. . PG - VG - PEG There are three possible base carrier liquids in use for e-liquid (the refill liquid used in electronic cigarettes): PG, VG, and PEG. PG is the most common, usually with a proportion of VG as that produces more vapor. PEG is more common in pre-filled cartomizers. These basic ingredients, mostly inert or with no major human-related function, are also called diluents or excipients. They are used for several reasons: - They are effective solvents for other ingredients such as flavorings, and disperse them well in the aerosol produced; - They produce a visible fog effect that replicates smoke but at very little temperature elevation, and therefore strongly add to the cigarette replication function of the system (and are the same materials used in fog machines, which have decades of safe use); - They are completely non-toxic and in some cases have 70 years' safe medicinal usage history and research. Intolerance to any one of these diluents is unusual, and severe intolerance is rare; one of the alternatives can be used instead. (Virtually all materials considered safe for human consumption will produce intolerance in a small number of individuals.) - Chemically, they are all (types of) alcohol, as can be seen from the suffix -ol that each has. They are not oils. Inhalation of alcohols has no specific effect on the lungs. For example, inhalation of alcohols has never been shown to cause lipoid pneumonia (as has been alleged by opponents); an entirely new effect currently unknown to medical science would first need to be shown. 1. PG or propylene glycol or propane-1,2, diol Glycols are alcohols, not oils. The non-toxic types such as PG have no inhalation concerns, and are widely used in medicines for inhalation (and it has been for 70 years without issue). PG presents flavors well and has the best TH of the three choices. It is the most common refill liquid base material, normally used with 10% to 30% VG. It is virtually inert in the body and can even be injected into the bloodstream without effect - it is used as the carrier liquid for injection of immiscible drugs like diazepam (that do not mix with water). It is a powerful bactericide and virucide, and has been used in hospital air spray projects. It is reported to kill all airborne pathogens at only 1ppm. It is commonly used as the antibacterial water treatment agent in large building airconditioning plant, for its bactericidal/virucidal properties; because it has a very low freezing point; is cheap to manufacture at industrial grade; and because the inevitable inhalation resulting is harmless. There is a large body of research on it going back 70 years that proves it is safe for inhalation. PG is usually the main ingredient in disco fog machine refill liquids. Due to strict employee health regulations that cannot be subverted, it is approved as safe for extended high-volume exposure for employees in discos, theatres and bars. PG has been used for decades as an excipient in inhalable medicines. Some of its common uses are in asthma inhalers, in the nebulizers used by lung transplant patients, and inhalable treatments for patients with lung diseases. It is used in inhalation therapies for patients with the most delicate lung conditions and in the most fragile health. Because it is made from non-organic sources (via synthetic processes) allergies are not believed possible although some experience reactions; the most common is a drying-out effect on the throat. Significant drying of the throat is reported by some (>1 in 100, <1 in 10), and for some the effect is painful. It is reported to pass with time. [Note also that cessation of smoking produces a painfully dry throat for some. The two conditions may, rarely, combined to produce a painfully sore throat for an unfortunate few.] A few experience other effects although these might be due to use of non-pharmaceutical grade materials; in a small number of cases there are strong reactions such as skin issues (thought to be around 1 in 1,000 or less). A note on MSDS warnings MSDS cautions are often quoted because they appear alarming, but it needs to be clearly understood that these are produced for transport and storage regulations, where firefighters or employees may be exposed to massive overdoses (read: immersion for extended periods or similar). Note for comparison the MSDS for water, salt etc., which also have the same cautions ('irritant to the eyes and lungs', and similar). The MSDS has no relevance for materials licensed for pharmaceutical, food and cosmetic use since the material is obviously safe for all uses, all purposes, and all forms of consumption. PG can be injected, inhaled and ingested (and is fully licensed for all those uses) with no observable effect on health. Where a material has multiple licenses for all forms of use in pharmaceuticals, and additionally has LD50 quantities either not stated or in the multiple-gallon range (as for all the materials on this page), the MSDS should be ignored. 