Nicotine

Table 1 summarises the current research regarding the nicotine levels found in e-cigarette cartridges, refill solutions, aerosols and environmental emissions. Nicotine is the primary addictive substance in tobacco products. The data indicate that the nicotine levels in e-cigarettes vary considerably. e-cigarette brands and models differ in the efficacy and consistency of nicotine yields, and the delivery of nicotine is not uniform either from puff-to-puff or across products of the same brand.4 ,6–11 Furthermore, the level of nicotine listed on the labels of e-cigarette cartridges and refill solutions is often significantly different from measured values4 ,7 ,8 ,12–18 and labelling may not adequately convey the amount or concentration of nicotine.19 For example, Goniewicz et al conducted a quantitative analysis of nicotine in aerosols generated from 15 e-cigarette brands (16 products) that were selected based on their market popularity. They found that total nicotine in aerosol varied by brand from 0.5 to 15.4 mg per 300 puffs (20 series of 15 puffs, 70 mL/puff, triplicate tests of each product) and that the nicotine in aerosol varied from 21% to 85% of the nicotine present in the cartridge. Westenberger repeatedly tested three individual cartridges with the same label and obtained results varying from 26.8 to 43.2 μg nicotine per 100 mL puff (estimated to be 8.04–13.0 mg nicotine per 300 puffs). Consequently, environmental nicotine emissions from e-cigarettes differ across brands. For example, McAuley studied nicotine emissions from aerosols of four different high-nicotine content e-liquids in cartridges and found 538–8770 ng/L of nicotine in indoor air compared with 5039 to 48050 ng/L from conventional cigarette smoke.11

Given these issues with nicotine content variability, all studies recommend that e-cigarette manufacturers implement quality standards regarding nicotine content.

Other chemical substances

Quantitative and qualitative studies have identified a wide variety of chemical components in the cartridges, refill solutions and aerosols of e-cigarettes. Tables 2–7 summarise the chemical substances that have been detected and/or quantified in e-cigarette refill solutions, cartridges and aerosols. Substances identified include tobacco-specific nitrosamines (TSNAs),9 ,20–22 aldehydes,20 ,22–25 metals,20 ,22 ,26 volatile organic compounds (VOCs),6 ,20 ,22 ,27 phenolic compounds,22 polycyclic aromatic hydrocarbons (PAHs),22 ,28 flavours,6 ,22 solvent carriers,6 ,22 ,27 tobacco alkaloids,7 ,9 ,10 ,13 and drugs (amino-tadalafil and rimonabant).18

These TSNAs, aldehydes, metals, VOCs, phenolic compounds, PAHs and tobacco alkaloids are harmful or potentially harmful constituents released during the smoking of conventional cigarettes, and their public health risks have been the focus of many studies. In contrast, e-cigarettes use solvent carriers, such as propylene glycol and glycerol, as humectants in e-cigarette solutions to produce aerosols that simulate conventional cigarette smoke. These humectants are oxidised to form the same aldehydes found in conventional cigarette smoke when a heating voltage greater than 3 V is used during the aerosol generation process.23 ,25 ,26 The data reported by Goniewicz et al indicate that e-cigarette brands and product models differ in yields of TSNAs, aldehydes, metals and VOCs.20 For example, the acrolein level in the aerosol generated from two different product models within the same brand is reported to be 4.4±2.5 μg/150 puffs for one model and 16.6±2.5 μg/150 puffs for the other model. An even greater variance in the acrolein level is observed when comparing products across brands, where an acrolein level was determined to be as large as 41.9±3.4 μg/150 puffs in aerosol of a product from a different brand. The relative standard deviations (SDs) reported for all measurements range from 0% to 100% of the mean values, indicating inconsistencies in the release of these chemicals across products. Similarly, in 2013 Etter et al found that e-cigarette sub-brands differ in levels of tobacco alkaloids.13 Within a brand, nicotine-N-oxide (one of the tobacco alkaloids) is at 0.16% (of nicotine content) in a tobacco-flavoured sub-brand, 0.09% in a menthol-flavoured sub-brand and 0.03% in an unflavoured sub-brand.

Nevertheless, analytical methods applied in these studies are inconsistent. Tables 8 and 9 summarise the instrumental methods developed for specific categories of target analytes by each study. Analytical methodology for qualitative and/or quantitative determination of a constituent in cigarette smoke generally encompasses two areas of effort: sample preparation and instrumental analysis. Sample preparation involves smoke/aerosol generation, sample extraction and sample collection. Instrumental analysis involves analysing the sample to identify and quantify analytes of interest. The instrument is commonly selected based on the scientific characteristics of the target analyte, the applicable features of the instrument and the instrument accessibility.

Taking instrumental TSNA analysis as an example, ultra-performance liquid chromatography coupled with mass spectrometry (LCMS) was used in Goniewicz's research,20 whereas high-performance liquid chromatography coupled with tandem mass spectrometry (LCMSMS) was used in Kim's study.21 The two instruments differ in performance characteristics such as detection limit, resolution and linear dynamic range.

