Understanding e-liquids

You have begun using electronic cigarettes or you would like to try vaping and you’d like to know what the refillable e-liquid is made up of, and in particular whether some of its ingredients could be harmful. Our answers are based on recent scientific data!

E-cigarette liquid: the essential ingredients

The e-liquid used in electronic cigarettes has relatively few ingredients: nicotine, propylene glycol, glycerine, flavours and additives. (1) Questions remain regarding the long-term safety of these inhaled ingredients, as well as the potential addictive nature of nicotine when it is inhaled using e-cigarettes.

1 - Nicotine
The majority of e-cigarettes deliver nicotine in aerosol form, to recreate the experience of smoking a real cigarette. The nicotine used in electronic cigarettes is an extract of the tobacco plant. The concentration of nicotine used in the mixture or in the cartridges can vary between 0 and 36 mg (in France the amount is limited to 20 mg/ml). The nicotine content displayed is accurate in the case of well known brands but can be less reliable in the case of lesser known makes.  What does this mean for your health?

Nicotine substitutes (patches, gum, nasal spray) are not dangerous for health even for those with heart disease. E-cigarettes release a dose of nicotine similar to that released by nicotine substitutes. By analogy, the amount of nicotine inhaled by vapers is not dangerous. The potential risks of nicotine coming from tobacco products which aren’t smoked (snus, snuff) have already been studied. Such risks include haemodynamic variations, endothelial dysfunction, cases of thrombogenesis and systemic inflammation, as well as other metabolic effects. (2) The question is raised of how fast the nicotine from electronic cigarettes reaches the brain and therefore the speed of possible onset or persistence of nicotine dependence.
To note: the European Union has recently drawn up a new regulation which establishes a limit in excess of 20 mg/ml of nicotine concentration in the liquids used for electronic cigarettes.

2 - Propylene glycol (PG)
What is it?
Propylene glycol is a solvent. It’s a viscous, colourless liquid and is practically odourless but possesses a faintly sweet taste. Propylene glycol is permitted as a food additive by the American Food and Drug Administration (FDA) and is also used in cosmetics and pharmaceuticals (as an excipient in syrups, creams…). It is found in the nicotine nasal spray Nicorette, an approved medication. It is the major ingredient of the liquid found in e-cigarettes, accounting for approximately 90% of the solution (9). It leads to the production of vapour and also enhances the flavours.

Is it a health risk?
Propylene glycol is not considered to be a carcinogen and is not toxic when reproduced. There is no evidence of it being acutely or chronically toxic when ingested. The only effect observed in the case of animals is as an irritant to eye or nose mucus (3)… However, this product is considered to be more toxic than vegetable glycerine in the long-term, when inhaled. In the United Kingdom, there is an occupational exposure limit value (OEL) of 150 ppm (447 mg/m3). There is no data on the long-term toxicity of PG on the amount and frequency of usage by vapers.

3 - Vegetable Glycerine (Glycerol, VG)

Glycerol is used as a food additive as well as an additive in pharmaceuticals, toothpaste, chewing tobacco… Glycerol is often used in electronic cigarettes, either on its own, or mixed 50/80 with propylene glycol. It enables the production of vapour and enhances the flavours, much in the same way as propylene glycol.

Is it a health risk?

Glycerol is considered to have low toxicity, and not to be carcinogenic or toxic when reproduced. It is no more than an irritant for the eyes and respiratory tract. However, studies have been primarily based on its ingestion and not on its inhalation. The long-term effects of inhalation are unknown.

Dehydrated glycerine produces acrolein, a powerful irritant. A temperature of 275°C is required for this process to take place; the atomizers in electronic cigarettes do not usually reach such temperatures but nevertheless this factor needs to be kept under control. McCauley et al. (4) reported the case of a 42 year-old woman who was suffering from exogenous lipid pneumonia caused by e-cigarettes. It was the glycerine added to the e-cigarette that was suspected of being the cause.

