Waterpipe Size Matters: The Effect of Waterpipe Size on Toxicant Exposures and Subjective Experiences

Abstract

Introduction

Waterpipe (WP) tobacco smoking remains popular among US young adults, yet specific regulations for WP devices are lacking. This study examines how WP device size (base volume and height) affects smoking behavior, toxicant exposures, and subjective experiences.

Aims and Methods

Thirty-eight individuals who smoke WP aged 21–39 participated in a randomized crossover study. Each completed three 45-min ad libitum smoking sessions using small, medium, and large WP sizes. Saliva nicotine and exhaled carbon monoxide (eCO) were measured before and after smoking. Puff topography was recorded during sessions, and subjective smoking experiences were collected afterwards.

Results

Significant differences were observed in saliva nicotine between the three WP sizes, with higher concentrations for the small WP relative to larger sizes (p < .05). Exhaled CO was significantly higher after smoking the large WP compared to smaller sizes (p < .05). Puff topography revealed significant differences and followed a dose-response pattern, with larger size associated with longer puff duration, greater smoke volume inhaled, and larger puff volume (p < .05). The large WP was associated with enhanced subjective experiences for satisfaction and concentration compared to the small WP (p < .05).

Conclusions

WP size substantially impacts smoking experiences and exposures. Regulation of WP size should be essential in comprehensive tobacco control policies aimed at curbing WP use among young adults. These findings will aid in the developing of WP size-specific standards to regulate the marketing and sales of WP devices. Such regulations should aim to minimize toxicant exposure by controlling the dimensions and design of WP components, which can significantly influence smoking behavior and toxicant intake.

Implications

This study underscores the critical role of waterpipe (WP) size in shaping smoking behavior, experiences, and exposures among young adults.

Introduction

Waterpipe (WP) smoking, commonly known as hookah or shisha, is a traditional method of tobacco consumption that has regained popularity in modern times, especially among young adults.¹ In 2023, an estimated 290 000 high school and middle school students in the United States were individuals who were currently smoking WP tobacco (past 30-day use).² According to the 2021 National Health Interview Survey, the highest prevalence of current WP tobacco use (including regular pipes, WPs, or hookahs) is observed among adults aged 18–44 years, with 1.5% for those aged 18–24 and 1.6% for those aged 25–44, compared to older adults.³

Evidence has shown that WP smoking can lead to nicotine dependence, increased risk of disease transmission and many known smoking-related diseases such as lung or oral cancer and cardiopulmonary diseases.^4,5 In addition, there are widespread misconceptions regarding the perception of harm attributed to WP smoking. For example, WP smoking has been considered less harmful and less addictive than cigarette smoking by people with and without WP experience, yet WP often carries many of the same health risks.⁶

The negative health effects and widespread misconceptions about WP smoking have driven the Food and Drug Administration to regulate the manufacturing, sale, and distribution of WP products, including WP components and their various designs.⁷ However, regulation and policy specific to WP remains limited both in the United States and worldwide with most countries relying on generic tobacco control policies that cover all tobacco products.⁸ The lack of regulation is compounded by the current proliferation of various configurations of WP components (eg size, designs, and accessories), which can affect smoking behavior, toxicant exposure, and harm perception.

Typically, WP devices include several key components that influence smoking behavior. The head bowl holds the flavored tobacco, the stem connects the bowl to the base, the hose allows for inhalation, and the base is a water-filled chamber that cools the smoke. However, WP devices vary greatly in size, impacting smoking behavior and exposure levels.