By instrumental design, LCMSMS uses two mass spectrometers connected in series, as opposed to one mass spectrometer for LCMS. LCMSMS provides increased sensitivity and resolution when detecting an analyte in a complex sample matrix, such as smoke, by mass filtering and fragmentation pattern recognition of a specific target mass that undergoes inert gas fragmentation in the collision chamber. This also results in LCMSMS having a wider linear dynamic range than LCMS. These advantages of LCMSMS are beneficial to the analysis of a cigarette smoke/aerosol sample because the target analyte levels are frequently near the limit of quantitation for LCMS, and the complex sample matrix is not well resolved by LCMS unless additional sample clean-up is completed before analysis. Such differences are expected to contribute to variations in quantitative and qualitative results.

Another noteworthy concern is that many of the analytical methods used in these studies have not been validated. Method validation is generally an important part of good laboratory practice (GLP) and good manufacture practice (GMP). It ensures good science, consistent application of method and comparability of data. Therefore, it would be helpful to develop validated analytical test methods to measure chemicals of interest in e-cigarettes.

Aerosol generation

The sample preparation for analysis of smoke/aerosol involves generation of the smoke/aerosol. The factors influencing e-cigarette aerosol generation include human use topography, machine aerosol generation parameters and aerosol generation techniques. Human use topography is important in determining true levels of human exposure to constituents in e-cigarettes. Smoking machine parameters for laboratory studies are important in understanding the way that constituent yields delivered by a tobacco product can change over a range of different smoking conditions. Studies of aerosol generation characteristics reveal that, compared with conventional cigarettes, e-cigarettes require a higher airflow rate and longer puff durations to produce aerosols. Furthermore, pressure drop (mm H₂O across e-cigarettes during each puff) varies greatly across cartridges in models across brands and within a brand.1 ,4 ,7 ,12 ,20 Hua et al determined electronic nicotine delivery systems (ENDS) require longer puff durations with large variation (1.9–8.3 s).32 Hua's result is consistent with Farsalinos’ research, which reported puff durations of 4.2±0.7 s.33 The results suggest that standard protocols used to test conventional cigarettes are not necessarily applicable to e-cigarettes. Table 10 presents a side-by-side comparison of e-cigarette and conventional cigarette smoking parameters1 that clearly indicates the remarkable difference between e-cigarettes and conventional cigarettes regarding the vacuum and pump speed applied for aerosol generation in a laboratory-built device. Table 11 summarises the parameters and instrumentation used for aerosol generation in the chemistry studies. The wide range in smoking parameters and instrumentation demonstrate the lack of standardisation in methodologies for aerosol generation used for e-cigarette analyses.

Performance

The performance of e-cigarettes depends fundamentally on the physical and electronic design of the device. Performance properties include airflow, pressure drop, aerosol density, puff strength (ie, vacuum required to produce aerosol) and number of puffs. Studies of e-cigarette devices have found that aerosol generation performance varies by brand and even by product within a specific brand.1 ,4 ,12 Evidence of this is demonstrated by Trtchunian's 2010 study showing that the total puff number ranges from 30±43 for one brand to 313±115 for another brand.1 The relatively high SD (±43 and ±115, respectively) from each mean (30 and 313, respectively) indicates large performance variance by product within a specific brand. Furthermore, in 2011, Williams et al12 recorded aerosol density for every other puff during the first 10 puffs of four e-cigarette models. They observed dissimilarity in aerosol density among models and between two samples of one model. Therefore, variations observed in these properties appear to be associated with both e-cigarette design and e-cigarette product qualities, suggesting a need for improved quality control during manufacture.

Aerosol particle size distribution

Pellegrino et al measured the ultrafine particulate matter (PM) fraction yield of e-cigarettes. Data showed that the PM emission values were lower in e-cigarettes compared with the conventional cigarette (eg, PM₁=14 vs 80 μg/m³ and PM₁₀=52 vs 922 μg/m³; PM₁ and PM₁₀ are particulate matter emission with particle size lower than 1 or 10 μm).6 Ingebrethsen et al discovered that aerosol dilution, which is necessary for detection of aerosol particle size when using an electrical mobility analyser, results in average particle diameters in the 50 nm range, compared with the 250–450 nm range determined by spectral transmission measurement, due partially to aerosol particle evaporation at the dilution levels. The authors suggested measuring e-cigarette aerosols in an undiluted state by using the spectral transmission method.29 A recent in vitro study by Zhang et al demonstrated that under the conditions of a single puff experiment, during which the aerosol does not reach steady state, an e-cigarette generates an aerosol having particle sizes in the range 100–600 nm, which is similar to that of conventional cigarettes.31 The authors noted that the single puff experiment generates much smaller particles than would be expected if the aerosol approached steady state; they also suggested that future studies could assay aerosols generated by human users. In another recent study by Schripp et al, the investigators concluded that the inhaled “aerosol size distribution alters in the human lung and leads to an exhalation of smaller particles. The effect is caused by the evaporation of the liquid particles in the lung and also in the environment after exhalation.”27 Varying results in particle size distributions of PM emissions from e-cigarettes across studies indicate that standardised and valid determination methods are necessary in order to evaluate the real exposure to users and the mechanisms (such as the evaporation of solvents and the agglomeration and deposition of particles) underlying such exposure.