This information is primarily for DIY purposes, and describes the grades suitable for inhalation.
Three base liquid carriers are used for e-liquid manufacture: PG, VG and PEG - propylene glycol, glycerol (which we tend to refer to as VG or 'vegetable glycerine') and polyethylene glycol.
These liquids are all alcohols (and not oils), as can be seen by the -ol suffix of their common names. There are a multitude of both common and chemical names for these materials, as is normal, and some of the more common chemical names (more precise definitions of their nature than the common names) are:-
PG: propane 1,2 diol
VG: propane 2,3 triol
When used as the carrier for other materials, they can be referred to as excipients or diluents, especially when they are essentially inert in the human organism, as these materials are. That is to say, they have no major function or effect* and are treated as carbohydrates, metabolised, and excreted. Both PG and VG are used as the base for medical inhalers; in fact PG is used as the excipient for the medicinal inhalers used by lung transplant patients, and both PG and VG are used in asthma inhalers, so it should be fairly obvious that these materials are completely harmless for inhalation, at least in the short to medium term. There are questions about high-volume long-term use but these can only be answered after 20 years use for vaping; currently, it seems unlikely that serious issues will result since we would have seen some indication of this in asthma patients, who have of course used such inhalants for decades.
* PG is reported to act on the organism in the same way as ethanol but with around one-third of the effect. This can be ignored with the very small quantities consumed in vaping.
MSDS
The MSDS sheets for these materials can be safely completely ignored. This is an important point to make since the MSDS makes all materials without exception sound like chemical warfare agents: see the MSDS for water and for salt, for example.
It is obvious that if asthma patients are encouraged to inhale them for decades, and they are supplied without reservation to lung transplant patients for inhalation, they are regarded as inert and harmless.
The purpose of the MSDS is for bulk storage contractors, transport contractors and fire officers. In case of mass release and inundation of a worker, for example, there need to be documented hazard procedures. You can drown in a tank of these materials but that is the only practical hazard. To evaluate the real threat from a material, the MSDS-quoted dose can be checked: if it quotes a risk from amounts measured in milligrammes, such as 100mg, then there is a possible risk; 45 gallons of PG or VG are only dangerous, like water, if you fall into a barrel of it. They are used for inhalation therapies by persons in the most fragile of health and with the most serious lung conditions.
Purity
In theory, only the pharma grade of these materials should be used for inhalation, since all other grades contain contaminants to an increasing degree (the difference between the grades is the percentage of contaminants). The problem here is that DIYers and perhaps many small-scale manufacturers do not have access to pharma grade materials, so food grade materials must be used. Indeed, it is reported that some pharmaceutical suppliers are actively trying to prevent access to their top-grade materials by ecig products manufacturers.
This is more an issue of excessive regulation or fear of legal problems reducing safety than anything else.
There may not be any practical difference between the pharmaceutical grade and food grade materials from some suppliers. The only difference might even be that the pharma grade is specially tested and comes with a test certificate. However we do know that there is a difference between low-grade and high-grade materials in vaping products (though we cannot know what certificates / grades were supplied) because more tests are now showing MEG contamination - (mono)ethylene glycol - which is toxic, as is DEG. In fact it looks as if DEG contamination has been replaced by MEG contamination in the marketplace, perhaps because suppliers know to test for DEG but are unaware that low grade PG may contain MEG.
Regulation
If e-cigarettes become regulated then it is likely that only pharma grade will be allowed for inhalation duty. The advantage of this is the minimal contaminant load; the disadvantage is that prices go up. Everyone has a different opinion on this, so in the current climate, where ingredients are unregulated / untested in some countries, perhaps vendors should simply publish what their policy is: either "we use pharma grade only", or "we use food grade", or "we use food grade and also test for X and X". Then people could choose accordingly. Some sort of proof would need to be offered if a vendor makes specific statements - talk is cheap, after all.
Grades
These carrier materials are made in four grades:
DO NOT BUY FARM GRADE OR INDUSTRIAL GRADE MATERIALS FOR INHALATION - contamination levels may be higher than allowable for safe inhalation.
There is a possibility that Food Grade should not be inhaled, because the digestive system can handle the contaminants in it that the lungs may not be able to. It is likely there will be significant differences between food grade products from different suppliers.
PG and VG are both sold in good quality and are labelled:
Propylene Glycol, USP (or BP or EU)
Glycerine, USP (or BP or EU)
It is possible that a USP product can be either pharma grade or food grade. The USP designation only refers to a general production regime and does not cover the exact contaminants as long as the minimum standard is met. So a USP product might be considered safe for inhalation; or it might, on test, prove inadvisable to inhale. The only product that is guaranteed to be suitable for inhalation is a product that is (a) designated by the manufacturer as Pharmaceutical Grade, and that (b) also has a provable license for inclusion in a specific inhalable medicine. Needless to say this is virtually impossible to locate for the individual buyer. A vendor would be able to buy e.g. Dow Optim glycerine or PuraGuard PG in quantity, for mixing down, and be assured the material is suitable. (This doesn't take into account the nicotine base carrier PG or glycerine, though.) It can be bought in 18 litre containers and up, so is suitable for e-liquid manufacturers, though manufacturers may try to prevent ecig product manufacturers from obtaining high-quality products.
