Dihydrogen monoxide is used in pesticides, as a solvent in industry settings and a coolant in nuclear power plants, but you consume it every day.

That’s because it’s just water. Di-hydrogen mon-oxide: H2O.

Chemicals tend to have pretty scary-looking names, but they’re all around us, all the time. “Propylene glycol” sounds almost alien the first time you hear the word. Surely that must kill you? It’s even used in anti-freeze!

But being afraid of a chemical just because it’s used in something that seems dangerous really doesn’t make any sense. The examples above with water make this obvious.

Plus, the simple truth is that everything is toxic if you consume too much of it.

In other words: the dose makes the poison. That’s why things like nitrosamines in vapour aren’t too worrying. The amounts are really tiny.

One of the most cited studies by supporters of vaping – for exactly this reason – is Igor Burstyn’s review of the evidence on the chemistry of e-cigarettes.

Burstyn set out with a simple goal: to compare the levels of chemicals detected in vapour to how much of them we class as acceptable in the workplace.

His study showed that most of the chemicals people say we should worry about in vapour are only in levels so low they’re unlikely to do any harm.

However, in the section on PG, Burstyn pointed out that:

Estimated levels of exposure to propylene glycol and glycerine are close enough to [workplace limits] to warrant concern.

His paper did use “worst-case” assumptions, but we can't ignore this point entirely. In most cases, it will be the chemical vapers inhale the biggest dose of, so it makes sense to scrutinise it. If PG causes health problems, vapers will be likely to get them.

So is it dangerous to inhale? Should vapers really be concerned about PG?

To answer this, here’s an update of our older post on this topic. The aim is to give a clear run-down of what the available evidence says about inhaling PG.

A Primer on PG

PG is widespread.

You almost certainly come into contact with PG, even if you don’t vape or spend time near people who are vaping. It’s used in everything from pharmaceuticals to food and from cosmetics to (low-toxicity) anti-freeze.

The chemical itself is technically an alcohol, since it contains two oxygen-hydrogen “hydroxyl” groups. It’s colourless, clear and near-odourless.

It’s “generally recognised as safe” by the US FDA, and has many useful properties. It’s a good solvent (things can be dissolved in it), it attracts water, and it has low toxicity and low vapour pressure.

These properties make it a great choice for use in various consumer products, including e-cigarettes.

However, the “generally recognised as safe” designation relates to ingestion, rather than inhalation. And the difference between the two could be very relevant.

When it comes to eating or drinking reasonable amounts of PG, there’s no cause for concern, but what about inhaling it?

Also see: Propylene Glycol vs Vegetable Glycerine: Five Key Differences

Animal Studies on PG Inhalation

A lot of the older evidence on PG comes in the form of studies conducted on animals. It's hard to draw firm conclusions based on animal studies (unless you’re mainly concerned about whether you should let your pet rat vape). But they still offer an indication of the likely risk.

There have been several studies of PG inhalation in rats. These range from just four days of exposure to 90 days or longer. Other studies have looked at rats and dogs, each for about a month. One study also looked at the effect on dogs over the course of nine months.

The results show that PG doesn’t pose a big threat to animals when inhaled. There were some small differences, though.

For example, rats have sensitive eyes and noses, and the irritation from PG led to some bleeding in one study. Also, female rats lost a little weight in another study.

Also, there were some small changes in haemoglobin and red blood cell levels in the dogs exposed the most. But just because you can detect a statistically significant difference in one or two specific things in a lab doesn’t mean it really matters in practice. In this case, the levels were still within the normal range.

Finally, examinations of the animals’ organs showed no differences between those inhaling PG and air.

One long-term study looked at the effect of high concentrations of PG on monkeys and rats.

The animals inhaled saturated PG vapour for between 12 and 18 months, but when their organs were examined, there was no apparent effect of the PG. A detailed look at their lungs found no sign of irritation of breathing the vapour. The only difference related to PG was some weight gain in the exposed rats.

Evidence like this is used to support the use of PG in inhalable medicines. Simply put: animal studies show no risk from inhaling PG, even in high concentrations.

Finally, since cigarettes also contain PG, it’s been studied with reference to that too.

Rats inhaled smoke either produced with PG-containing cigarettes, VG-containing cigarettes, cigarettes with both or ones with neither. Although there was plenty of damage from the smoke, the PG and VG seemed to make no difference.

