers during their heaviest smoking period (mean[SD] number of cigarettes smoked/d = 17.8 [10.9]). Current smokers reported having medium currentcigarette dependence severity (mean [SD] Fagerström Test for Cigarette Dependence score, 4.4[2.2]) and included 53 smokers who never regularly vaped e-cigarettes. Among the 66 (55.5%)current e-cigarette users, the modal e-cigarette device (55.7% pod style) and mean (SD) nicotineconcentration typically used (29.3 [20.6] mg/mL) were similar to the products used in this studyprotocol, and 30 participants were current dual users of both combustible and e-cigarettes (Table).




In the 10 salt nicotine solutions, the mean (SD) nicotine concentration value was 23.4 (0.9)mg/mL, pH was 6.6 (1.1), propylene glycol/vegetable glycerin vehicle proportion was 52.5 (2.6)/47.5(2.6), density was 1.1 (0.02) g/mL, and estimated protonated nicotine was 97.8% (2.3%). The free-base nicotine solutions had a mean (SD) nicotine concentration value of 23.8 (1.7) mg/mL, pH of 8.9(1.1), propylene glycol/vegetable glycerin vehicle proportion of 52.5 (2.6)/47.5 (2.6), density of 1.1(0.03) g/mL, and estimated protonated nicotine of 17.5% (25.7%) (eTable 1 in Supplement 2).

Table. Participant Characteristics

Variable No. (%)a

Female sex 39 (32.8)

Age, mean (SD), y 42.1 (14.4)

Race/ethnicity

White 35 (30.2)

Black 46 (39.7)

Asian 9 (7.8)

Multi-racial 12 (10.3)

Otherb 14 (12.1)

Hispanic 23 (19.8)

Tobacco product use characteristics

Ever combustible cigarette smokingc 105 (88.2)

Current e-cigarette vaping statusd 66 (55.5)

Current other tobacco product usee 27 (25.0)

Biomarkers, mean (SD)

Carbon monoxide, ppm 6.4 (5.9)

Cotinine semiquantitative levelf 4.0 (1.6)

Combustible cigarettes

Age started smoking regularly, mean (SD), yg 18.7 (6.9)

Current cigarettes/d, mean (SD)h 11.0 (6.8)

Cigarettes/d when smoking heaviest, mean (SD)g 17.8 (10.9)

No. of days smoked in past 30 dh 23.4 (10.7)

FTCD, mean (SD)h 4.4 (2.2)

Usually smoke(d) menthol cigarettesg 47 (53.4)

e-Cigarettes

PSECD, mean (SD)i 10.1 (4.9)

Puffs per day, mean (SD)i 85.5 (90.4)

Nicotine concentration typically used, mean (SD), mg/mLi 29.3 (20.6)

Duration of e-cigarette use, mean (SD), moj 21 (16.2)

No. of days vaped in past 30 di 23.4 (8.6)

e-Cigarette device type typically usedj

Cig-a-like 2 (3.3)

Tank/pen 5 (8.2)

Advanced personal vaporizer/mod 11 (18.0)

Pod-based 34 (55.7)

Other 9 (14.8)

Preferred e-cigarette flavorj

Fruit 28 (45.9)

Dessert 7 (11.5)

Mint 6 (9.8)

Menthol 9 (14.8)

Tobacco 9 (14.8)

Other 2 (3.3)

Abbreviations: e-cigarette, electronic cigarette; FTCD,Fagerström Test for Cigarette Dependence; PSECD,Penn State Electronic Cigarette Dependence Index.a Overall N = 119. Sample size ranges from 110-119

across variables due to differential patterns ofmissing data across variables.

b Includes American Indian or Alaskan Native, MiddleEastern, Pacific Islander (including Hawaii),and other.

c Smoked �100 cigarettes lifetime and in the past 30days (n = 22 were former smokers who did notsmoke at all in past month, n = 83 were current[past-month] smokers).

d Vaped �3 days per week for past �2 months.e Past 30-day use of “chewing tobacco, snuff or dip,”

“dissolvable tobacco product,” “bidis,” “kreteks,”“regular pipe tobacco,” “snus,” “big cigars,” “littlecigars or cigarillos,” or “hookah water pipe.”

f Test strip (range, 1–6; 0 = 0-10, 1 = 10-30, 2 = 30-100, 3 = 100-200, 4 = 200-500, 5 = 500-1000,6 = >1000 ng/mL).

g Former or current smokers only (n = 105). Numbersrange from 83-105 due to missing data acrossvariables.

h Current smokers only (n = 83). Numbers range from80-83 due to missing data across variables.

i Current users of e-cigarettes only (n = 66). Numbersrange from 54-66 due to missing data acrossvariables.

j Ever users of e-cigarettes only (n = 64). Numbersrange from 54-64 due to missing data acrossvariable.