Unlike standardized cigarettes, WPs can differ by over 20 cm in height and hundreds of milliliters in bowl volume.⁹ Moreover, the social café environment, where WP smoking is communal and prolonged, complicates this further. The largely unregulated WP market leads to diverse devices, influencing user behavior and health outcomes. For example, many online WP retailers advertise small WP (a.k.a., mini hookahs) as “best hookahs for beginners,” and “an excellent choice for a first hookah” which makes them appear as perfect “starter sizes” for otherwise WP-naïve individuals.¹⁰

Previous research by Hauser and colleagues (2020) demonstrated that smaller WPs generate higher concentrations of smaller, more harmful particles, highlighting the critical role of WP size in smoke toxicity.⁹ While this study provides important insights, these assessments have not yet been implemented in clinical behavioral studies and are currently limited to machine smoking.

Using a clinical laboratory model, our study aims to fill the gaps in understanding the effects of various WP sizes on smoking behavior, toxicant exposures, harm perception, and subjective experiences among young adults who smoke WP. We hypothesize that smoking a larger WP will lead to more pronounced effects on puffing parameters, exposure to carbon monoxide (CO) and nicotine, harm perception, satisfaction, and craving compared to smoking a smaller WP. Our findings will inform regulatory entities on the potential impact of WP specific-regulation and their expected effect on limiting toxicant exposures. Furthermore, data generated from this study can help in the development of standardized WP measures, improving smoking cessation interventions by enabling consistent quantification of exposure.

Materials and Methods

Design

This study was a 2X3 crossover within-subject design, where the within-subject factors were time (pre- and post-WP smoking) and study condition (small- vs. medium- vs. large-WP sizes). Each participant underwent three independent smoking sessions that differed by WP size. The total volumes of the glass base, stem height and overall height for each WP were respectively as follows: large (1750 ml; 86 cm; 98 cm), medium (680 ml; 45 cm; 57 cm), and small (600 ml; 23 cm; 30 cm) as illustrated in Figure 1. The selection of WP sizes and dimensions was informed by the prevailing commercially available range, representing the most common sizes found in the market. This ensures the study’s alignment with real-world conditions and enhances its overall relevance.

Figure 1.

Three WP study conditions (large, medium, and small WP sizes).

Participants

A total of 38 participants were recruited from the University of Texas at Arlington and the surrounding area by distributing flyers across the campus and at hookah cafes and lounges in proximity to the university and via word-of-mouth. The participants in the study were generally healthy adults aged 21–39 years, reported regular WP use (smoked WP at least once a week for the past 6 months) and provided written informed consent for participation in the study. Exclusion criteria included: (1) history of chronic diseases; (2) regular use of prescription medications (other than vitamins or birth control); (3) breastfeeding or testing positive for pregnancy (confirmed by urine testing) at screening; and (4) regular use (≥5 times/mo) of other tobacco products (e-cigarettes, cigarettes, cigars, smokeless tobacco, and other). Data collection occurred from April 2022 to August 2023.

Definition of Regular Waterpipe Smoking

Regular WP smoking was defined using a structured set of questions. Participants were first asked if they smoked hookah (Yes/No). If they responded “Yes,” they were then asked when they started smoking hookah (less than 6 months ago, at least 6 months ago but less than a year, or at least a year ago). Participants who indicated they had been smoking for at least 6 months were then asked about their smoking frequency over the past 6 months (less than once a week, at least once a week but not every day, every day or on most days, or not at all in the past 6 months). Participants who reported smoking at least once a week in the past 6 months were classified as individuals who smoke WP regular regularly.^11,12

Procedure

Eligible participants completed three 2-h visits (which encompassed all survey completion, study measurements, and the smoking session) that differed by WP size (ranging from small-, medium-, and large-WP). The sessions were separated by a 48-h washout period, and the session order was counterbalanced using the Latin Square Method¹³ to account for the first-order carryover effect between the study conditions. Participants were asked to abstain from any tobacco use ≥12 h prior to the study visit. Abstinence was confirmed by breath eCO < 5 ppm.¹² WP tobacco brands and flavors were standardized to Al Fakher Double Apple since it is considered one of the most popular WP tobacco brands and flavors on the market.^14,15 A glazed ceramic bowl (7.6 cm diameter) with five holes in the base was packed with 15 g of flavored tobacco and subsequently enclosed with a perforated foil covering the top of the bowl.¹² One disk of quick-light charcoal (Starbuzz), which is among the most commonly used commercially and within various analytical and clinical studies due to user preference and convenience,^11,16–18 was placed on top of the packed WP bowl.