Studies like these are used to support the use of PG in inhalable medicines.

Simply put: animal studies show no substantial risk from inhaling PG, even in high concentrations. There may be irritation and other minor effects, but overall it isn’t very harmful to mammals or other primates.

Is PG Safe to Inhale for Humans?

So some animals could inhale PG – at least for a couple of years – with minimal issues, but what about us humans?

The amount of evidence in this area is limited. The available studies are looking at things like at exposure at work and the potential use of PG mist in medical settings.

A short-term study on 27 non-asthmatic volunteers exposed them to PG in a flight simulator. This involved quite a lot of PG, but they were only exposed for one minute.

The results showed that the effect of PG was minimal. There was some irritation of the upper airways, some coughing and a slight decrease in one measure of airway obstruction.

Because PG is anti-bacterial, several studies have looked into its use for these purposes. The most striking example is a study which pumped vaporised PG into a children’s ward of a hospital. The PG reduced infections, and had no reported negative effects.

Since PG is used in some inhaled medicines, there have been some studies looking into its safety for that. One example compared a higher-PG formulation of a nasal spray with a reduced PG version over the course of four weeks, and another study compared them for six weeks.

The higher PG version led to more throat irritation, and burning and stinging of the nose than the lower PG one. Like the other studies, it shows that PG is an irritant, but not much else happens when people inhale it over the short term.

Studies on PG in Theatrical Fog Machines

The best evidence we have on the safety of PG for vaping outside of vaping itself is from theatrical fog or smoke machines. The mixture used in these machines almost always contains propylene glycol.

Even though other glycols (or even oils) can be used too, the evidence on them can be roughly applied to vaping. Because they work in a similar way, the minor contaminants in vapour are also found in fog machines. Additionally, people are exposed in bursts rather than continuously in both cases.

The main difference is the obvious. We directly inhale vapour, but those exposed to fog machines inhale it passively. The people closest to the fog machine inhale the most, as you’d expect, so these are likely to give us the best idea about vaping.

The other issue is that vapers inhale PG whenever we're awake, but people exposed to fog machines only encounter it at work. So this evidence isn’t perfect, but it’s still more relevant than other non-vaping studies.

One study looked at 101 people who worked close to fog machines and found results similar to the previous animal and human studies. In this case, though, those who worked closest to the source of fog had slightly reduced lung function. Wheezing and chest tightness increased with the workers’ total exposure over the previous two years. Coughing and dry throat were also reported as short-term effects of exposure to the PG fog.

A bigger study looking at actors exposed to theatrical fogs followed 439 people for two years. As well as looking at whether they developed relevant symptoms or lung problems, they also looked in detail at how much the actors inhaled.

The results showed no significant changes in lung function or the vocal cords for those exposed to glycols. Also, exposure to glycols wasn’t associated with increased rates of asthma. But those exposed to the highest levels were more likely to report nose, throat and breathing-related symptoms. These actors also had some inflammation of the vocal cords.

Researchers from the National Institute for Occupational Safety and Health (NIOSH) also looked at the issue in the early 90s. Overall, they found similar results to the studies referenced above. They concluded that there is no evidence that theatrical fogs cause asthma, but they can have irritant effects. As a result, they said it’s prudent to avoid direct exposure wherever possible.

They also offered some recommendations relevant to vaping (though obviously not intended for it):

“The glycols used should be at the level of ‘food grade’ or ‘high grade.’ Glycol-based systems should also be designed to heat the fog fluids only to the lowest temperature needed that achieve proper aerosolization. This would help to avoid overheating the fluid and minimize the generation of decomposition products.”

For vaping, this means that e-juice should only use high-quality PG, and that overheating is best avoided. This is pretty much true for vaping anyway. Food or pharmaceutical-grade PG is always used by reputable manufacturers, and overheating e-juice produces such a horrible taste that all vapers avoid it.

Vaping Studies Addressing PG

But really, we’re talking about vaping. So, unsurprisingly, the most relevant evidence on PG for vapers is the stuff directly related to vaping.

Long-term evidence isn't available, but these studies do address exactly what we're interested in. There are other factors at work in these studies too (e.g. flavourings, nicotine and contaminants), but PG always plays a key role.

