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E-Liquids

E-liquid formulations typically comprise an excipient/carrier/solvent (propylene glycol and/or glycerol), nicotine and sometimes flavourings.

Typical constituent levels in e-liquids are:

  • nicotine 0-5%
  • glycerol 0-95%
  • PG 0-95%
  • water 0-5%
  • flavourings <0.5%

When the user puffs on the device, the formulation is drawn by capillary action onto the vapourizing mechanism, a heating coil wound around a fibre glass/ceramic core. A current is applied to the heating coil and this vaporizes the formulation (at about 150oC, with the actual temperature depending on the device and the battery power), which then condenses in the ambient airflow, producing an aerosol that can be inhaled by the user.

Nicotine

Nicotine is a naturally occurring alkaloid found in several varieties of plant, but at the highest concentration in the tobacco plant. Its effects on the body have been extensively studied for many years and are the subject of numerous scientific papers and public health reports. Nicotine activates receptors in the brain resulting in the release of a wide range of chemicals associated with various effects on mood and cognition. This results in nicotine acting both as a mild stimulant and a mild relaxant. Studies have also shown that nicotine can have beneficial effects on motor abilities, attention and memory. Nicotine can be addictive, whatever form it is in.

Nicotine at high doses has acute toxic effects (in the tobacco plant it functions as a natural insecticide) and accidental exposure, especially through the skin or swallowing, can cause dizziness, nausea and vomiting and can be fatal if a particularly high dose is delivered quickly.

At low doses non-nicotine users may experience dizziness and nausea quite rapidly whereas regular nicotine users quickly become tolerant to the short term effects. Nicotine exposure is associated with transient increases in blood pressure and heart rate. Nicotine also has an effect on increasing general metabolism, may play a role in weight control and may also play some role in protecting against some neurodegenerative diseases.

All the nicotine in e-liquids is extracted from tobacco and this process may also extract other minor alkaloids from the tobacco, such as nornicotine, anatabine and anabasine and cause the formation of impurities, such as β-nicotyrine, cotinine and nicotine-N-oxide. Although any impurities are undesirable, the European Pharmacopeia permits low levels in pharmaceutical grade nicotine (up to 0.3% each of certain specified impurities, and no more than 0.1% each of unspecified impurities, up to a total of 0.8%)[1]. A recent analysis of a range of e-liquids concluded that only half of those tested contained acceptable levels of impurities[2]. This analysis also found the concentrations of nicotine measured in the e-liquids ranged from 85-121% of the labelled content.

Excipients

Propylene glycol (PG) and glycerol are used in e-liquid formulations as carriers (excipients) for the nicotine and flavourings and form the majority of the aerosol formed during the heating and cooling process. Both PG and glycerol are widely used in the pharmaceutical, cosmetic and food industries and are designated as ‘Generally Recognized as Safe’ (GRAS; for oral or dermal use) by the US Food and Drug Administration. PG and glycerol are also used as humectants in conventional tobacco cigarette blends. The available literature for exposure to PG[3][4][5] and glycerol[6][7][8] by inhalation suggests that there are no systemic issues but further research is needed to assess the effects on the lungs of inhalation of PG and glycerol.

Flavours

Most commercial e-liquids are flavoured, with a very wide range of flavours available. The most frequently used flavours in e-liquids are tobacco, mint/menthol, fruit, coffee, vanilla, and chocolate (in that order)[9].


  1. European Directorate for the Quality of Medicines and Healthcare. European Pharmacopeia 7.0 Opens new window. European Directorate for the Quality of Medicines and Healthcare, 2012.
  2. Etter, J.F., Zäther, E. and Svensson, S. 2013. Analysis of refill liquids for electronic cigarettes Opens new window. Addiction 108(9): 1671-1679.
  3. JECFA, 2003. Safety evaluation of certain food additives Opens new window. WHO Food Additives Series 50. Prepared by the 59th meeting of the Joint FAO/WHO Expert Committee on Food Additives.
  4. OECD, 2001. 1,2-Dihydroxypropane: Initial Assessment Report for 11th SIAM  Opens new window(USA, January 23-26, 2001).
  5. Suber, R.L., Deskin, R., Nikiforov, I., Fouillet, X. and Coggins, C.R.E. 1989. Subchronic nose-only inhalation study of propylene glycol in Sprague-Dawley rats Opens new window. Food and Chemical Toxicology 27(9): 573-583.
  6. JECFA, 2002. Safety evaluation of certain food additives and contaminants Opens new window. Prepared by the 57th meeting of the Joint FAO/WHO Expert Committee on Food Additives. WHO Food Additive Series 48. WHO, Geneva.
  7. OECD, 2002. Screening information dataset (SIDS): Glycerol Opens new window. SIDS Initial Assessment Report for SIAM 14.
  8. Renne, R.A. et al 1992. 2-week and 13-week inhalation studies of aerosolized glycerol in rats Opens new window. Inhalation Toxicology. 4(2): 95-111.
  9. Etter, J.F. and Bullen, C. 2011. Electronic cigarette: users profile, utilization, satisfaction and perceived efficacy Opens new window. Addiction 106(11): 2017-2028.
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