Sensitivity to PG
Throat irritation in new e-cigarette users
Glycerine viscosity issues
Questions are sometimes raised about PG  sensitivity, leading to discussions about a change to VG - but this brings up its own issues, particularly regarding viscosity problems in carto tank systems. Some of these topics are examined below.
This article was written specifically for the benefit of e-cigarette store owners, one of whom brought up these issues; in general, shop owners/managers are keen vapers but not technical experts. There is no comparison between the relative size of the knowledge base for ecigarette vendors and tobacconists (vendors of cigarettes and tobacco): the amount of knowledge required by an ecigarette supplier is hundreds of times greater than that for a tobacconist. An ecig vendor needs at least two years of experience to reach a basic level of competency, since they are customer-facing staff who will have two major issues to contend with:
1. A continual volume of questions about the products, and about general vaping issues (two things that are interdependent: few vaping issues can be discussed without reference to the products).
2. A significant number of new users, who deserve the best advice.
In order to assist transition from smoking to vaping, a mentor requires knowledge of the many thousands of options in order to achieve the best success rate: zero-option vaping has a success rate of as low as 10%, whereas full-option vaping may succeed in 75% of cases under optimal circumstances . As an example, there are more than 7,000 refill variants offered by just one vendor; a few of these will work and most won't, for any smoker attempting to switch - and the optimal choices will be different for each smoker.
Perhaps at a later date there will be large chains of ecigarette stores, who will be able to run staff training courses; until then we may find a knowledge shortfall at the shop counter.
First, ensure that persons who are new to ecigs do not inhale direct to the lungs, as many smokers do. Draw the vapour into the mouth - hold - inhale (if required) . Tobacco smoke contains anaesthetics designed to reduce throat and lung irritation, ecig vapour does not. Despite the fact that vapour is simply a water-based fog, inhaling anything other than pure air causes a reaction, which can be mitigated by correct technique. All cases of 'lung irritation' are caused by incorrect technique, since an e-cigarette is not used like a cigarette (in multiple ways).
Inhaling directly to the lungs is incorrect technique with an e-cigarette and can lead to irritation or coughing. While any/all variations of every aspect of ecig use may work for experienced users, who will certainly be able to use a direct lung inhale if they wish, beginners need correct advice in order to transition successfully.
Beginners need correct advice, and not providing it helps no one. Indeed, it would be easy to achieve extremely poor results with an ecigarette by isolating the beginner from correct advice (or even deliberately withholding it). This is clear from studies where deliberate minimising of information and product variations was employed, which generally results in success rates as low as 10% or issues such as 'lung irritation', neither of which have any relevance to correct use.
About 1 in 10 people are sensitive to PG, meaning that they suffer from upper respiratory tract irritation and/or excessive drying-out . Most will experience reducing symptoms if they persist with vaping, as they become tolerant to it. For others, the symptoms may persist, and there could be a benefit in changing to VG  or PEG-based  refills.
About 1 in 1,000 are sensitive to VG and must avoid it. However at this point we don't know if these people are sensitive to a particular type of VG, since there are at least 5 different ways of making it (animal source, single vegetable source, mixed vegetable source , synthetic source, biodiesel byproduct, and more). If it seems worth the effort, other types could be tried, although the main problem at this time is identifying the source type. In theory a synthetic glycerine should be problem-free and this is the basis for its use in many inhalable medicines.
PEG is a viable alternative base especially as it presents flavours better than either PG or VG. It has been extensively used in prefilled cartos for this reason, especially when supplied from the Boge-Dekang factories. However it is the most usual base type to experience contamination issues with DEG, a toxic glycol made in the same manufacturing facilities from similar feedstocks and therefore liable to cross-contamination issues. If PEG can be located in a high-quality form and efficiently tested as absolutely free from DEG, it is a good alternative base material .
Propylene glycol has one of the largest and oldest research data volumes of any regularly consumed non-food material (i.e. pharmaceuticals, medical products, and industrial products with human contact). It has at least 70 years of research and has been used and consumed for over 70 years.
