What dangers are related to smoking unflavoured, plain PG/VG e-liquid?

Smoking or vaping

You can't smoke e-cigs or e-cig refill liquid unless you set it on fire somehow. This is a bad idea as you will then be inhaling smoke, which leads to an elevated risk for a range of diseases.

You can vape it, inhale it, or use it - your choice of wording, as preferred. The official term is 'vape' - see the Oxford Dictionary. You cannot smoke an asthma inhaler, a kettle, or a vaporizer as there is no smoke. They produce mist. Inhaling the aerosols they produce is not smoking in any way, shape, or form.

Vapor ingredients

There are five basic ingredients in all e-cig refill liquids, listed here in descending quantity: PG and/or glycerol, flavoring, water, and nicotine. (There is always some water.) However, most of the water seen in e-cig vapor comes from the lungs - see exhaled breath on a cold morning. Water is needed to produce the mist made visible by agglomeration of water molecules by glycerine, and, less efficiently, by PG.

Flavorings may contain numerous compounds, of course. To be quite straightforward about this, e-cig vapor may contain up to 150 compounds [1]. This sounds a lot, but is an enormous reduction in several separate ways compared to cigarette smoke:

Tobacco and tobacco smoke contains 9,600 compounds identified to date [2].

The most harmful compounds in tobacco smoke are absent from e-cig vapor.

Potentially harmful compounds present in both are on average found at 1,000 times lower levels in vapour.

The compounds present in vapor have no known toxic effect at the levels present in normal vaping.

Studies that have found potentially harmful amounts of some compounds are invariably found to be fraudulent: without exception, they nearly set the device on fire to produce smoke constituents. An example of this is one study that ran a C4 head at 4.8 volts with 5-second draws at close intervals. No person could inhale the result. Describing the near-smoke resulting from this misuse as showing that vaping is toxic is just a simple fraud, perpetrated by professional liars who are well-paid by those who wish to protect smoking at all costs.

Likely harm of vapor constituents

The presence or absence of nicotine is not significant to health on average. For the average person, it has no chronic effects positive or negative. In uncommon cases, it may be advisable to avoid dietary mega-supplementation of this type - perhaps with some CVD patients. For other uncommon cases, it may be of benefit - for persons at risk of neurodegenerative or auto-immune disease for genetic reasons, for example.

Flavorings are where the current concerns lie. Investigation of these is proceeding, but the honest position is that we cannot know the health impact of inhalation of flavors until at least two decades have passed. Some flavors are clearly inadvisable to inhale, and some have a medical license for inhalation (such as menthol). The majority lie somewhere in between.

PG and glycerine are the base materials for most medical inhalers. People have been inhaling them for a very long time, with no significant effects, and they are regarded as inert. PG has been inhaled for 70 years medically. A few people are sensitive to PG or to glycerine by inhalation. Rarely, some are very sensitive to PG (in these rare cases it can cause wheezing and/or skin issues).

However, it is a different matter to inhale a material hundreds of times a day, for many decades. The impact of this type of usage cannot be measured for many decades. It will conceivably lead to COPD stage 1 (a reversible state) in some subjects. This is simple logic, as people should not inhale anything other than air, and water in very small quantities [3].

Conclusion

Vaping PG or glycerine or a PG/glycerine mix is going to be less unhealthy than vaping normal refill liquids, principally because flavorings will be absent.

A 2/98 mix looks optimal: 2% PG, 98% VG (the convention is to always place the PG percentage first, in the description of a base ingredient list for vaping). This is because vapourised glycerine is currently seen as a better option than vapourised PG for extended usage, within the medical inhaler industry [4]; but some PG inclusion seems a good idea since it is one of the most powerful aerosol bactericides and virucides known. Also, high-volume inhalation of pure glycerine/water aerosols may be suboptimal for some individuals (or in some environments). Since we are almost exclusively talking about ex-smokers here, and all ex-smokers without exception have damaged lungs, this is an issue that may be significant.

Vaping is to be avoided except for smokers or ex-smokers. It is a substitute for smoking [5], and should be seen as such rather than something else like a social or personal activity.

Here are some links to other answers where I have explained some of the issues in more detail:

Is the e-cigarette harmful to your lungs?

Are e-cigarettes that don't have nicotine unhealthy?

Update

[New question details resulted from merging two identical questions]

There have been no long-term studies of vaping because it has only been popular since 2005 when e-cigs were first sold in the UK. In late 2006 they became available in the USA. So, we have about 10 years of experience with them (in 2016). Clinical studies that examine health impact require at least 20 years, and 30 years is better (such studies do not need any kind of enrolment until the study is carried out).

As there are no studies of normal vaping, studies of discrete, less common subsets of behaviors are certainly not available. This refers to the fact that vaping is not a homogenous activity: it differs according to hardware, refills, and choices; low power indirect inhale, and high-power direct inhale. That would include your question regarding unflavoured refills.

The first reliable long-term data will be available around 2025, after 20 years; and the full story will be seen in 2035, after 30 years.

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[1] This is the professional opinion of a vapor scientist.

[2] Rodgman, Perfetti 2013.

[3] In normal life, it is impossible to avoid inhalation of significant quantities of water aerosols, either during a rainstorm or taking a shower, for example. The lungs are designed to handle water inhalation and expel it, and to expel water vapor as a normal function, and they are very efficient at this task. The lungs always contain water vapor, it is part of the local environment and processes.

The lungs are a two-way gas exchange mechanism that operates with liquid on one side of the interface, and very high humidity on the other.

[4] See Dow Chemical, for example. Search <dow optim> for resources.

[5] Search THR, Tobacco Harm Reduction.