Prior to each session, the WP glass base was filled with water, maintaining 2.5 cm of WP down-stem immersion,¹⁹ which is ideal for sustaining unrestricted airflow.²⁰ Participants sat in a recliner chair and were allowed to smoke the WP ad libitum, for up to a 45-min maximum duration. During the smoking session, puff topography was recorded. Prior to and immediately following each WP session, 2 mL of saliva was collected, exhaled CO (eCO) was measured, and subjective measures were assessed. Throughout the study, tobacco brands and flavor remained constant, and the use of charcoal was limited to one disc to minimize their impact on study outcomes. Additionally, to ensure standardization, we employed identical rubber seals, head bowls, and disposable hoses, and maintained consistent metal stem insertion depth across all WP sizes.

Measures

Demographics

Participants completed questionnaires assessing age (years), sex (male vs. female), race (White, Asian, Middle Eastern/North African, Black or African American), ethnicity (Hispanic vs. non-Hispanic), highest degree achieved, and student/employment status.

WP smoking frequency and patterns

Participants self-reported their WP smoking frequency by responding to the following question: “On average how many hookahs (heads/bowels) do you usually smoke per month?.” Participants self-reported their WP smoking patterns by responding to the following questions: “At what age did you start smoking hookah?,” “What is the average time you usually spend during a hookah smoking session?” (“less than 30 min,” “30–60 min,” or “more than 60 min”), “Do you usually share the same hookah with others?” (“yes” or “no”), and “Where do you usually smoke hookah?” (“at home,” “at a friend’s place,” or “at a public place”). To assess WP size preference, participants were asked “What size of hookah do you usually use for smoking?” (“small,” “medium” or ‘large’). These measures were collected at 1^st visit before the start of the smoking session.

Other tobacco product use

Participants self-reported use of any tobacco/nicotine products (≥5 times/mo) by answering the following question: “Do you regularly use any of the following tobacco/nicotine products?: electronic cigarettes, cigarette, cigars, cigarillos/little cigars, smokeless tobacco, other, and none.” If they answered ‘Yes,’ they were further asked to describe their usage over the past year with the following options: 1–5 times/mo, 6–10 times/mo, or more than 10 times/mo. This measure was collected at first visit before the start of the smoking session.

WP puff topography

Puff topography was measured using a validated instrument from the American University of Beirut.¹¹ This device converts pressure signals to data.²¹ Measures collected included total smoking time, cumulative puff time, puff duration, average flow rate, number of puffs, total volume inhaled, average puff volume, and maximum puff volume.

Saliva nicotine

Lab staff collected approximately 2 mL of saliva via using a Quantisal disposable collection kit before and immediately after the completion of each smoking session. Saliva samples were stored at –80°C and analyzed using liquid chromatography–tandem mass spectrometry (LC–MS/MS). The assay’s limit of detection was 0.2 ng/mL and the limit of quantitation was 0.7 ng/mL.²²

Exhaled carbon monoxide (eCO)

ECO levels were measured before, and within 10 min after each WP smoking session via Breath-CO monitor (Vitalograph, Lenexa, Kansas, United States).¹²

Subjective measures

This study utilized three subjective measures. The selection of these measures was driven by the suitability for our study’s aims and prior use in similar studies assessing WP tobacco products.^12,18,23 Participants used a computer tablet to respond to these measures for each study condition using the QuestionPro software. Individual items for each measure are summarized below.

The WP Evaluation Scale (WES).