The first large-scale clinical studies were conducted in the 1940s, when PG was used to mist the air in hospital wards. It was discovered that PG is one of the most powerful bactericidal and virucidal aerosols in existence: hospital wards where the air was PG-treated in studies over several years noted that communicable diseases were reduced by 95% compared to control wards with no PG mist, including for diseases transmitted by virus (e.g. influenza, acute coryza). All airborne pathogens are reported killed by a PG level of just 0.5ppm. This effect has still not been explained even today, since at such low levels it is presumed that a PG molecule cannot even come into contact with all the pathogens that are killed.
PG is one of the glycol family, of which there are many variants. Glycols are an alcohol, not an oil. Some glycols are highly toxic, some are in effect inert in the human body and therefore harmless. DEG or diethylene glycol is toxic, and kills by irreversibly destroying liver, kidney and other organ function. A large scale poisoning that resulted in more than 100 deaths in 1937, due to DEG inclusion in a medicine, caused the original foundation of the FDA in 1938 - so the FDA owes its existence to a toxic glycol.
However, some of the family are completely harmless: PG and PEG being two examples. PG is not actually inert, it is treated by the body as a complex carbohydrate; some is metabolysed to lactic acid, and it is all excreted. Because PG is harmless, and because it has a very low freezing point (like all glycols), it is used as an antifreeze in human-contact and animal-contact applications instead of the cheaper, toxic DEG, which is used where toxicity is not an issue. For example it is used for winterising RV and boat lavatory systems, where human contact may be a possibility, instead of the toxic but cheaper glycols - it is completely harmless and its pathogen-killing capability is also useful; farmers can add it to water troughs in winter, since it will stop the troughs freezing and is completely harmless even if drunk in quantity.
Because PG is in effect inert, and is one of the best excipients available (a diluent or solvent for other materials), it is used extensively in medicines for inhalation and injection. It has always been used in asthma inhalers; in the nebulisers used by lung transplant patients; and as the excipient for injectable medicines that are immiscible (cannot mix with water): diazepam and phenytoin are examples. It should be fairly obvious that if PG is considered inert, is injectable in large quantities, is inhaled by patients in the most fragile health and with the most serious lung conditions, is drunk by animals without any consequences, and has been used for medical applications for 70 years, then there are few if any reasons to consider its use as having any measurable risk. There are of course many who are ignorant of these facts or are prepared to lie about them for payment.
Another common use of PG is in air-conditioning plant. Because it has all the required attributes, and in fact performs exceptionally well in this regard, it is used as the treatment agent in the cooling water. It is harmless, inert, acts as an anti-freeze, and is exceptionally potent as an airborne pathogen killer. This is why Legionnaire's Disease is not a risk in well-maintained large buildings: the PG-treated water in the aircon plant kills all airborne and water-borne pathogens. PG is is exceptionally well-dispersed as an aerosol, which allows it to do its work both in the the water phase and the aerosol phase.
Since it is obvious that the Legionnaire's Disease virus is dispersed in the air of a building with poorly-maintained aircon plant and can infect widely as a result, and PG disperses very well as an aerosol, then it is reasonable to assume that everyone in large buildings inhales PG and has done so for decades. You can assume that hospitals are very well maintained in this regard.
PG has no health concerns regarding its use in any form, whether ingested, inhaled or injected. Some people find it dries out the upper airway tract, especially when newly exposed; this generally recedes with familiarity. A very few are intolerant to it, as may be expected with any/all materials of any kind; they may find skin issues result from over-consumption. Of course, we cannot know the result of long-term high-volume inhalation, which could conceivably lead to reversible COPD stage 1 issues or the like. Compared to the certain death of as many as half of all continuing smokers, this is an invisible issue of no consequence.