2. VG or vegetable glycerine or glycerol or propane-1,2,3, triol Glycerol is an alcohol, not an oil. There are no inhalation concerns, and glycerol (glycerine, glycerin) is widely used in medicines for inhalation. VG produces more vapor than PG. It produces less TH and does not present flavors as well. VG is an alternative carrier liquid, though more commonly used as an additive to PG liquids. It is more viscous (thick) than PG. If used as the sole base material, it will need diluting with 10% - 20% DW (distilled water). Other diluents include alcohol; EM (ethyl maltol) may also be added. VG can be made in around five different ways, and for this reason there are a number of people who may have an allergy or intolerance for one or more brands. It can be made from animal or vegetable sources, or as a byproduct of biodiesel production, or synthesized and made in a similar way to PG and PEG, from other chemicals. Vegetable sources include coconut oil and palm oil. It is thought that glycerine produced as a by-product from biodiesel production should not be used for inhalation, since it is more likely than any other type to be contaminated by the toxic phorbol esters of the Jatropha plant, increasingly used in biodiesel production . VG is bacteriostatic, that is it does not support growth; preservatives such as sulphites may be added to it to prevent microbial deterioration within the water content, and therefore there are questions such as whether the additives (or contaminants) are the cause of occasional intolerance (which is far less common than intolerance to PG). There is less research on inhalation. It is thought to be less inert than PG, and to be treated by the body as a complex carbohydrate - that is, metabolized by being broken down into carbohydrates then used/excreted. However, Dow Chemical make all grades of all these products, and advise that their pharma grade synthetic glycerine is the best choice of all their products for inhalation, and it is supplied to pharmaceutical companies for that purpose. Glycerine is therefore used as the base for inhalable medicines and has multiple licences for inhalation. Glycerine is replacing PG as the excipient in inhalable medicines (i.e. as the main diluent). This may be because there are less problems with throat / windpipe dehydration. There is a suggestion that it may be less suitable than PG for inhalation by those with lungs severely compromised by smoking-related diseases, as it may be less easily absorbed by the lungs, and unlike PG it has no antibacterial/antiviral action. 3. PEG or polyethylene glycol Glycols are alcohols, not oils. The non-toxic types such as PEG have no inhalation concerns. PEG is said to present flavors better than any other base liquid. It is the least common carrier liquid. It can be made in many different formulations, the one used for e-liquid is called PEG400. This formulation appears to be chosen as it has the right viscosity, as PEG can be thinner, thicker, or even in the form of a wax or powder. As far as we know there is little research on inhalation but it cannot be toxic as it is used in foods and medicines. For example it is used as the main ingredient in a very large number of laxatives, where it is supplied in the form of a powder, with electrolytes, for mixing to a drink that has a safe laxative effect. Because it handles flavors so well it is used by several of the major cartomizer-only (2-piece ecig) firms that mainly sell to first-time buyers, as it works well in cartos. We think it comprises 1% or less of bottled eliquid sales volume but it is far higher in the pre-filled carto market - perhaps as much as 33%. PEG is the most likely of all the three base materials to be contaminated with DEG as the manufacturing process can be carried out using the same equipment, so there is a risk of cross-contamination. Uses All these base liquids are used in medicinal preparations of one sort or another, and are considered non-toxic. They are all used in foods, skin creams and some (if not all) in medical inhalers, so any MSDS warnings can be completely ignored. PG for example is used as the mist carrier in nebulizers for lung transplant patients, so it can be assumed that it is as harmless as it is possible for a material to be in the human body. PG Propylene glycol can be used to make an expensive, non-toxic antifreeze - for example, to make it less dangerous to livestock or pets (who will lap up antifreeze due to the slightly sweet taste), and for winterizing boat and RV lavatory systems that may have human contact by touch or inhalation. PG is used for this purpose because it has a very low freezing point, is non-toxic, and is cheaper or more acceptable than other options (such as alcohols). Many glycols have antifreeze applications due to their very low freezing point and cheapness at industrial grade (i.e. with some degree of contaminants). Some glycols are toxic, some are non-toxic. The types that are toxic for animals and humans tend to be cheaper (like DEG), the non-toxic types are more expensive (like PG). Basically any/every glycol has been used as an antifreeze. There is 70 years of experience with glycols in medical and industrial uses, for example: a. PG was used extensively in hospital ward aerosol trials to reduce infection in the 1940s. The only observed effect after years of continuous trials was that bacterial and viral infections in treated wards was reduced by 95%. This is now redundant because large buildings have their aircon plant treated with PG and the effect is presumably building-wide. b. Aircraft engines in WW2 used glycols for antifreeze and engines of all types still do. There is a vast amount of practical experience and research on PG going back 70 years. Any 'new trial' is both (a) pointless, given the quantity of data available; and (b) possibly agenda-based for some reason. There is enough research and usage data to know exactly what the health issues are: essentially non-existent for temporary high-volume use and permanent low-volume use. Ongoing high-volume use is not researched but the e-cigarette user population will provide this data; thus far (approaching a decade of