The WP Evaluation Scale (WES) is an 11-item questionnaire originally intended for cigarette assessment²⁴ and modified for the WP.^11,12,25 This scale assesses participants’ perception of the smoked WP for (1) “Satisfying”; (2) “Tastes good”; (3) “Makes you dizzy”; (4) “Calms you down”; (5) “Makes you concentrate”; (6) “Makes you feel more awake”; (7) “Reduces hunger for food”; (8) “Makes you nauseous”; (9) “Makes you feel less irritable”; (10) “Enjoyment of the sensations of the smoke in your throat and chest”; and (11) “Immediate reduction of your craving for smoking.” WES was answered on a 7-point Likert scale anchored at the extremes (1 = not at all; 7 = extremely).²⁵ The WES was administered after each smoking session.

The Duke Sensory Questionnaire (DSQ).

The DSQ consisted of nine items: (1) “How much did you like the puffs?”; (2) “How satisfying were the puffs?”; (3) “How high in nicotine were the puffs?”; (4) “How similar to your usual hookah size were the puffs from this hookah?”; and “Rate the strength of the puffs on…” (5) “…tongue,” (6) “…nose,” (7) “…back of mouth and throat,” (8) “…windpipe” and (9) “…chest.” All questions were answered on a 7-point Likert scale similar to the WES. The DSQ was administered after each WP smoking session.²⁶

Harm Perceptions.

Participants were assessed regarding WP size-specific harm perception, using a question adapted from Popova & Ling, “Compared to your usual hookah size, this hookah size you just smoked is…” with answers on a 7-point scale ranging from one (“a lot less harmful”) to four (“equally as harmful”) to seven (“a lot more harmful”).²⁷ Similarly, participants were asked to assess the perceived harm of “hookah” and “smoking cigarettes” to their health, following the same 7-point scale. These questions were administered after each WP smoking session. At the first visit, before the start of the smoking session, participants were asked about the WP size that they thought “produced the most smoke” and the size considered “worse for health.”

Data Preparation and Analysis

Descriptive statistics of the study sample were summarized with means (standard deviation [SD]) or proportions. Means (SD) and frequencies were calculated for eCO, saliva nicotine concentration, topography, and subjective measures and compared by WP size using two-tailed paired samples t-tests or chi-square as appropriate. We also reported the standard error of the mean (SEM) as appropriate. Consumed tobacco for each WP size was calculated by subtracting the remaining tobacco weight after each smoking session from the baseline tobacco weight (15 g). Data for eCO measures were missing for one participant, and data for saliva nicotine samples were missing for two participants due to technical difficulties. Accordingly, analyses for eCO outcomes were based on 37 participants, and saliva nicotine outcomes were based on 36 participants. In order to compare the effects of WP size on saliva nicotine and eCO concentration, data were entered into a repeated measures ANOVA with two within-subject factors: time (pre-WP and post-WP) and WP size (small, medium, and large). Analyses controlled for age and sex. Huynh–Feldt corrections were used to adjust for violations of the sphericity assumption. The significance level was set at α  = .05. All analyses were performed using IBM SPSS V.29.

Results

Participants had a mean age of 24.9 (SD = 3.15) years, with the majority being men (n = 28; 73.7%). The mean age for first starting WP smoking among participants was 19.1 (SD = 2.37) years. Full details about the baseline characteristics of the study participants are presented in Table 1. There was a significant difference in the amount of consumed tobacco following each WP smoking session, with the large WP being associated with a lesser amount of tobacco consumed (3.71 g) compared with the medium- (4.32 g) and small-WP sizes (5.44 g) (p < .05).

Table 1.

Baseline characteristics for the overall sample (n = 38).

