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There is a growing but still limited body of information on the composition of the contents of e-cigarette cartridges, refill fluid and vapor generated by the devices. The evidence suggests wide variation in composition of contents and vapor but to date, no studies indicate evidence that e-cigarettes expose users to concentrations of toxins sufficient to cause harm or no harm. Primary refill fluid ingredients are generally propylene glycol (PG), glycerin and nicotine.

Cahn and Siegel (2011) identified 16 studies that sought to characterize the components in e-cigarette liquid and vapor, and found that the principal components are propylene glycol (PG), glycerin and nicotine. Trehy et al. (2011) found PG and glycerin together comprise about 95% of the volume of e-cigarette liquid.

The degree to which different brands and different cartridges within the same brand of e-cigarettes expose users to measurable levels of nicotine and other compounds varies widely (Westenberger, 2009; Cobb et al., 2010; Goniewicz et al., 2012; Cheng, 2014).

The accuracy of labeling of nicotine content is highly variable (Chiah et al., 2012, Trtchounian et al., 2010). Trehy et al. (2011) found some e-cigarette refill bottles labeled ‘no nicotine’ containing nicotine in significant amounts. Cameron et al. (2013) found the nicotine content of seven e-cigarette nicotine fluids available in the US to be equivalent to or lower than indicated by the manufacturers, and the quality of labeling was widely variable: some were handwritten with no concentration level, warning nor directions for use whereas others were well-sealed with printed labels.

The notation ‘mg/ml’ is often used on labels for the nicotine concentration, and may be sometimes shortened to ‘mg’: the true content is only possible to calculate if the volume of the cartridge or refill bottle is known.

Etter et al. (2013) used gas and liquid chromatography to analyze 20 models of 10 of the most popular brands of refill liquids for nicotine content, content of the known nicotine degradation products and found the nicotine content in the bottles corresponded closely to the labels on the bottles. The levels of nicotine degradation products represented 0–4.4% of those for nicotine, but for most samples the level was 1–2%.

Pharmacologically-active agents may be added to some e-cigarette cartridges or fluid bottles by manufacturers or by users but the extent of this practice is unknown (Hadwiger et al., 2010; Etter & Bullen, 2011). Most e-cigarette fluid contains flavors and fragrances (such as cinnamon, menthol and chocolate) as well as food dyes. With the exception of menthol, found as an additive in most tobacco cigarettes, none of these substances is generally found in products used for inhalation.
E-cigarette vapor contains a number of potentially toxic compounds. Testing on some devices has found tobacco-specific nitrosamines (TSNAs) (Kim & Shin, 2013) and polycyclic aromatic hydrocarbons present in cartridge fluid, but generally in very low levels, similar to those in nicotine replacement therapy (Westenberger, 2009; Laugesen et al., 2009; Goniewicz et al., 2012; Burstyn, 2013).

FDA-commissioned testing of e-cigarette cartridge fluids found diethylene glycol in one of the 18 e-cigarette cartridges tested (Westenberger, 2009). Formaldehyde, acetaldehyde, and acroleine (potentially toxic carbonyl compounds) have been detected in e-cigarette vapor in 12 brands of e-cigarettes but at levels substantially lower than in cigarette smoke. These compounds may be formed by the oxidation of propylene glycol or glycerol when in contact with the heating coil.
Fine and ultrafine particulates were detected in some Italian e-cigarettes by Pellegrino et al. (2012) but the levels were far lower than in regular cigarettes (e.g. PM10 levels were 52 mcg/ml in e-cigarettes versus 922 mcg/ml in conventional tobacco cigarettes).

Cadmium, lead and nickel have also been detected in vapor but in trace levels only, comparable with levels found in Nicorette inhaler (Goniewicz et al., 2013).

Metal and silicate particles were detected in fluid and vapor from non-nicotine containing e-cigarette cartomizers obtained from one manufacturer over a two-year period, leading to exposure to amounts of these particles equal to or higher than users of tobacco cigarettes might typically experience (Williams et al., 2013).