N.B. there have been cases of glycerol contaminated with diethylene glycol, which is toxic. It is for this reason that the FDA requires tests to be carried out on glycerol used in pharmaceuticals, to ensure its purity. E-liquids can contain glycerol that is of pharmaceutical quality (USP), or not. This quality ensures a priori an absence of impurities in the glycerol.

4 - Flavourings

The liquids in all electronic cigarettes contain flavourings, some natural and in other cases artificial (in particular certain flavourings used by the tobacco industry). The most common flavour is that of tobacco, followed by menthol and fruit. There are also chocolate, vanilla, coffee, cannabis flavours, among others.

Health consequences: we know that some of these flavourings are toxic in certain doses or when used in certain ways, in particular cinnamon is to be avoided. Menthol can interact with nicotine. The other food flavourings are known to be innocuous when ingested. It is not yet known what the effects of heating and repetitive inhalation of these additives is over a number of years. Nevertheless, it is thought that the amounts absorbed could be higher than when taken orally and if that is the case then certain flavourings could give cause for concern. Liquorice for example, which should be monitored for its contribution to arterial hypertension, or diacetyl, a butter flavouring permitted for consumption, but responsible for serious obliterative bronchiolitis when inhaled in an industrial environment.

5 - Ethanol 

Ethanol is used as a solvent for flavourings as well as a thinning agent in e-liquids. It can also be responsible for the sensation of a hit. This alcohol ingredient can be made from ethanol but also from whisky, rum, vodka…

Health consequences: the consequences for recovering alcoholics should be kept in mind. (6)

Electronic cigarette liquids: impurities and contaminants

Various studies, including that of the FDA (7), have shown that the liquids used in electronic cigarettes, apart from their main ingredients, can contain toxic elements and impurities.

A recent meta-analysis of 29 studies (which, between 2007 and 2013, evaluated the chemical composition of e-cigarettes) shows that numerous chemical substances and ultrafine particles known to be toxic, carcinogenic and/or irritants to the respiratory tract and which cause cardiac problems, were identified in the aerosols, cartridges and refill liquids. (8)

The FDA and HNZ (9, 10) have both reported traces of TSNA (nitrosamines), which are carcinogenic substances, in the e-liquids but it represents a tiny proportion (0.008 mg/e-cigarette cartridge containing 16 mg of nicotine) compared to the 6.3 mg in a classic Marlboro cigarette. Similar quantities, in trace form and therefore non-toxic, are present in nicotine substitutes. A study carried out in 2008 by a manufacturer, Ruyan, found that in a 16 mg nicotine cartridge the level of tobacco-specific nitrosamines was four times higher than in Nicorette gum (but 765 times lower than in a Marlboro). (11) It is better to opt for pharmaceutical nicotine which has a limited amount of impurities, similar to that of nicotine substitute products, as non-pharmaceutical nicotine can present higher levels of nitrosamines found in tobacco.

Phenolic compounds and polycyclic aromatic hydrocarbons have also been found in electronic cigarette liquids. (12)

Some contaminants, most of which are from tobacco flavourings, have also been detected in e-liquids: a small quantity of diethylene glycol (around 1%), a known carcinogenic agent, and an anti-gel ingredient have also been found in one of 18 cartridges analysed by the FDA, which was then removed from the market (13). In some electronic cigarettes, anatabine, anabasine and norcotinine, nicotine-like substances which can be active, have been found.

The New Zealand study (14) has shown that cadmium, arsenic, chromium, nickel and lead were undetectable in e-cigarette liquids. However, Williams et al. (13) have shown that the levels of lead, chromium and nickel in electronic cigarette liquids are equivalent, and in some cases higher, than those found in cigarette smoke. (15) In addition, certain traces of metals found in the cartridges are partly from nanoparticles produced in the manufacturing process and which can penetrate deep into the lungs.

All this points to a need for strict controls on liquids, cartridges and electronic cigarettes, as well as manufacturing standards, and above all new studies in order to establish reliable estimates of the quantity of chemical products found in e-cigarettes and to evaluate their possible toxicity.


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