Primary ResultsThe MLMs revealed that salt vs free-base nicotine formulations produced significantly higher ratingsof appeal (mean [SE]: 55.6 [1.7] vs 43.6 [1.7]; difference in means effect estimate b = 12.0; 95% CI,9.9-14.1), sweetness (mean [SE]: 52.2 [1.6] vs 43.0 [1.6]; b = 9.3; 95% CI, 7.1-11.4), and smoothness(mean [SE]: 63.7 [1.6] vs 46.3 [1.6]; b = 17.4; 95% CI, 15.2-19.6). Salt vs free-base nicotine producedsignificantly lower ratings of bitterness (mean [SE]: 30.4 [1.8] vs 43.7 [1.8];b = −13.3; 95% CI, −15.4 to−11.2) and harshness (mean [SE]: 36.0 [1.6] vs 56.9 [1.6]; b = −21.0; 95% CI, −23.2 to −18.7) (Figure 2).To determine whether sensitization or habituation during the procedure affected the results,

Figure 1. Study Flow Diagram and Schematic

808 Individuals completed preliminary telephone screen

124 Attended in-person visit

119 Completed study protocol included in analyses119 Data available for appeal outcomea

119 Data available for sweetness outcome119 Data available for bitterness outcome119 Data available for harshness outcome111 Data available for smoothness outcomeb

684 Excluded616 Ineligible68 Eligible but did not attend in-person visit

5 Excluded2 Discontinued for positive breath alcohol1 Declined to participate1 No longer met eligibility criteria1 Removed for protocol error

119 Randomized in unique order of exposureto the 20 study products

Study flow diagramA

Schematic of study procedureB

Extremely

“How much did youlike the e-cigarette?”

Not at all

Product rating:

Repeat: 20x trialsRandom order without replacement

Product administrationParticipant takes 1standardized puff

Participant rates appeal andsensory attributes

Full-flavoredtobacco

Greenapple

Regularmenthol

Subtletobacco

Coolmenthol

Freebase

Salt

Chocolate VanillaStrawberryPeppermint Spearmint

Nic

otin

e fo

rmul

atio

n

eCigarette indicates electronic cigarette.a Mean of like, dislike (reverse-scored), and willingness

to use again ratings on visual analog scale.b Smoothness measure introduced into the study after

the first 8 participants had already completed theprotocol.




analyses were retested after adjusting for trial number (1-20), which produced the same pattern ofnicotine formulation effects across all outcomes (eTable 2 in Supplement 2).

Secondary ResultsStratified by FlavorThe interactions between nicotine formulation and flavor were nonsignificant for all outcomes,except harshness (F = 3.1; P = .001). (Figure 3; eTables 2 and 3 in Supplement 2). The harshness-reducing effect of nicotine salt was significant for all flavors except chocolate (b = −5.6; 95% CI, −13.0to 1.8) (Figure 3). Chocolate appeared to be an outlying flavor driving the interaction, as post-hocanalyses excluding chocolate flavor trials found nonsignificant interaction between flavor andnicotine formulation effects on all outcomes (eTable 2 in Supplement 2).

Stratified by Smoking StatusParticipants classified as never smoking e-cigarette users (n = 14; mean [SD] age, 22.3 [1.9] years) orever smokers (n = 105; mean [SD] age, 44.8 [13.2] years) were compared in tests of interactionsbetween nicotine formulation and smoking status. Interactions were significant for smoothness (B,10.4; 95% CI, 3.4-17.5; P = .004) and harshness (b: −12.1; 95% CI, −19.0 to −5.2; P = .001) andnonsignificant for other outcomes (eTable 3 in Supplement 2; Figure 4). The smoothness-enhancing(b: 26.7; 95% CI, 20.2-33.2 vs B: 16.3; 95% CI, 13.9-18.6) and harshness-reducing (b: −31.7; 95% CI,−37.4 to −25.9 vs b: −19.5; 95% CI, −22.0 to −17.1) effects of nicotine salt were stronger in never vsever smokers.