As a practical example, the FDA has warned that glycerine marked USP may contain toxic components from biodiesel byproduct manufacture, and currently there appears to be no way of determining whether or not a USP product contains biodiesel byproduct glycerine. Only synthetic pharma grade glycerine can be be assumed to be free of such contaminants at this time.
PG
There appears to be a current problem (2015) with some supplies of food grade PG that has detectable levels of MEG. This is not suitable for inhalation.
VG
The use of the term VG to describe the glycerine we use for inhalation is probably obsolete now, and stems from the early days of e-cigarette use when there was little use of Glycerine USP and DIYers had a choice between vegetable-source glycerine and animal-source glycerine [1]. If synthetic glycerol is used [2] its origin is irrelevant, as it is absolutely pure.
[1] Glycerine can be made from a multiplicity of sources including animal carcases, biodiesel manufacture byproduct, palm oil, coconut oil, a combination of vegetable sources, or synthetically.
[2] To see more info on synthetic glycerine, google 'dow optim'.
Note
Glycerine made as a byproduct of biodiesel production should NEVER under any circumstances be used for any human consumption of any kind, and especially not inhaled. It is likely to be carcinogenic because some of the vegetation used for modern biodiesel production is toxic (e.g. the Jatropha plant). How to exclude this material from the production chain is another matter entirely, and this is a matter for serious concern at present.
PEG
It is better to avoid PEG for DIY purposes if purchased 'ad hoc', i.e. not from a trusted vendor of inhalation products, as it is difficult to source in the correct viscosity (PEG400); but mainly because it is the most likely of all the excipients to be contaminated with DEG, a poison. It should probably not be used by anyone without access to a testing facility for DEG.
Clarifications
This post contains general advice about purity. In practical terms this is a mix of advice for retail buyers and for manufacturers, and may be confusing because of this. Here is a further post that explains the different issues for retail buyers and for manufacturers:
Buying glycerine retail or for manufacture
Three base liquid carriers are used for e-liquid manufacture: PG, VG and PEG - propylene glycol, glycerol (which we tend to refer to as VG or 'vegetable glycerine') and polyethylene glycol.
These liquids are all alcohols (and not oils), as can be seen by the -ol suffix of their common names. There are a multitude of both common and chemical names for these materials, as is normal, and some of the more common chemical names (more precise definitions of their nature than the common names) are:-
PG: propane 1,2 diol
VG: propane 2,3 triol
When used as the carrier for other materials, they can be referred to as excipients or diluents, especially when they are essentially inert in the human organism, as these materials are. That is to say, they have no major function or effect* and are treated as carbohydrates, metabolised, and excreted. Both PG and VG are used as the base for medical inhalers; in fact PG is used as the excipient for the medicinal inhalers used by lung transplant patients, and both PG and VG are used in asthma inhalers, so it should be fairly obvious that these materials are completely harmless for inhalation, at least in the short to medium term. There are questions about high-volume long-term use but these can only be answered after 20 years use for vaping; currently, it seems unlikely that serious issues will result since we would have seen some indication of this in asthma patients, who have of course used such inhalants for decades.
* PG is reported to act on the organism in the same way as ethanol but with around one-third of the effect. This can be ignored with the very small quantities consumed in vaping.
MSDS
The MSDS sheets for these materials can be safely completely ignored. This is an important point to make since the MSDS makes all materials without exception sound like chemical warfare agents: see the MSDS for water and for salt, for example.
It is obvious that if asthma patients are encouraged to inhale them for decades, and they are supplied without reservation to lung transplant patients for inhalation, they are regarded as inert and harmless.
The purpose of the MSDS is for bulk storage contractors, transport contractors and fire officers. In case of mass release and inundation of a worker, for example, there need to be documented hazard procedures. You can drown in a tank of these materials but that is the only practical hazard. To evaluate the real threat from a material, the MSDS-quoted dose can be checked: if it quotes a risk from amounts measured in milligrammes, such as 100mg, then there is a possible risk; 45 gallons of PG or VG are only dangerous, like water, if you fall into a barrel of it. They are used for inhalation therapies by persons in the most fragile of health and with the most serious lung conditions.
Purity
In theory, only the pharma grade of these materials should be used for inhalation, since all other grades contain contaminants to an increasing degree (the difference between the grades is the percentage of contaminants). The problem here is that DIYers and perhaps many small-scale manufacturers do not have access to pharma grade materials, so food grade materials must be used. Indeed, it is reported that some pharmaceutical suppliers are actively trying to prevent access to their top-grade materials by ecig products manufacturers.
