Is vaping harmful to us?
Dow Chemical is currently persuading their pharmaceutical industry B2B clients to shift from using PG (propylene glycol) as the most often used excipient in inhalable medicines and therapies to glycerol (glycerine, glycerin, VG) instead, with their product Dow Optim. This is a pure, synthetic glycerol.
They would not be doing this unless they believed that glycerol (propane 2,3 triol) has fewer implications for health, and, probably more importantly here, less minor irritation issues than PG (propane 1,2 diol), which has been inhaled medically for 70 years and is regarded as inert for inhalable (it has been the principal excipient used in asthma inhalers and lung transplant patients' nebulizers for decades). In other words, they think glycerine, if it can be manufactured as a pure material, has less potential for any kind of issues than PG; and reduction of minor throat irritation (the drying-out effect) is likely to be a factor.
Someone asked me if pulmonary edema might be an issue with glycerol (aka 'wet lung' or ARDS), so I asked Prof Polosa, who is the expert on this and similar issues [1]. He stated that no case of it had been noted after at least 10 years of use, so the possibility is very low or non-existent. He also pointed out that a suggested association with lipoid pneumonia was not possible either, since these materials (glycerol and glycols for inhalation duty) are alcohols, and medical opinion is that it is impossible to associate inhalation of alcohols with lipoid pneumonia.
So it appears that glycerine/glycerol is safe to inhale. That is to say, it is GRAS (USA), Generally Recognised As Safe, and AS (UK), Acceptably Safe, according to the classification system used by the pharmaceutical regulation bodies FDA and MHRA. Otherwise, these materials would not have a safe record of up to 70 years of inhalation use (PG) and at least one decade (glycerol) in medicinal inhalers; would not have universal pharmaceutical licenses for inhalation; and would not be free of contra-indications or significant cautions for use, as they are; and would not be promoted for pharmaceutical inhalation purposes by Dow and others.
------------
Vaping
Many people asking this question will be doing so for information about vaping (electric/electronic vaporizer or 'e-cig') use. The information above is equally applicable to medical or consumer inhalation products.
------------
Cautions
1. Emphysema patients
I have one minor concern in this area: inhalation of '100% VG' base refill liquids for electronic vaporizers by ex-smokers with severely compromised lungs. There appears, to me, to be a small possibility that all-glycerine refills may not be the optimal choice for ex-smokers with lungs already seriously damaged by smoking: those with emphysema or COPD. Emphysema patients are at risk for (regular) pneumonia, and such patients have a known risk of contracting pneumonia and subsequently dying. Emphysema may or may not be diagnosed as COPD [2]. Because this known risk exists, and because we know of two cases of ex-smokers with damaged lungs being reported as having presented with pneumonia after (self-reported) smoking cessation and vaping initiation, and because I have reason to believe that one of these cases was associated with an all-VG refill, it seems to me that caution is indicated.
While we know simply from the statistics (2 cases in 25 million vapers) that such incidents are not just rare but extremely rare, nevertheless these are the first and only cases of morbidity of any type associated loosely with vaping. Even though the actual causality is smoking and COPD in some form, caution seems valid.
Since PG is used as the base for medical inhalers given to lung transplant patients, and since we know it is one of the most powerful aerosol bactericides and virucides known (and reportedly kills all airborne pathogens at 0.5 ppm), perhaps a 2% PG inclusion in otherwise all-glycerol based refills will be of benefit to someone who might be in a position to be considering this matter. The resulting mix can be written 2/98 PG/VG according to the convention, which places the PG content first, as in 70/30 PG/VG (it is not correct to write the largest component first - the PG content should be placed first, to avoid confusion).
Emphysema and COPD patients must without fail report all lifestyle choices in this area to their pulmonary consultant.
2. Inhalation of unnecessary materials
Only smokers should be contemplating inhalation of vapor products. There is likely to be in the region of a 1,000 times reduction of risk when switching from smoking cigarettes to vaping (plus or minus one order of magnitude); but if starting to vape without smoking, the elevation of risk is not advisable even though it may not be high when using low-power low-flavour set-ups. Like drinking coffee, vaping cannot be harmless.
3. Abuse
Any consumer or medical product can be abused. This means to use it to excess or outside of normal parameters. These products lend themselves to variable power and quantity adjustment. In particular, they can be over-driven to create high temperatures and high refill usage; for example, by the use of high power or high-temperature settings, the refills can be subject to 300 degree Celsius (572 F) heat exposure. Currently, we do not know what the health impact is from the consumption of thermal degradation products that may result from this process. Such products are likely to be different with PG only, VG only, or a mix.
A benchmark device such as the eGo runs with an atomizer temperature of around 70 C (158 F). It may be possible that a sensible temperature maximum could be a set of around three times this value: 200 c (392 F, suitably close to 400 F for machine settings). Vapers using TC (temperature control) settings may wish to consider setting TC max to 200 C / 400 F as a way to limit risk. Setting TC to 600 F is essentially the same as having no temperature limit. Aldehydes and other thermal breakdown products will be seen in increased quantities above 200 C. These compounds are not likely to be beneficial or innocuous in the human organism, and the pyrolytic aldehydes may be associated with nicotine dependence.
Pure nicotine has zero potential for dependence: it is impossible to clinically demonstrate it in never-smokers, no matter how large a dose is administered daily for however long in clinical trials. Nicotine requires potentiators to create dependence, and some of these high-temperature compounds appear to fit that requirement as they are closely linked with the MAOIs present in tobacco smoke. Everyone knows that vapers' dependence on nicotine reduces exactly as it does for ex-smokers, but continued consumption of some of these high-temperature compounds may reduce that effect.
[1] Prof Polosa of Catania is almost certainly the leading expert (if not the only real expert) - no one else is actively investigating and publishing in this area, as he is while being a pulmonary airway obstruction disease specialist.
[2] It seems there may be some 'crossover' between the diagnosis of emphysema and COPD, in some places; therefore if there is any chance of any confusion, the safest path ought to be considered.