Variables	Overall sample
Gender (%)
Female	26.3
Male	73.7
Race (%)
White	5.3
Asian	69.4
Middle Eastern/North African	16.7
Black or African American	13.8
Hispanic (%)	7.9
Age (in y)a	24.9 (3.15)
Age of first smoking waterpipea	19.1 (2.37)
Student status (%)	81.6
Waterpipe bowls/moa	6.0 (4.77)
Average time spent smoking waterpipe (%)
Less than 30 min	7.9
30–60 min	55.3
More than 60 min	36.8
Waterpipe size usually used (%)
Small	13.2
Medium	55.3
Large	31.6
Waterpipe size thought to produce the most smoke (%)
Small	5.3
Medium	36.8
Large	57.9
Waterpipe size thought to be worse for health (%)b
Small	19.2
Medium	15.4
Large	57.7
Sharing waterpipe with others (%)	97.4
Location of waterpipe smoking (%)
At home	36.8
At a friends’ place	23.7
At a public place	39.5
Intention to quit waterpipe smoking (%)
No	44.7
In the next month	10.5
In the next 6 mo	7.9
In the future	36.8
eCO (ppm)	1.62

eCO, exhaled carbon monoxide (parts per million).

^aData presented as mean (SD).

^b n = 26.

WP Smoking Topography

Significant differences were observed among the large, medium, and small WP sizes, displaying a dose–response pattern with larger WPs associated with greater measures of smoking topography. These measures included cumulative puff time, puff duration, average flow rate, number of puffs, total volume, average puff volume, and maximum puff volume (Table 2). Specifically, the large WP was associated with longer cumulative puff time and puff duration (6.50 vs. 5.09 vs. 5.14 min and 2.48 vs. 2.20 vs. 2.18 sec, respectively; p < .05). A larger average flow rate and total volume were observed for the large WP (18.40 vs. 16.38 vs. 14.78 and 117.69 vs. 79.73 vs. 77.92 L, respectively; p < .05). Larger average puff volume and maximum puff volume were also seen in the large WP (0.78 vs. 0.63 vs. 0.57 L and 1.63 vs. 1.30 vs. 1.19 L, respectively; p < .05). The number of puffs, though not significantly different, followed a similar dose-response pattern with a larger WP size associated with a greater value (186.82 vs. 165.11 vs. 164.35, respectively; p > .05).

Table 2.

Waterpipe smoking topography measures by size (n = 38).

Topography parameters	Small		Medium		Large
	Mean	SD	Mean	SD	Mean	SD
Smoking time, min	38:17a	05:35	35:29	07:06	36:23	07:07
Cumulative puff time, min	05:14	02:40	05:09c	03:00	06:50b	04:03
Puff duration, s	2.18	1.10	2.20c	1.06	2.48b	0.98
Average flow rate	14.78a	6.06	16.38c	6.03	18.40b	7.23
Number of puffs	164.35	91.66	165.11	125.08	186.82	140.90
Total volume, L	77.92	51.80	79.73c	49.35	117.69b	64.46
Puff volume, L	0.57	0.40	0.63c	0.46	0.78b	0.42
Maximum puff volume, L	1.19	0.49	1.30c	0.61	1.63b	0.63

^aSmall versus medium (p < .05).

^bSmall versus large (p < .05).

^cMedium versus large (p < .05).

Saliva Nicotine

There was a significant time-by-condition interaction for mean saliva nicotine concentration (F (1,33) = 29.21, p < .05). This demonstrated that changes in saliva nicotine concentration across time did depend on WP size. Mean pre-WP smoking saliva nicotine concentration for the small WP was 7.8 ng/mL (SEM = 3.30) and increased to 1066 ng/mL (SEM = 253) post-WP smoking resulting in a nicotine boost of 1058 ng/mL. For the medium WP, mean pre-WP smoking saliva nicotine concentration was 8.1 ng/mL (SEM = 4.3) and increased to 576 ng/mL (SEM = 120) post-WP smoking resulting in a nicotine boost of 568 ng/mL. For the large WP, mean pre-WP smoking saliva nicotine concentration was 11 ng/mL (SEM = 4.4) and increased to 688 ng/mL (SEM = 124) post-WP smoking resulting in a nicotine boost of 677 ng/mL. Average saliva nicotine concentration at post-WP smoking was significantly higher for the small WP size relative to the medium- and large-WP sizes (p < .05)