In general, the studies on vaping agree with what you’d expect based on the existing research on PG. In clinical, cell and animal studies, the main short term effect of PG-based vapour is inflammation.

People enrolled in quitting studies often get coughs and dry mouth (as many vapers will have experienced themselves). These usually fade with time.

Cell studies show inflammation and some cell death, but the toxic effect of vapour on cells appears to be more related to chemicals other than either PG or VG. Based on what we know, inhaling PG through vaping is just going to be the same as inhaling it any other way.

Formaldehyde and PG

PG is implicated in the formaldehyde issue, so it’s worth addressing here too. PG can break down into formaldehyde, depending on how much it’s heated. It only happens a tiny bit under normal conditions, so vapers in the real world don't need to worry. But several studies have shown that cranking up settings to the extreme can increase this quite a lot.It isn’t a concern in practice because formaldehyde tastes disgusting. It might not stand out as part of the horrible taste of smoke, but it's very noticeable in otherwise delicious vapour. When you get a “dry puff,” you know about it, and change your settings or re-soak your wicks immediately.

What Are the Risks of PG for Vapers and Bystanders?

So what does all of this mean for the likely risks of PG in e-liquid? Should vapers be worried?There are two issues we need to look at to work out if PG from vaping is likely to pose a risk to vapers or bystanders. First, the amount of PG expected to pose a significant risk when inhaled. Second, the amount of PG released from vaping.

Workplace Exposure Limits for PG

The amount of PG you can inhale without significant risk isn’t exactly clear-cut. What’s a “significant” risk? Exactly how much inhaled PG would lead to this risk? Answering these questions is difficult, especially when evidence is limited.

But people looking to establish workplace limits for chemicals consider these issues closely. Regardless of your job, there will be some chemical exposure that could theoretically pose a risk to your health.

The job for the people who establish the limits is to draw a line somewhere. Avoiding potentially harmful chemicals altogether is basically impossible. Chemicals are all around us, and you’re exposed to small doses of harmful ones pretty much all the time. They can’t make the workplace absolutely risk free, but they need to establish reasonable levels to prevent significant risks to workers’ health.

They base these decisions on the available evidence, but tend to build in a margin of error too. So even though they don’t apply directly to vaping, their conclusions serve as a useful guideline for our purposes.

The limits present a maximum amount that workers can be exposed to in the air without serious risk, averaged over an eight-hour day. This means they do assume 16 hours unexposed, and two full days unexposed at the weekend. Vapers don't have this unexposed time, but the limits still give a ball-park estimate of the risky quantities.

In the UK, Workplace Exposure Limits (WELs) for chemicals specifically address inhalation. For PG, the WEL is 474 mg per cubic metre for total vapour and particle content and 10 mg per cubic metre for particulates alone (p25).

The US Threshold Limit Value (TLV) is 10 mg per cubic metre too, as quoted in Igor Burstyn’s review. He points out that this is the default, precautionary limit set for organic mists not known to be toxic.

He also references the exposure limit recommended in the Netherlands: 50 mg per cubic metre. This is the most up-to-date assessment, and is exclusively based on available evidence. The other guidelines are more cautious, but the Dutch guidelines are only as protective as is justified by the data.

Risks of PG for Bystanders: Will PG Cause “Passive” Vaping?

Before we address the risks to vapers themselves, it’s worth looking at the risks for bystanders. After all, we choose to take the risk, but people around us don’t.

The amounts of PG detected in passive vaping studies varies. They've ranged from being completely undetectable right up to 2.2 mg per cubic metre (after one hour of sampling). Several studies (here, here and here) have found PG in the air at levels of between 0.1 and 0.4 mg per cubic metre.

This gives a clear picture of the likely risks to bystanders. Even the most PG detected in passive vaping studies is over four and a half times less than workplace limits. Using the Netherlands’ limit, it would be almost 23 times lower.

For bystanders, the fact that these limits only address exposure during working hours isn’t too important. If you live with a vaper but don't work together (or vice-versa), you’d spend around 8 hours exposed each workday. This is just what the limits are designed for. The weekend would be “extra” exposure, but aside from this, the workplace limits apply quite well.

Should Vapers Be Worried About PG Exposure?

