Evidence on the content and emission of electronic cigarettes is limited. As nicotine is the addictive substance in tobacco cigarettes, nicotine delivery from electronic cigarettes is essential if these products are to be effective for smoking cessation or harm reduction. There are three key elements that influence nicotine delivery from e- cigarette vapour to human body: the nicotine content in the cartridge, which determines the amount of nicotine vapourised; the efficacy of vaporization, which affects levels of nicotine transferred from a cartridge into aerosol; and the bioavailability of nicotine, which determines the dose and speed of absorption of nicotine from the aerosol and subsequent transfer into the blood stream and hence to nicotine receptors in the brain.  All of these characteristics vary across brands, manufacturers, and product designs.
Smoking a cigarette delivers nicotine throughout the lung and leads to absorption into both the systemic venous circulation from the oropharynx and large airways, and the pulmonary circulation from the small airways and alveoli. The latter route of absorption generates a rapid peak in systemic arterial nicotine levels and hence rapid delivery to the brain. No other nicotine product has yet been demonstrated to mimic the speed and high dose delivery characteristics of cigarettes. Since nicotine absorbed from the intestine is heavily metabolised on first pass through the liver, conventional nicotine replacement therapy (NRT) products rely on venous absorption from skin, nose or mouth, which avoid this hepatic metabolism but produce relatively low plasma levels, relatively slowly. It is not yet clear whether electronic cigarettes produce vapour that is sufficiently fine to reach the alveoli, but available pharmacokinetic data suggests that absorption is primarily from the upper airway, that is, slower than a cigarette, and achieving systemic venous blood levels of similar order of magnitude to a conventional NRT inhalator. Data on the arterial nicotine levels achieved by electronic cigarettes is not available.
It is also evident however that different electronic cigarette products are highly variable in the amount of nicotine they deliver in vapour,[21, 25] and that the nicotine content indicated on a cartridge is not a reliable guide to likely nicotine delivery. Although there have been concerns that use of electronic cigarettes could lead to an overdose of nicotine, a study carried out using electronic cigarette brands available in the UK suggests that there is low risk of overdose of nicotine or even inhaling toxic doses of nicotine using electronic cigarettes. Newer generation PV devices may deliver higher doses of nicotine, but the absorption kinetics still indicate that absorption remains almost, if not completely, via the systemic rather than pulmonary vasculature.
Electronic cigarettes: A report commissioned by Public Health England
Authors: Professor John Britton and Dr Ilze Bogdanovica
UK Centre for Tobacco and Alcohol Studies
Division of Epidemiology and Public Health, University of Nottingham
© Crown copyright 2014
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