Exhaled Carbon Monoxide

A significant time-by-condition interaction was observed for eCO level (F (1, 34) = 71.88, p < .05) demonstrating that changes in eCO level across time depended on WP size. The mean eCO level for the small WP increased from 1.65 ppm (SEM = 0.26) pre-WP smoking to 71.59 ppm (SEM = 9.90) post-WP smoking with an overall change of 69.95 ppm. For the medium WP, the mean eCO level increased from 1.62 ppm (SEM = 0.22) pre-WP smoking to 64.65 ppm (SEM = 8.61) post-WP smoking with an overall change of 63.03 ppm. For the large WP, the mean eCO level increased from 1.61 ppm (SEM = 0.27) pre-WP smoking to 98.05 ppm (SEM = 12.84) post-WP smoking with an overall change of 96.44 ppm. Mean eCO post-WP smoking was significantly greater for the large WP relative to the small- and medium-WP sizes (p < .05).

Subjective Measures

Figures 2 and 3 depict mean ratings for WES and DSQ, respectively. For the WES, significant differences were observed between the three WP sizes for some indices including “satisfying,” “taste,” “concentrate,” “awake,” and ‘less irritable.’ For each of these items, scores were greater after smoking the large WP (p < .05), except for feeling ‘less irritable’ which was greater after smoking the medium WP (p < .05). For the DSQ, significant differences were observed between the three WP sizes for some indices including “puffs high in nicotine,” “strength of puffs on the throat” and ‘strength of puffs on windpipe’. For all three indices, values were greater after smoking the small WP compared to the medium- and large-WP sizes (p < .05).

Figure 2.

Means (±SEM) for post-WP smoking subjective responses for Waterpipe Evaluation Scale by size condition (n = 38); WP, waterpipe. *indicates small versus medium (p < .05). † small versus large (p < .05). ‡ medium versus large (p < .05).

Figure 3.

Means (±SEM) for post-WP smoking subjective responses for Duke Sensory Questionnaire by size condition (n = 38); WP, waterpipe. *indicates small versus medium (p < .05). † small versus large (p < .05). ‡ medium versus large (p < .05).

Among all WP sizes, there were no significant differences in harm perceptions compared to cigarette, and WP smoking in general (p > .05). Additionally, there were no significant differences between the harm perception of the WP size used in the study compared to the participants’ usual WP size (p > .05). Overall, 57.7% of participants perceived the large WP to be worse for health, whereas 19.2% believed that the small WP was worse for health, and 15.4% believed that the medium WP was worse for health (p < .05). Participants’ evaluations regarding smoke generation across WP sizes indicated that 57.9% favored the large WP as the highest smoke producer, followed by 36.8% for the medium WP, and 5.3% for the small WP (p < .05).

Discussion

This study stands among the first clinical investigations to examine how WP size influences puff topography, toxicant exposures, and subjective responses in young adults who smoke WP. The findings reveal that using a larger WP size leads to higher puff indicators and elevated eCO levels when compared to smaller WP sizes. Similarly, in terms of subjective experience, smoking a larger WP was linked to a more satisfying experience compared to smaller sizes. Conversely, smoking a small WP was associated with a greater increase in saliva nicotine concentration compared to using a larger WP. These results underscore the crucial role of WP device size in shaping subjective experiences and toxicant exposures. Our study extends previous work that assessed different components of WP including heat source and tobacco flavor and additives,²⁸ by specifically focusing on the impact of WP size.