Sensitivity AnalysisSensitivity analyses tested the generalizability and robustness of the findings. Consistent with theprimary results, salt vs free-base nicotine increased appeal, sweetness, and smoothness and reducedbitterness and harshness in participants without significant previous experience using e-cigarettes(n = 53) (eTable 4 in Supplement 2), after adjusting for variability in the nicotine concentration ofe-cigarette solutions (eTable 5 in Supplement 2), and regardless of whether participants usedpod-style e-cigarettes (n = 34) or other types of e-cigarettes (n = 27) (eTable 6 in Supplement 2).

Discussion

The results of this study indicate that the addition of an acidic compound to e-cigarettes, whichchanges nicotine from free-base to salt,6 improves the appeal and sensory experience of vaping.Surveillance and market research indicate national sales of high-nicotine pod-style e-cigaretteproducts that contain nicotine salt formulations, daily use prevalence, and addiction have increased

Figure 2. Mean (SE) Appeal and Sensory Attribute Ratings, by Nicotine Formulation of Electronic Cigarettes

70

60

50

40

30

20

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Appeal Sweetness Smoothness Bitterness Harshness

Free-base formulation

Salt formulation

Sensory attribute

P <.001P <.001

P <.001

P <.001

P <.001

The number of participants was 119 for all outcomesexcept harshness (n = 111). Appeal refers to the meanof liking, willingness-to-use-again, and disliking(reverse-scored) (range, 0-100) scores.











since 2013, particularly in young people.1-3,21-24 Research on the reasons high-nicotine pod-stylee-cigarette products are widely used have predominately focused on their marketing, design, flavors,and cultural trends,3,25,26 without systematically investigating the effects of nicotine formulation.

The present study addresses this gap using a tightly controlled double-blind trial designed toexperimentally control for variation in marketing, device design, flavors, cultural trends, preexistinguser preferences, and other factors in product choice. The use of e-cigarette solutions that werecustom-designed to match nicotine concentration level, flavors, and other constituents acrossnicotine salt and free-base conditions permitted isolation of nicotine formulation effects. There wasspecificity in the results: nicotine salt enhanced desirable sensory attributes and suppressedundesirable sensory attributes of e-cigarette aerosol and did so independently of nicotineconcentration.

Previous studies applying methods similar to the present study found that increasing nicotineconcentration in e-cigarettes with free-base nicotine formulations generated aerosol that wasperceived by users as harsher, more bitter, less sweet, less smooth, and less appealing overall.7,9,27,28

Prior to the advent of nicotine salt formulations, e-cigarette solutions predominately containednicotine concentrations 2 to 10 times lower than JUUL1 and were often paired with high-wattagedevices to obtain aerosol that was both satisfying and capable of delivering high nicotine levels.4

Based on the current findings, it is plausible that nicotine salt formulations make modern diminutivepod-style e-cigarette devices capable of producing aerosol that is rich with nicotine and enjoyable

Figure 3. Nicotine Formulation Effect Estimates, Stratified by Flavor

Green apple

e-Cigarette flavor

Peppermint

Spearmint

Strawberry

Chocolate

Cool menthol

Subtle tobacco

Full-flavored tobacco

Regular menthol

Vanilla

Effect estimate (salt vs free-base formulation)

–5 0 5 10 15 20 25 30

AppealA

e-Cigarette flavor

Green apple

Peppermint

Spearmint

Strawberry

Chocolate

Cool menthol

Subtle tobacco


Regular menthol

Vanilla


–5 0 5 10 15 20 25 30

SweetnessB

e-Cigarette flavor

Green apple

Peppermint

Spearmint

Strawberry

Chocolate

Cool menthol

Subtle tobacco


Regular menthol

Vanilla


SmoothnessC

–5 0 5 10 15 20 25 30

Green apple

Peppermint

e-Cigarette flavor

Spearmint

Strawberry

Chocolate

Cool menthol

Subtle tobacco


Regular menthol

Vanilla


–35 –30 –25 –20 –15 –10 –5 0 5

BitternessD

Green apple

Peppermint

Spearmint

Strawberry

Chocolate

Cool menthol

Subtle tobacco


Regular menthol

Vanilla


–35 –30 –25 –20 –15 –10 –5 0 5

e-Cigarette flavor

HarshnessE

The number of participants was 119 for all outcomes except harshness (n = 111). Effect estimate (salt vs free-base). Error bars are 95% CIs. Appeal refers to the mean of liking,willingness-to-use-again, and disliking (reverse-scored) (range, 0-100) scores. e-Cigarette indicates electronic cigarettes.




to the user despite containing small batteries with modest electrical output. By improving thesensory experience of vaping, individuals who first try e-cigarettes with nicotine salt formulationsmight find the nicotine-rich aerosol they emit palatable and easy to inhale. Thus, the likelihood of usecontinuation (instead of desistance) might be higher with nicotine salt than free-base e-cigarettes,which may prolong exposure to nicotine’s addictive properties and encourage long-term vapingpatterns. This continuation could be a factor in the rise in e-cigarette use prevalence, frequency, anddependence symptoms in the US since 2015, assuming our findings may be generalizable outside ofLos Angeles and to youths, new users unaccustomed to vaping, and across different products. Whilegeneralizability remains a question, secondary analyses suggest the results extend to smokerswithout significant prior experience vaping, across various flavored products, and to young adultnever smokers.