We are only aware of one genuine PG-related health issue: cats have an unusual blood chemistry and high-volume ingestion of PG is reported to damage the blood cells. This is why there is no PG in cat food, as against dog food (PG is a powerful humectant, which here keeps the food moist and prevents drying-out). On the other hand, tens of thousands of vapers with hundreds of thousands of cats in their homes have reported absolutely zero health impact on their pets from '2nd-hand vapour'; so we can assume that the minuscule amount of PG in exhaled vapour has no health impact at all for cats, otherwise we would certainly have seen this reported on the giant ecig forums with millions of posts on these types of issues. It is also an additional checkmark for PG and vapour safety: even if an animal's blood is negatively affected by a vapour constituent, there is so little present even in closed rooms that it is not an issue, and this has been tested by millions of user-years (at 2014 we probably now have approaching 50 million user-years of experience with ecig products).
Glycerine, or glycerol as it may perhaps be better termed, to reflect the fact it is an alcohol and not an oil (see the suffix -ol), can be obtained from multiple feedstocks. In the past, vegetable-based glycerine was the best choice for vaping, from coconut oil or palm oil, or a mix; the best available now is the pure synthetic form of pharmaceutical grade (e.g. Dow Optim).
Because PG has an initial drying-out effect on the throat or upper airway tract for some people, several inhalable medicines (such as asthma inhalers) increasingly use glycerine instead of PG. The health impact is assumed to be the same (essentially zero). Glycerine is treated as a carbohydrate by the body, metabolysed, and excreted. No lung issues are reported for medicinal use, when using the pure pharma grade of course.
General safety issues
PG (chemical name: propane 1,2,diol) and glycerine (chemical name: propane 2,3,triol) are alcohols, and are not dissimilar molecules as can be seen by their proper names. Inhalation of alcohols cannot cause lipoid pneumonia, as has been alleged by the chemically illiterate. However, some think that inhalation of glycerol may exacerbate a tendency to (ordinary) pneumonia in those with emphysema caused by smoking. Thus, there is a case to be made that those with irreparably damaged lungs and already at risk for pneumonia, as emphysema patients are, should avoid high-VG refill liquids with their ecig. Actually they should of course be using Snus as their THR product of choice.
PG, VG (and all similar widely-used compounds) come in a variety of grades:
The difference is mostly the amount of contaminants, although the feedstock can be a factor. Although it seems obvious that only pharmaceutical grade PG or glycerine must be inhaled, as other grades are guaranteed to be contaminated, this does not seem to have percolated down to some e-liquid manufacturers.
In the case of glycerine, there is now an especially-important caution: biodiesel byproduct glycerine must not be used for refill liquid manufacture. This is because such feedstocks increasingly contain jatropha plant glycerine, which is toxic: the phorbol esters of the jatropha plant are carcinogenic. Unfortunately, although this issue has been widely publicised with regard to pharmaceutical glycerine products, it is difficult to see how jatropha glycerine can be absolutely excluded from the supply chain unless only synthetic glycerine is used (as, realistically, it should be exclusively now - this is not the 19th century).
The concept of VG being 'thick', that is to say gloopy, or more correctly too viscous, is erroneous: it means that it has not been sufficiently diluted. Glycerine must be diluted by around 20% before it becomes usable for vaping. It can be diluted by 5% to 20% with distilled water before flavourings are added. The final viscosity of a correctly-made glycerine-based refill liquid (aka 'e-liquid') is similar to a PG-based one - if not, it has been mixed wrongly and needs correcting. If it is too viscous then it needs further dilution.
For example a correctly-made VG liquid can certainly be used in a carto tank system. The precise viscosity of the final mix determines the precise size of the feed hole into the carto, just as it does for a PG-only or PG/VG mix. Different 'PG' liquids (i.e. PG/VG mixes, commonly around 80/20) may need the hole smaller or larger to avoid flooding or dry hits; this is normal (it's why no-button pulls are sometimes needed: sucking on the driptip without operating the on/off switch, in order to pull more liquid through a system with a feed hole too small for the refill viscosity). If no-button pulls are frequently needed, the liquid viscosity is too high and it needs diluting, or the feed hole needs to be enlarged.