Analysis of puffing behavior revealed a clear trend; larger WP sizes correlated with heightened puffing parameters. Furthermore, this trend in puffing behavior was mirrored in the levels of eCO, with smoking the larger WP resulting in significantly higher eCO levels compared to the medium and small sizes. These findings echo previous studies, underlining that deeper and more frequent puffing translates to notably elevated eCO levels among individuals who smoke WP.¹⁸ Essentially, the size of the WP emerges as a pivotal factor influencing eCO levels, highlighting a direct size correlation where larger WP sizes result in greater levels of eCO, which can cause both acute and chronic poisoning. Documented cases of WP-induced CO poisoning further highlight these risks.^29–31

On average, larger WPs consist of longer metal stems and greater base volumes. Therefore, larger WPs require deeper and more forceful inhalation to draw smoke through the longer stem and hose. This vigorous inhalation increases the volume of air and smoke mixture inhaled, which subsequently elevates the amount of eCO and other combustion-related products. Additionally, the larger bowl volume in these WPs holds more smoke, necessitating deeper inhalations to clear the bowl effectively. This increased volume and the need for deeper inhalation lead to greater CO intake and more pronounced inhalation efforts. These mechanical factors combined explain the higher eCO levels observed in users of larger WPs. These findings underscore the critical need for further research and regulation to mitigate toxicant exposure related to WP smoking.

In contrast, smoking the small WP was associated with greater concentrations of saliva nicotine compared to the medium- and large-WP sizes. In this study, each WP bowl was carefully prepared with a standardized amount of 15 grams of tobacco. Interestingly, our findings indicated a correlation between saliva nicotine concentration and the amount of tobacco consumed during each WP session. Specifically, smaller WP sizes were linked to higher tobacco consumption, as evidenced by the varying amounts of tobacco consumed across WP sizes (ranging from 5.44 g for small WP, 4.32 g for medium WP, to 3.71 g for large WP). These findings emphasize the need for comprehensive measures to address the impact of WP sizes on tobacco consumption and associated health risks.

Regarding nicotine levels, Hauser et al (2020) discuss how the longer stem and insertion depth in larger WPs can lead to increased evaporation and agglomeration of smaller particles before they reach the water.⁹ This process can result in a lower number of smaller nicotine particles in the smoke that is inhaled. In contrast, smaller WPs, with shorter stems and less complex paths, may retain more small nicotine particles in the smoke. This retention could explain why nicotine levels in our study were observed to be higher in users of smaller WPs compared to larger ones. The mechanical design of smaller WPs also may result in less filtration and agglomeration, leading to higher nicotine retention in the smoke. Smaller WPs are often marketed as beginner-friendly and easy to use. The ease of use and accessibility of smaller WPs attract novice individuals.^10,15 This marketing strategy, combined with the higher nicotine levels produced by smaller WPs as found in our study, could rapidly lead to dependence, especially in young adults who are most likely to use WP. This relationship warrants further investigations into the influence of WP size on nicotine dependence and withdrawal symptoms in future studies.

Another interesting finding in our study is how participants perceived risk regarding WP smoking. Prior to the initial smoking session, most participants found the large WP size to be “worst for health” and to “produce the most smoke.” The variation in WP height from 38.5 inches to 12 inches may have influenced harm perception with some participants feeling more intimidated by the large WP, especially with the majority of our sample reporting regular use of the medium WP (55.4%). Ironically, after smoking each condition, participants were more likely to report a preference for the large WP, followed by the medium- and small-WP sizes, for “future use” and “use if the only product in the market.” This finding suggests that the WP size itself—rather than health perceptions—may have a greater influence on smoking behavior and preferences among individuals who use WP. Participants’ subjective experiences also revealed this preference with enhanced subjective measures of satisfaction, taste, concentration, and awareness for the large WP. High rates of satisfaction reflect the reward obtained from smoking that has been associated with experimentation and progression to cigarette regular smoking.³² Additionally, our study shows that smoking the small WP was associated with subjective experiences such as “puffs high in nicotine” and greater “strength of puff” on “throat,” “windpipe,” and “chest.” These experiences may be related to the high consumption of tobacco during the small WP session in comparison with large WP. The increased burning of tobacco is likely to create a poor/burnt taste, higher smoke heat, wispy clouds, and a less satisfactory experience.³³