Tobacco companies have historically used acid additives in combustible cigarettes based onindustry research demonstrating that lowering pH increases the smoothness and palatability oftobacco smoke.29 Increasing the alkalinity of cigarette smoke increases perceived bitterness,30 andlowering pH can suppress the bitter-enhancing effects of some alkaline compounds.31 Theproportion of free-base to salt nicotine in tobacco aerosol increases volatility, oral and upper airwaydeposition, stimulation of pharynx nicotinic receptors, and sensations of harshness.5 Long-termsmokers may be familiar with such effects due to experience inhaling cigarette smoke, which mightexplain why the relative differences in perceived harshness and smoothness between salt and free-base formulations were less pronounced in ever vs never smokers in this study.

The health implications of vaping vary by age and smoking status. For older adult smokers whoare unable to quit smoking, having nicotine salt e-cigarettes on the market might be advantageousif the sensory properties of these products facilitate transition from cigarettes to e-cigarettes.32 Bothever and never smokers in this study found the nicotine salt products significantly more appealingthan free-base nicotine products, although young adult never smokers were more sensitive to theharshness-reducing effects of nicotine salt formulations than ever smokers. For never smokers andyoung people, having palatable and smooth nicotine salt e-cigarettes on the market that encouragechronic vaping patterns might be disadvantageous. Risks from long-term e-cigarette use includeexposure to respiratory and cardiovascular toxins, potential for disrupted growth of brain pathwaysunderlying mood and attention regulation, and nicotine dependence.32,33 Regulations reducing the

Figure 4. Mean (SE) Appeal and Sensory Attribute Ratings of Electronic Cigarettes, by Nicotine Formulation in Ever and Never Smokers

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NS ES NS ES NS ES NS ES NS ES

Free-base formulation

Salt formulation

Appeal Sweetness Smoothness Bitterness Harshness

Sensory attribute

P <.001P <.001

P <.001P <.001

P <.001

P <.001

P <.001P <.001P <.001

P <.001

P = .004a

P = .001a

Never smokers (n = 14): smoked <100 combustible cigarettes in lifetime. Ever smokers (n = 105): smoked �100 combustible cigarettes in lifetime. Appeal refers to the mean of liking,willingness-to-use-again, and disliking (reverse-scored) (range, 0-100) scores. ES indicates ever smoking participants; NS, never smoking participants.a P value for test of interaction between nicotine formulation and ever smoking status.




availability of nicotine salt e-cigarettes may benefit the health of youths and never smokerpopulations who are unaccustomed to inhaling free-base nicotine. Because never smokers andyouths might be deterred by the bitterness and harshness of e-cigarettes with free-base nicotineformulations that lack acid additives, they might be less likely to become long-term users ofe-cigarettes if free-base nicotine products were the only e-cigarettes on the market. Regulatoryagencies could decline to authorize (or unilaterally prohibit) sales of new or existing e-cigaretteproducts that contain benzoic acid or other acid additives demonstrated to change nicotine from freebase to protonated nicotine salt.6

The current findings merit consideration alongside accumulating evidence on the direct biologicaleffects of exposure to protonated (salt) vs unprotonated (free-base) nicotine. Nicotine salt might bemore likely to alter lung epithelium inflammatory responses, which could increase risk of respiratoryillness.34 Protonated nicotine is less likely to diffuse across membranes, which could reduce nicotineabsorption rate and other health effects.34 e-Cigarette devices with nicotine salt vs free base tend tohave lower power output,1,35 which could reduce emissions of toxins caused by overheating that occurswith high-powered devices.32 On the whole, acid additives may increase the appeal and regular use ofe-cigarettes while also having nuanced effects on the inherent harms of e-cigarette exposure.