A suitable diluent can be anything thinner than the VG element of the liquid, and could be alcohol, PG or distilled water. There is no downside from adding 10% water, or even more: it is still effectively nebulised .
 PG = propylene glycol, the most common ecig refill excipient (diluent or base material). PG is completely non-toxic and has multiple medical licenses for injection, inhalation and ingestion. It is regarded as an inert excipient although it is treated by the organism as a carbohydrate.
VG = vegetable glycerine, used with PG or by itself.
PEG = polyethylene glycol, another suitable excipient.
 If you give smokers one option when supplying an e-cigarette and refill system as an alternative to smoking, the success rate measured by clinical trials is as low as 10% at 12 months (90% of the participants relapse to smoking). If a very limited range of options is presented, the success rate can rise to around 30%. In contrast, if an expert mentor provides support, using the full range of e-cigarette and refill products that are actually available, a success rate as high as 75% is reported anecdotally (apparently such methods cannot be used in clinical trials, which must use one hardware and one or two refill products, and therefore cannot measure the proper range of real-world results).
 An ecig can be used in 'cigar mode' if required - this means to draw into the mouth, and expel from the mouth or the mouth and nose. Inhalation is not required if the refill nicotine strength is sufficiently high, because much of the nicotine absorption from e-cigarette use, unlike cigarette use, is through the buccal and nasal membranes (mouth and nose) as well as the lungs. Therefore, provided that the refill strength is sufficiently strong, and perhaps if the user has previously-existing lung issues such as COPD, there may not be a need to inhale (although of course inhalation works better to replicate the satisfaction of cigarette smoking, together with full nicotine delivery). At any rate, cigar smokers are well-served by this method.
 But note that 'quitter's throat' is a common symptom of smoking cessation: a very sore throat, possibly due to the cessation of inhalation of the anaesthetics in cigarette smoke. It is more common than for example the 'quit zits' (a symptom of smoking cessation that results in skin eruptions even in those who never had acne in youth). There is a possible issue here with sufferers from both the 'quitter's throat' together with a PG sensitivity: this fairly rare co-occurrence can result in a painfully sore throat.
PG or propylene glycol is the most common refill base liquid. Normally, it has additions of glycerine, flavouring and nicotine if required, to comprise the refill in total. PG is completely non-toxic and is regarded as inert, though it is a complex carbohydrate that is metabolysed and excreted as normal. It has multiple medical licenses for inhalation, injection and ingestion.
Inhaling PG causes throat drying for about 1 in 10 users, which is why many asthma inhalers have changed to a glycerine-based composition (PG was the most popular excipient for inhalable medicines until synthetic glycerine of pharma grade became widely available and economic).
About 1 in 1,000 people (and therefore fairly common among millions of users) may experience additional intolerance to PG, manifesting in the form of skin issues or similar. At this point we don't know if this is specifically a PG issue or related to impurities in the base material.
 We still use the term 'VG' or vegetable glycerine as a general term for glycerines suitable for vaping, though strictly speaking it is now incorrect. Originally, glycerine was only available from two source feedstocks: animal or vegetable, and the vegetable form was preferable. Now there are multiple types, and the best choice is a fully-synthetic form such as Dow Optim, since it avoids (in theory) all allergy or toxicity issues due to its synthetic derivation and purity.
Note that there is now a new contender, biodiesel byproduct glycerine, and this must be absolutely avoided due to the possibility of the vegetable source having contained Jatropha plant material, which is toxic; this plant is commonly used in biodiesel production.
 PEG or Polyethelyene Glycol is one of the three commonly-used base materials for e-liquid. Its principal use is in pre-filled cartos.
 For example palm oil, coconut oil, or a mix of the two.
 We understand from the factories that the transport barrels are a likely source of cross-contamination; inefficient (or zero) cleaning protocols between use for DEG and then PEG.
 The technically correct term for the active element within an ecigarette is a nebuliser, not an atomiser: an atomiser is a device where liquid is turned into a spray by an air jet, using a venturi tube / pump.