Electronic cigarette liquid and device parameters and aerosol characteristics: A survey of regular users
Introduction
Electronic nicotine delivery systems, and specifically ‘electronic cigarettes’ (ECs), represent a varied group of devices with user adjustable parameters, including heating element variables (such as heater coil configurations and power settings), and concentrations of nicotine, a variety of flavorants, and other ingredients in liquids. Early ECs (i.e. ‘first-generation’ ECs) were relatively simple devices that restricted the user from altering power levels and heater coil configurations, and typically limited liquids to only those sold for a specific device. ECs evolved from these simpler first-generation devices to more sophisticated 2nd and 3rd generation devices. Key changes include heating element control by specifying wattage, voltage, or temperature, adjustable heater coil configurations (e.g. number, diameter, length, and materials of coils), and refillable tanks that allow the user to control liquid composition. Because ECs have many variables, it is necessary to understand how regular EC users utilize their devices and their parameters to construct relevant regulatory policy or maximize efficacy of ECs for smoking cessation.
One key function of ECs is to deliver nicotine, which is influenced by user-adjustable parameters. Previous laboratory research has demonstrated that concentration of nicotine in EC liquids can have a significant impact on the EC-using experience and the composition of the aerosols they produce. Expectedly, increases in liquid nicotine concentration have been shown to increase concentration of nicotine in EC aerosols (Talih, Balhas, Eissenberg, et al., 2015) and plasma nicotine levels following EC use, with plasma nicotine levels following high nicotine concentrations (i.e. 3.6% nicotine) exceeding levels typically achieved from conventional tobacco cigarettes (CC) (Ramôa, Hiler, Spindle, et al., 2016). The nicotine concentration in EC liquids has also been demonstrated to have a negative relationship with puff topography, such that EC users will take shorter duration puffs from ECs containing higher nicotine concentrations compared to lower concentrations (Lopez, Hiler, Soule, et al., 2016). Lastly, previous research showing that liquids with greater proportions of propylene glycol (PG), relative to vegetable glycerin (VG), increases nicotine concentration in EC aerosols, which may suggest that PG/VG concentrations could alter plasma nicotine concentrations following EC use (Talih, Balhas, Salman, et al., 2016).
It has also been suggested that EC device parameters affect the level of nicotine emitted by ECs. Shihadeh and Eissenberg (2015) provide a framework for understanding the level of nicotine emitted by ECs, emphasizing that many different EC variables impact nicotine emission. Variables in this framework include: liquid container design (e.g. cartomizer, tank, drip tip, disposable), heating element parameters (electrical resistance, voltage, and surface area of the heating coil), liquid variables (nicotine concentration, solvent composition, flavoring, and other additives), and usage behaviors (puff duration, inter-puff interval, and number of puffs). Since these variables likely affect nicotine delivery, they also likely affect the abuse liability of ECs. Additionally, these variables can influence the potential harm of ECs. For example, voltage level (directly associated with wattage when coil resistance is held constant), is associated with emissions of carcinogens from ECs (Kosmider, Sobczak, Fik, et al., 2014). Other research has demonstrated that constituents in EC liquids – namely PG and VG – can degrade into carcinogens when used in an EC. Yet, whether regular EC users typically use ECs in a manner that would lead to high levels of carcinogen emission is not well studied.
It is important to know what factors motivate the initiation and continued use of ECs in order to more completely understand their abuse liability and efficacy as potential smoking cessation tools. Farsalinos, Romagna, Tsiapras, et al. (2013) reported that the majority of EC users in their sample use multiple different flavors within a day, and that the EC would be ‘less enjoyable’ if flavor availability were limited.Therefore, taste might drive both the initiation and continuation of EC use. It is unclear from these data, however, what the relative contribution of taste is compared to other possible motivations for initiation and continuation of EC use. Users of ECs have also identified “throat hit” as an important stimulus characteristic, with reports suggesting that liquid and device parameters (e.g. use of higher liquid nicotine concentrations) may be associated with this effect (Etter, 2016a), though whether device power (i.e. wattage) is associated with throat hit is not yet known.
In August 2016, a Food and Drug Administration (FDA) rule came into effect that placed ECs under FDA regulation. Now that there is a regulatory mechanism for ECs it is important to study usage patterns of regular EC users. Usage patterns of regular EC users may, for example, highlight common behaviors that put individuals at undue health risk, which would provide an important regulatory target. Knowing common EC settings and usage behaviors will also inform future research, such that studies can utilize the most common EC user settings to determine if these settings confer significant health risks to the user. The extant literature is lacking estimates from a broad sample of regular EC users on key variables that likely influence the reinforcing effects of ECs, their potential harms, and motivations for initiation and continued use of ECs. This poses several problems: 1) it is difficult to conduct generalizable research on ECs without knowing what adjustable parameter settings are commonly used, 2) it is difficult to assess health risks associated with ECs without knowing whether settings that could cause harmful effects are commonly used, and 3) it is difficult to employ meaningful regulation without first understanding how ECs are being used. To address this gap in the extant literature, a survey of regular nicotine-dispensing EC users in the United States was conducted. The primary goal of the survey was to determine how adjustable parameters are typically used, the factors that motivate EC liquid selection, and the factors that influence initiation and continued EC use. Also examined was the association between nicotine dependence and (1) frequency of EC use (2), adjustable settings, and (3) liquid preferences. Results of this survey will support the translation of laboratory research findings to the natural ecology, and to inform EC-related public health policy efforts.
Section snippets
Participants
A non-EC internet marketplace (Amazon's Mechanical Turk [mTurk]) was used to recruit study participants. A listing on the site invited EC users to complete the survey during the spring of 2016. Participants were first asked 10 screening questions to verify EC use and establish eligibility. To complete the full survey, participants were required to: report daily use of a nicotine containing EC throughout the past month, use ECs as their primary means of administering nicotine, have a 95% or
Results
A total of 1586 participants answered screening questions, and the 522 individuals that met inclusion criteria completed the entire survey. Participants were comprised of 290 (55.7%) females with an average age of 33.0 (SD = 10.2). Descriptive statistics are shown in tabular form for usage behaviors and settings (Table 1) and liquid variable preferences (Table 2).
Fifty-nine percent of individuals endorsed currently using at least one other tobacco product in addition to ECs. Endorsement of
Discussion
This study addresses critical gaps in our understanding of EC use by providing detailed information on EC usage behaviors, device parameter settings, and liquid variables among regular (i.e. daily) EC users. EC users utilize a wide variety of devices, power levels (i.e. watts, volts, ohms) and nicotine concentrations. Our data suggest that no predominant combinations of user-adjustable products have emerged from the broad range of commercial options. Consistent with previous findings (
Role of funding sources
Funding for this study was provided by the University of Kentucky and the National Institute on Drug Abuse (T32DA035200). These funding sources had no role in the study design, collection, analysis or interpretation of the data, writing the manuscript, or the decision to submit the paper for publication.
Contributors
Mr. Harvanko designed the study and wrote the protocol, collected study data, conducted the statistical analyses, and prepared the first draft of the manuscript. Drs. Kelly and Ashford and Mrs. McCubbin assisted in designing the study protocol and contributed to, and have approved, the final manuscript.
Conflict of interest
All authors declare that they have no conflicts of interest.
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