LimitationsThis study has limitations. First, this study’s methods assess immediate sensory reactions to aerosolin a controlled setting and do not address product appeal due to neuropharmacologically mediatednicotine reinforcement, marketing, and other factors. Second, while nicotine formulation effectslargely generalized across differently flavored products in this study, the extent to which theseresults generalize to other e-cigarette products is unclear, including nicotine salt products with acidsother than benzoic acid. In addition, the ratio of free-base to salt nicotine in e-cigarettes lies on acontinuum,36 and further studies of dose-response effects of free-base to salt nicotine arewarranted. Third, it is unknown whether these results in adults will generalize to adolescents. Last,because there is no consensus on the optimal measurement of e-cigarette product appeal, differentmeasurement strategies and rating scale anchors merit inclusion in further research.

Conclusions

In this randomized clinical trial of adult current nicotine or tobacco product users, controlledexposure to e-cigarette puffs with salt vs free-base nicotine formulations increased product appealand improved the sensory experience of vaping, particularly among never smokers. Regulatorypolicies limiting acid additives in e-cigarettes might reduce the appeal of high-nicotine e-cigarettesamong populations deterred from vaping e-cigarettes that emit harsh aerosol.

ARTICLE INFORMATIONAccepted for Publication: November 13, 2020.

Published: January 12, 2021. doi:10.1001/jamanetworkopen.2020.32757

Open Access: This is an open access article distributed under the terms of the CC-BY-NC-ND License. © 2021Leventhal AM et al. JAMA Network Open.

Corresponding Author: Adam M. Leventhal, PhD, Institute for Addiction Science, University of SouthernCalifornia Keck School of Medicine, 2001 N Soto St, 302-C, Los Angeles, CA 90032 ([email protected]).

Author Affiliations: Department of Preventive Medicine, University of Southern California Keck School ofMedicine, Los Angeles (Leventhal, Peraza, Schiff, Whitted, Barrington-Trimis, Mason, Anderson); Department ofPsychology, University of Southern California, Los Angeles (Leventhal); Institute for Addiction Science, Universityof Southern California, Los Angeles (Leventhal, Madden, Lebovitz, Barrington-Trimis); School of Social Work,University of Southern California, Los Angeles (Madden); Department of Pediatrics, University of Oklahoma,Oklahoma City (Tackett).





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mailto:[email protected]

Author Contributions: Dr Madden had full access to all of the data in the study and takes responsibility for theintegrity of the data and the accuracy of the data analysis.

Concept and design: Leventhal.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Leventhal, Madden, Peraza, Lebovitz, Anderson, Tackett.

Critical revision of the manuscript for important intellectual content: Leventhal, Madden, Schiff, Whitted,Barrington-Trimis, Mason, Anderson, Tackett.

Statistical analysis: Leventhal, Madden, Peraza.

Obtained funding: Leventhal.

Administrative, technical, or material support: Leventhal, Schiff, Whitted.

Supervision: Leventhal, Tackett.

Conflict of Interest Disclosures: None reported.

Funding/Support: This project was supported in part by Tobacco Centers of Regulatory Science (TCORS) awardU54CA180908 (Leventhal) from the National Cancer Institute (NCI) and Food and Drug Administration (FDA) andgrants K24DA048160 (Leventhal) and K01DA04295 (Barrington-Trimis) from the National Institute on DrugAbuse (NIDA).

Role of the Funder/Sponsor: The funding organizations had no role in the design and conduct of the study;collection, management, analysis, and interpretation of the data; preparation, review, or approval of themanuscript; and decision to submit the manuscript for publication.

Disclaimer: The content is solely the responsibility of the authors and does not necessarily represent the officialviews of NCI, NIDA, or FDA.

Data Sharing Statement: See Supplement 3.

Additional Contributions: We thank all who participated in this project and Jon Samet, MD (University ofColorado), for comments to earlier versions of this manuscript. No compensation was received.

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SUPPLEMENT 1.Trial Protocol

SUPPLEMENT 2.eMethodseTable 1. Constituents of e-Cigarette SolutionseTable 2. Effects of Nicotine Formulation on Appeal and Sensory Attributes in Sensitivity AnalyseseTable 3. Effects of Nicotine Formulation in Overall Sample and Stratified by Smoking Use Status and InteractionEffectseTable 4. Effects of Nicotine Formulation in Current Smokers without Significant Previous Experience Usinge-CigaretteseTable 5. Effects of Nicotine Formulation With and Without Adjusting for Nicotine concentrationeTable 6. Effects of Nicotine Formulation in e-Cigarette Users, Stratified by e-Cigarette Device Type UsedeReferences

SUPPLEMENT 3.Data Sharing Statement




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