presence on the market, ~10 million users at this point: Q2 2013) there appears to be no issue. PG is not 'antifreeze' - it is a chemical that is virtually inert to humans while exhibiting all the properties needed in an antifreeze: very low freezing point, antibacterial, and cheap to make at industrial grade. It is more expensive than 'ordinary' antifreeze, which is toxic, and PG is therefore only used for applications where human or animal contact is likely, since it is completely non-toxic. Therefore it can be used as a winterizing antifreeze for boat and RV lavatory systems, where some human contact may be possible; and for applications close to livestock. If something is used as the carrier for medicines in lung-transplant patients' nebulizers, and the base for injected medicines that do not mix with water, it is reasonable to assume that there are few things less toxic. It seems that because PG is now used in the air-conditioning plants of large buildings to kill pathogens and act as an antifreeze, the incidence of Legionnaire's Disease is now lower. And, since the pathogens that cause the disease can obviously migrate from the air-con cooling water into the airspace of large buildings, it also seems likely that PG is present in the airspace, as it is notably well dispersed as an aerosol. Thus, people in large buildings may breathe PG, at very low concentrations. Glycerine VG has been widely used in inhalable medicines since 2000. The terms glycerine or glycerol are more accurate now than the term 'VG' that was introduced for the excipient as used in e-cigarette refills in the early days; at that time, the two main sources were vegetable or animal, and the vegetable type was the preferred option. Today the best option is synthetic pure glycerine as this will have the least possible safety issues. This is the type supplied for use in inhaled pharmaceuticals, a typical brand is Dow Optim. It is not as widely available as vegetable glycerine though (it is reportedly available in containers down to 4-gallon size in the US). There are two safety questions concerning VG that do not affect the other two liquids: 1. Biodiesel by-product glycerine must not be used for inhalation due to the risk of contamination by the phorbol esters of the Jatropha plant; apparently these carcinogens are difficult to test for. 2. If heated sufficiently, glycerine can degrade into acrolein, which is toxic. However all tests so far indicate that, although it is theoretically possible, this does not happen in a regular e-cigarette. Acrolein was not detected in tests carried out by Intellicig. The hypothesis is that if the temperature within an atomizer can rise sufficiently high to produce acrolein, there would be no liquid left to convert (an atomizer being a liquid-cooled heating element). Also, the addition of other ingredients such as water may lower the boiling point enough that the VG boils off before any acrolein can be created. In addition, negative pressure within the atomizer or cartomizer body also has a cooling effect. Acrolein has a strong, characteristic odor so would be noticed if in large amounts. We know less about PEG although it seems, so far, to be inert in the body. DEG Diethylene glycol is one of the many glycols, and toxic to humans and animals. It is a poison that kills by damaging the liver and kidneys among other effects. It is similar to PG and PEG in appearance, taste and characteristics, and it is produced in the same way from similar raw materials and as a by-product of similar manufacturing processes, although PEG seems especially implicated. It is therefore likely to be found in any materials of this type due to poor manufacturing controls or cross-contamination - for example, using drums for PEG400 that previously contained DEG. For this reason it is the main contaminant that should be tested for. Because PEG is more likely than the others to be contaminated with DEG, and because pre-filled cartomizers are the main area of use for PEG, it follows that such cartos need testing more than other materials. Note however that (a) significant quantities would need to be consumed before this type of toxin has a measurable effect, and it is probably quite likely that consuming a number of contaminated cartos would not reach anywhere near this amount; (b) it is unclear whether any/all of the DEG contaminant in the (very small number of) tested items that have showed such contamination would transfer from the liquid into the vapor, and one test seems to indicate that a 1% DEG contamination of PEG liquid in a carto is not measurable in the vapor; and therefore (c) you would actually need to drink the contents of many cartos before there is any risk. This is not a practical route to poisoning. The FDA was formed in 1938 as a direct result of a wide-scale poisoning incident in 1937 in which more than a hundred people died after a large quantity of DEG was used in a patent medicine. Additional Clarifications Because this post is addressed to both consumers and manufacturers, there may be some confusion about the way a specific issue should be interpreted by either. This post has some clarifications about differentiating between the type of buyer: Buying Glycerine: consumer or manufacturer? ------------------------------- notes  The FDA have warned about the use of biodiesel by-product glycerine in medicines, because of the risk of toxic contaminants. At present it seems difficult (perhaps very difficult) for either the FDA or industry to identify the toxic phorbol esters as testing is reported to be problematic.