Etter et al.’s (2013) analysis of 20 models of 10 of the most popular brands of refill liquids found Cis-N-oxide, trans-N-oxide, myosmine, anatabine and anabasine to be the most common additional compounds, but did not detect ethylene glycol nor diethylene glycol; for several brands the levels of impurities were above the level set for nicotine products in the European Pharmacopoeia, but below the level likely to cause harm.

Burstyn I. Peering through the mist: systematic review of what the chemistry of contaminants in electronic cigarettes tells us about health risks. BMC Public Health. 2014 Jan 9;14(1):18.

Cahn Z, Siegel M. Electronic cigarettes as a harm reduction strategy for tobacco control: a step forward or a repeat of past mistakes? J Public Health Policy. 2011 Feb;32(1):16-31.

Cameron JM, Howell DN, White JR, Andrenyak DM, Layton ME, Roll JM. Variable and potentially fatal amounts of nicotine in e-cigarette nicotine solutions. Tob Control. 2014 Jan;23(1):77-8.

Cheah NP, Chong NW, Tan J, Morsed FA, Yee SK. Electronic nicotine delivery systems: regulatory and safety challenges: Singapore perspective. Tob Control. 2014 Mar;23(2):119-25.

Cheng T. Chemical evaluation of electronic cigarettes. Tob Control 2014 May;23:ii11–ii17.

Cobb NK, Byron MJ, Abrams DB, Shields PG. Novel nicotine delivery systems and public health: the rise of the "e-cigarette". Am J Public Health. 2010 Dec;100(12):2340-2.

Etter JF, Bullen C. Electronic cigarette: users profile, utilization, satisfaction and perceived efficacy. Addiction. 2011 Nov;106(11):2017-28.

Etter JF, Zäther E, Svensson S. Analysis of refill liquids for electronic cigarettes. Addiction. 2013 Sep;108(9):1671-9.

Goniewicz ML, Knysak J, Gawron M, Kosmider L, Sobczak A, Kurek J, Prokopowicz A, Jablonska-Czapla M, Rosik-Dulewska C, Havel C, Jacob P 3rd, Benowitz N. Levels of selected carcinogens and toxicants in vapour from electronic cigarettes. Tob Control. 2014 Mar;23(2):133-9.

Goniewicz ML, Kuma T, Gawron M, Knysak J, Kosmider L. Nicotine levels in electronic cigarettes. Nicotine Tob Res. 2013 Jan;15(1):158-66.

Hadwiger ME, Trehy ML, Ye W, Moore T, Allgire J, Westenberger B. Identification of amino-tadalafil and rimonabant in electronic cigarette products using high pressure liquid chromatography with diode array and tandem mass spectrometric detection. J Chromatogr A. 2010 Nov 26;1217(48):7547-55.

Kim HJ, Shin HS. Determination of tobacco-specific nitrosamines in replacement liquids of electronic cigarettes by liquid chromatography-tandem mass spectrometry. J Chromatogr A. 2013 May 24;1291:48-55.

Laugesen M. Safety Report on the Ruyan E-cigarette Cartridge and Inhaled Aerosol. Health New Zealand Ltd. Christchurch, New Zealand, 2008 (accessed 1.2.13).

Pellegrino RM, Tinghino B, Mangiaracina G, Marani A, Vitali M, Protano C, Osborn JF, Cattaruzza MS. Electronic cigarettes: an evaluation of exposure to chemicals and fine particulate matter (PM). Ann Ig. 2012 Jul-Aug;24(4):279-88.

Trehy M, Ye W, Hadwiger M, Moore T, Allgire JF et al. Analysis of electronic cigarette cartridges, refill solutions and smoke for nicotine and nicotine related impurities. Journal of Liquid Chromatography & Related Technologies 2011, 34:1442-58.

Trtchounian A, Talbot P. Electronic nicotine delivery systems: is there a need for regulation? Tob Control. 2011 Jan;20(1):47-52.

Westenberger B. Evaluation of e-cigarettes. Rockville, MD: US Food and Drug Administration, Center for Drug Evaluation and Research, Division of Pharmaceutical Analysis; 2009. (accessed 1.2.13) logo
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