The only thing left to consider is what the likely PG exposure means for us vapers: should we be worried about PG?

Igor Burstyn worked out a vapers' daily PG exposure in his paper. He calculated that in the worst-case situation (vaping 25 ml of 95 % PG e-liquid per day – an unlikely feat), levels of PG in the inhaled air could reach the equivalent of 6 mg per cubic metre.

This was calculated based on a day's worth of vaping crammed into 8 hours. The figure is just how much PG would need to be in the air surrounding a vaper for 8 hours to lead to the same daily intake. In other words, he converted the vaping exposure into a workplace-like exposure to allow comparison. For less extreme assumptions (5 ml per day of 50 % PG e-liquid), it would work out to around 1 mg per cubic metre.

As Burstyn comments, these are below the most stringent workplace limit. But they are also close enough to warrant some concern. Here, the two unexposed days assumed by workplace limits are also relevant. Vapers will be exposed every day, so it’s not completely applicable.

One of the studies on people exposed to fog machines at work involved average exposures of 0.7 mg per cubic metre of air, ranging from 0.02 to 3.22 mg per cubic metre. This study found some negative effects on lung function, albeit small ones.

This is much more like a vapers’ average exposure (and again includes some days unexposed), suggesting that there will be some risk from vaping PG. However, other glycols are also included in the fog machine mix, so these could be contributing to the health impacts too.

Also, the bigger, longer-term study on the risks of fog machines didn’t find any impacts on lung function. This was for actors inhaling around the same amount of PG (0.73 mg per cubic metre) on average. The only negative effects seen in that study were for those exposed to higher concentrations for short periods of time. Levels for these times were an average of about 26 mg per cubic metre.

So, what does this mean? Since vapers’ daily exposures are within workplace limits, and the studies looking at people exposed to fog machines found no serious health impacts, it appears that PG isn’t much of a concern after all.

But there's another side to the story. One study did find impacts on lung function, and the difference between directly inhaling and being passively exposed means that it still could pose a risk to vapers’ lungs over the long term. (Update: A more recent studie by Professor Polosa appears to show no harm to lungs from vaping.)

There is more to consider in this case, though. For vapers, absolute risk (i.e. compared to nothing) isn’t really as important as relative risk (compared to smoking). With that in mind, the risk from PG really pales in comparison to that of smoking. Public Health England’s report on e-cigarettes confirmed that vaping is far safer than smoking. For most vapers, this is the most important factor.

PG isn’t Perfect (But it’s Better Than Smoke)

The evidence on PG is pretty encouraging. Animals seem to tolerate it well, and the effects seen so far in vapers and humans exposed in other situations are mild. Also, most vapers also won’t exceed existing workplace maximums for PG exposure. In short, the risk isn’t likely to be particularly big at all.

However, studies on fog machines suggest there probably is some risk to PG inhalation. It might not be a huge risk, and it may be dwarfed by the risk from smoking, but the risk does exist, nonetheless.

And then there’s the issue of the lack of evidence for the long-term safety of vaping. This is brought up a lot, but it is important. PG has been around for a while, but long-term safety studies at vaping-like levels of exposure are still thin on the ground. To a certain extent, we’re still not completely sure of the risks.

This shouldn’t really have much of an impact on your view of vaping, though. Nobody claims e-cigarettes are completely safe, and you'd expect PG to be partially responsible for any risks that do exist.

If you want to reduce the potential harm from PG inhalation, you should minimise the amount of it you inhale. For example, sub-ohm tanks vaporise more juice per puff and so will expose you to more PG. This is especially true because vapers often reduce their nicotine level when they get a more capable atomizer that delivers nicotine more efficiently.

As Dr. Farsalinos suggests, it’s better to vape less with higher nicotine than to decrease your nicotine and vape more often to feel satisfied. So to minimise the risk from PG, it’s probably best to choose a more capable vape tank but not reduce your nicotine to compensate; reduce your number of puffs instead.

If you don’t want to take any risk at all, the best advice is to stop vaping. But if you don’t mind a small amount of risk, then the substantial improvements over smoking should be more than enough to set your mind at ease. PG isn't an angel, but it probably isn't going to kill you either.

Also see:

https://www.ecigarettedirect.co.uk/ashtray-blog/2017/02/vapers-pg-allergy-sensitivity.html