Although our study addresses important gaps in the WP smoking literature, our results should be considered in the context of some limitations. First, this study is limited in scope, examining a sample of mostly college students; nevertheless, WP smoking is very common among this group.³⁴ Furthermore, the laboratory environment unavoidably differs from the typical WP smoking atmosphere. To minimize this, our laboratory mimicked a WP café environment with the furnishing of a reclining chair and allowance of participants to use their cellphones if they wish to facilitate a more natural smoking experience. Moreover, due to the limited sample size, our ability to control for potential covariates was constrained; these factors can be more thoroughly assessed in larger epidemiological studies. In addition, it is crucial to acknowledge the extensive diversity among WP devices available in the market, even within the same size category. Nevertheless, the anticipated variability within devices of the same size category is expected to be overshadowed by more pronounced differences between various size categories, such as large, medium and small WPs.

In summary, we demonstrated that using the larger WP was linked to more pronounced puffing behavior, higher levels of CO exposure, and enhanced subjective experience. In contrast, the smaller WP size resulted in the highest saliva nicotine exposure. Our study represents a crucial first step in informing the development of WP size-specific standards to regulate the marketing and sales of WP devices in the US and beyond. The World Health Organization advisory note on WP tobacco smoking has highlighted the regulatory needs based on the variant WP components.³⁵ Our findings emphasize the significant impact of WP size on smoking behavior and toxicant exposure, underscoring the broader implications for global tobacco control efforts.

Future studies should expand our findings to include detailed analyses of toxicant yield and emissions, including carcinogenic polycyclic aromatic hydrocarbons and pulmonary disease-causing volatile aldehydes, based on various WP size configurations and other components such as the heat source (eg various types of commercially available charcoal and electrical heating elements). Such in-depth analysis is essential for developing WP-specific standards that minimize toxicant exposure by controlling the dimensions and configurations of WP components to protect public health.

Author contributions

Danny Dabroy (Conceptualization [Equal], Data curation [Equal], Formal analysis [Equal], Investigation [Equal], Methodology [Equal], Resources [Equal], Software [Equal], Writing—original draft [Equal], Writing—review & editing [Equal]), Steven Barrientos (Conceptualization [Equal], Data curation [Equal], Formal analysis [Equal], Investigation [Equal], Methodology [Equal], Validation [Equal], Visualization [Equal], Writing—original draft [Equal], Writing—review & editing [Equal]), Mohammad Ebrahimi Kalan (Conceptualization [Equal], Formal analysis [Equal], Investigation [Equal], Methodology [Equal], Validation [Equal], Writing—original draft [Equal], Writing—review & editing [Equal]), Ahmed Alarabi (Conceptualization [Equal], Data curation [Equal], Formal analysis [Equal], Methodology [Equal], Validation [Equal], Writing—review & editing [Equal]), Kevin Schug (Data curation [Equal], Formal analysis [Equal], Methodology [Equal], Validation [Equal], Writing—review & editing [Equal]), Roman Goff (Data curation [Equal], Formal analysis [Equal], Investigation [Equal], Resources [Equal], Validation [Equal]), Sabrina Islam (Data curation [Equal], Formal analysis [Equal], Investigation [Equal], Resources [Equal], Validation [Equal]), Caroline Cobb (Conceptualization [Equal], Data curation [Equal], Formal analysis [Equal], Methodology [Equal], Validation [Equal], Writing—review & editing [Equal]), and Ziyad Ben Taleb (Conceptualization [Equal], Funding acquisition [Equal], Investigation [Equal], Methodology [Equal], Project administration [Equal], Resources [Equal], Supervision [Equal], Validation [Equal], Writing—original draft [Equal], Writing—review & editing [Equal])

Funding

This study is funded by the National Institute on Drug Abuse of the National Institute of Health (1R03DA054417).

Declaration of Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

Data available upon request—The data underlying this article will be shared upon reasonable request to the corresponding author.