This is more an issue of excessive regulation or fear of legal problems reducing safety than anything else.
There may not be any practical difference between the pharmaceutical grade and food grade materials from some suppliers. The only difference might even be that the pharma grade is specially tested and comes with a test certificate. However we do know that there is a difference between low-grade and high-grade materials in vaping products (though we cannot know what certificates / grades were supplied) because more tests are now showing MEG contamination - (mono)ethylene glycol - which is toxic, as is DEG. In fact it looks as if DEG contamination has been replaced by MEG contamination in the marketplace, perhaps because suppliers know to test for DEG but are unaware that low grade PG may contain MEG.
Regulation
If e-cigarettes become regulated then it is likely that only pharma grade will be allowed for inhalation duty. The advantage of this is the minimal contaminant load; the disadvantage is that prices go up. Everyone has a different opinion on this, so in the current climate, where ingredients are unregulated / untested in some countries, perhaps vendors should simply publish what their policy is: either "we use pharma grade only", or "we use food grade", or "we use food grade and also test for X and X". Then people could choose accordingly. Some sort of proof would need to be offered if a vendor makes specific statements - talk is cheap, after all.
Grades
These carrier materials are made in four grades:
- Pharmaceutical Grade - for human consumption including by inhalation
- Food Grade - for human consumption by ingestion but not inhalation
- Agricultural (Farm) Grade - for use in connection with animals
- Industrial Grade - for use in machinery, manufacturing and similar purposes
DO NOT BUY FARM GRADE OR INDUSTRIAL GRADE MATERIALS FOR INHALATION - contamination levels may be higher than allowable for safe inhalation.
There is a possibility that Food Grade should not be inhaled, because the digestive system can handle the contaminants in it that the lungs may not be able to. It is likely there will be significant differences between food grade products from different suppliers.
PG and VG are both sold in good quality and are labelled:
Propylene Glycol, USP (or BP or EU)
Glycerine, USP (or BP or EU)
It is possible that a USP product can be either pharma grade or food grade. The USP designation only refers to a general production regime and does not cover the exact contaminants as long as the minimum standard is met. So a USP product might be considered safe for inhalation; or it might, on test, prove inadvisable to inhale. The only product that is guaranteed to be suitable for inhalation is a product that is (a) designated by the manufacturer as Pharmaceutical Grade, and that (b) also has a provable license for inclusion in a specific inhalable medicine. Needless to say this is virtually impossible to locate for the individual buyer. A vendor would be able to buy e.g. Dow Optim glycerine or PuraGuard PG in quantity, for mixing down, and be assured the material is suitable. (This doesn't take into account the nicotine base carrier PG or glycerine, though.) It can be bought in 18 litre containers and up, so is suitable for e-liquid manufacturers, though manufacturers may try to prevent ecig product manufacturers from obtaining high-quality products.
As a practical example, the FDA has warned that glycerine marked USP may contain toxic components from biodiesel byproduct manufacture, and currently there appears to be no way of determining whether or not a USP product contains biodiesel byproduct glycerine. Only synthetic pharma grade glycerine can be be assumed to be free of such contaminants at this time.
PG
There appears to be a current problem (2015) with some supplies of food grade PG that has detectable levels of MEG. This is not suitable for inhalation.
VG
The use of the term VG to describe the glycerine we use for inhalation is probably obsolete now, and stems from the early days of e-cigarette use when there was little use of Glycerine USP and DIYers had a choice between vegetable-source glycerine and animal-source glycerine [1]. If synthetic glycerol is used [2] its origin is irrelevant, as it is absolutely pure.
[1] Glycerine can be made from a multiplicity of sources including animal carcases, biodiesel manufacture byproduct, palm oil, coconut oil, a combination of vegetable sources, or synthetically.
[2] To see more info on synthetic glycerine, google 'dow optim'.
Note
Glycerine made as a byproduct of biodiesel production should NEVER under any circumstances be used for any human consumption of any kind, and especially not inhaled. It is likely to be carcinogenic because some of the vegetation used for modern biodiesel production is toxic (e.g. the Jatropha plant). How to exclude this material from the production chain is another matter entirely, and this is a matter for serious concern at present.
PEG
It is better to avoid PEG for DIY purposes if purchased 'ad hoc', i.e. not from a trusted vendor of inhalation products, as it is difficult to source in the correct viscosity (PEG400); but mainly because it is the most likely of all the excipients to be contaminated with DEG, a poison. It should probably not be used by anyone without access to a testing facility for DEG.
Clarifications
This post contains general advice about purity. In practical terms this is a mix of advice for retail buyers and for manufacturers, and may be confusing because of this. Here is a further post that explains the different issues for retail buyers and for manufacturers:
Buying glycerine retail or for manufacture
