In terms of health safety, are the natural flavors preferable to artificial ones?
Not necessarily. A flavor, whether natural or artificial, is elaborate from a particular subtrate (gum, ethanol, sugar syrup, PG, acetin, oil…) and contains different types of aromatic molecules (ketones, terpenes, thiols, lactones, pyridines…).
In terms of health, the questions are:
- What are the reaction products obtained from the pyrolysis of these aromatics?
- Do they represent a potential risk to the organism?
When using an e-liquid made of synthetic flavors, most flavor enhancers (ethyl maltol, acetin, ethylvaniline, malic acid, monosodium glutamate… ), acidifiers ( citric acid, lactic acid, …) and preservatives (sorbate potassium …) can produce chemical compounds whose pulmonary toxicity is still unknown at this time.
And for e-liquids made of natural flavors, even if they are free of enhancers and preservatives, the presence and content of particular compounds shall be closely watched. If I take the example of a very common molecule called pulegone, that is the major mono-terpene component of many natural varieties of mint, its natural metabolic oxidation metabolism produces a hepatotoxin called menthofuran.
Are there some flavors to discard, whether natural or artificial, when choosing an e-liquid?
Yes, indeed. One must keep in mind that the respiratory system is not equipped with as powerful enzymes and metabolic detoxification pathways as those present in the digestive system. Therefore, food flavorings intended for ingestion may exhibit higher toxicity upon inhalation.
This is the case of Diacetyl, an aromatic molecule present in butter, fresh cream, dairy products and alcoholic beverages. Cumulated exposition to this organic volatile compound can cause severe bronchiolitis. The use of flavorings containing Diacetyl is not recommended in the manufacture of e-liquid.
A recent study by Dr. Farsalinos  showed that there could be a cytotoxic potential with liquids containing flavorings obtained by maceration techniques (but at much lower levels than of conventional cigarette). The cytotoxic potential is obviously highly correlated to the type of tobacco used (presence or absence of pesticides…) and the implementation of the maceration procedure.
Finally, some types of molecules classified as allergens (alimentary and cosmetics) and found in many natural or artificial flavors are likely to show more or less toxic effects by inhalation: benzyl benzoatem cinnamate, linalool, coumarin, farnesol, eugenol, geraniol…
Again, our experience is limited and doesn’t allow us to conclude on the toxicity or the absence of toxicity of certain types of aromatic molecules when inhaled. Future studies are required to better define allowable doses and circumstances of use of such aromatic compounds in the e-liquid.
Some vendors use the term “Organic glycerin”. How different is it from the commonly used glycerin and what are the benefits?
Vegetal glycerine (VG) is a by-product of the esterification of vegetable oils; the oils used are generally coconut, soybean or palm oil, containing a high percentage of fatty acids and releasing large amounts of glycerol. Other oils may be extracted from cottonseed, corn, olives…
The term “organic glycerine” is used when vegetable oils are derived from agricultural crops free of synthetic chemicals and whose crops are in sheltered areas with respect to any external contamination.
In an e-liquid glycerin, whether organic or not, must have a purity equivalent to at least a USP/EP Grade (the same applies to propylene glycol). This quality specifies its high purity and shows that the product meets all applicable specifications of United State (USP) and European (EP) Pharmaceuticals.
Now, is the organic glycerin only a marketing concept or is there really a significant benefit on the health? Only long-term studies may give an answer.
Why do we find different colours in the e-liquid sold in the market? Some e-liquids are always transparent for example, while some others are very dark.
The staining of an e-liquid depends on the type of flavor used and on the technique(s) of manufacture (chemical synthesis, cold extraction, solvent extraction, distillation…). A natural flavor of blueberry, for example, may have the blue-purple colour of blueberries or be completely transparent, depending on the method of production (extraction with solvents such alcohol/water or based on biotechnological techniques with a cocktail of enzymes). The colour of an e-liquid may also possibly change over time, due to the chemical interaction with the nicotine and/or the oxidation with ambient air.
If I had to choose a single selection criterion to select an e-liquid brand, which one would it be? (apart from the taste)
If one only takes into account the health aspect and consumer safety, the essential selection criterion should reside in the repository as established by the manufacturer, inherent in its production process and packaging. I think a manufacturer should have good notions in organic chemistry (especially in order to ensure the quality and safety of his aromas), and conduct regular chemical and microbiological analyzes on both raw materials & finished products.
Finally, I am not indulging in one-upmanship with flavors in an e-liquid because it increases the potential risk of toxicity (how small it may be) and blurs the natural sensitivity of our taste buds… but these latter are personal considerations over taste and, fortunately, everyone has their own!
We acknowledge Vincent Cuisset (VDLV), for this interview.
 Farsalinos, K. E., Romagna, G., Allifranchini, E., Ripamonti, E., Bocchietto, E., Todeschi, S., … Voudris, V. (2013). Comparison of the Cytotoxic Potential of Cigarette Smoke and Electronic Cigarette Vapour Extract on Cultured Myocardial Cells. International Journal of Environmental Research and Public Health, 10(10), 5146–5162. http://doi.org/10.3390/ijerph10105146