The experience of vaping

It’s one of the hottest topics in public health right now. It’s divided scientists and researchers across the globe. It’s even divided the community on some aspects of it. Vaping. A consumer driven market that has enabled 2.6 million people (UK) to either reduce the harms of smoking or stop smoking full stop.

The question that rattles around in my head regularly is why is this topic so divisive and why are so many intent on heavily restricting it? To that end, I try to go back to basics, with a little help from some friends.

Vaporisers and vaping is such a complicated topic with far-reaching consequences, the many discussions I have had with a variety of people from all walks of life, including medical professionals, highlights a lot of confusion and a general lack of clear knowledge on the subject, especially with those that seek to “regulate” it. Begs the question, how can you regulate something if you don’t understand it?

We know that smoking can be harmful, and yet millions choose to smoke despite the best efforts from the hardened folks in the tobacco control industry and their associated front organisations. Smoking prevalence is stubbornly stuck at an annoying %25.4 (UK) with the various tactics, campaigns and regulations not making a significant impact on the figures. Yet now, in some areas, both in the UK and globally smoking prevalence is beginning to decline sharply. Strangely enough, this decline hasn’t been attributed to tobacco control and rightly so. In fact, according to the latest information from the Smoking Toolkit Study the use of vapor products in combination with professional support, is by far the most dominant product used for those wishing to quit smoking.

Vaporisers, e-cigarettes, personal vaporisers, “ENDS”, there’s a whole range of names for them. Why do they work, what are they, why do so many choose to use them, what is in them, are they dangerous. There are so many questions that are asked by so many people. Some of those making the policy decisions to heavily restrict vaping as we know it seem to completely misunderstand the users and the product. Those that take the time to understand us as users of the products are much more positive towards the innovations coming in the market.

So why is it so difficult to understand?

Let’s start with the basics.

What are they?

According to some sources a vaporiser is:

Of course, the devices come in a variety of shapes, sizes and configurations but the basics are effectively there. An electrical power source (battery), coil & wick, some kind of reservoir or tank and mouthpiece. Pretty much everything else, OLED displays and so forth are just fluff.

The devices work by pumping power into the resistant coil in a similar way that a lightbulb works, when power is applied to the coil it glows (especially when dry, the source of all that nasty formaldehyde). The coil gets hot and vaporises the juice-soaked wicking material to produce vapour which is then inhaled by the user. There’s a lot more to it than that, the coil can be of varying resistance from ~1.8Ω to 0.xΩ, there’s regulated and mechanical devices allowing for broad customisation and a variety of choice for the user. Not forgetting that there are different ways the vapor can be inhaled, either by “lunging it” as many on sub-ohm devices or drippers do, or the more conventional “mouth-to-lung) method traditionally associated with the act of smoking.

Vaping mimics the act of smoking. This is one of the key reasons _why it works _for so many. Granted, it doesn’t work for all, but in the UK alone (at last count) were 2.6 Million users of these devices, with 1.1 Million using them exclusively.

What is in them?

Of course, whenever someone says chemicals everyone starts to get a little panicky. Everything is made up of chemicals. Some are completely harmless, some are mostly harmless (don’t panic!), some are relatively harmless and some will only cause harm in extremely large doses. Nicotine is one such chemical compound that is terribly misunderstood, “pure” nicotine is a poison and will kill you if you ingest too much. The exact amount that will do this is based on some very old research and repeated ad infinitum to the point of it being “true”. The human body is a wonderful machine, it knows what it wants and how much of that it can handle, nicotine is no different. The body will react to expunge as much of the substance as quickly as possible, in a variety of ways.

Therein lies one of the many misconceptions surrounding vaping, that somehow nicotine is inherently evil and will kill anyone that spills a tiny drop on themselves. Nicotine is hardly benign, but in the diluted form used by vapers (and even the levels in cigarettes) poses substantially less of a risk than those working with “pure” nicotine, of course in cigarettes there’s other “chemicals” which at the levels in tobacco carry an increased risk of harm. One comment I’ve seen is that when refilling our tanks we should “wear protective gloves and clothing”; which is a bit extreme to say the least.

Apart from nicotine (which is entirely optional by the way), there’s propylene glycol, glycerol (vegetable glycerin usually) and some flavourings. There may even be sucrose, alcohol and/or water depending on what the final flavour is going to be.

PG is a very well understood compound, following extensive research in the 1940s. It has been classified as GRAS (Generally Recognised As Safe), including by inhalation, for decades1. It is used in a variety of applications, including medicines, e.g. asthma inhalers, and cosmetics, e.g. baby wipes, shampoo, and is used in anti-freeze to reduce the toxicity.

Glycerol, or as is commonly noted vegetable glycerin is another well understood “chemical”, classified as GRAS with extremely low toxicity, including by inhalation2 and is often added to e-liquids to add a little sweetness to the flavour, but is mostly used for “da cloudz, man”. Sometimes AG (Aqueous Glycerine) is used instead of VG simply because AG is diluted with ionised water to “thin” it out. VG is very thick and doesn’t always “wick” very well, while AG behaves more like PG simply because it has similar properties to VG but is not as thick. This dilution allows a high glycerol based (AG/VG) liquid to produce large volumes of vapor while still allowing the juice to soak the wicking material in a reasonably consistent manner.

Flavourings. The final part of an e-liquid. As smokers, our sense of taste is heavily muted thanks to the cocktail of chemicals being inhaled, but somehow for some reason we enjoyed smoking. The human body is a wonderful biological machine and is capable of incredible self-repair. Taste buds are no exception, albeit the full range of taste senses may not return, a large proportion does. Flavours are important. Not just in vaping, but in everyday life too. We consume flavoured beverages regularly, we eat a variety of flavoursome foods. As smokers, the stronger the flavour the better. As our sense of taste returns, we start noticing more subtleties in our everyday food and drink.

Are they dangerous?

The answer to that is purely a matter of perspective. At the end of the day, there are some risks associated with vaping. We have all seen some ridiculous stories about users “burning holes” in their lungs, or exploding devices and so forth. I guess the actual question should be are they inherently dangerous rather than just “are they dangerous”.

One of the biggest problems with vaping is there are wide variations in the quality of the batteries used.

Source – http://www.ecigarettedirect.co.uk/ashtray-blog/2015/11/winter-vaping.html

When a battery fails, either due to misuse or some other flaw, it will fail spectacularly. Tom Pruen details some battery failures, and highlights some mod-design flaws here and here.  Of course, the battery isn’t the only concern. The coil and wicking material can also suffer from degradation, usually this presents itself as a reduction in the amount of vapor generated or the taste isn’t the same.

This leaves the e-liquids themselves. We know that there are ingredients that shouldn’t be used, even if the amount present in the vapor is orders of magnitude less than the contents of tobacco smoke but have been associated with certain lung-related impairments. This area is probably the one (and only) Achilles heel as there are numerous e-liquid vendors out there with a dazzling array of flavours and strengths available, and that doesn’t touch on those that decide to make their own juices.

Fortunately, there are some relatively inexpensive tests that can be performed to help isolate, or reduce the amount of these no-no ingredients, and of course regulations and guidelines exist (or soon will do) to reduce the potential concerns further.

Why do they work?

There are a number of reasons why vapor products work, ranging from customisation options, flavour variety to the more fundamental socio-economic factors such as initial and ongoing costs.  The simple fact is, for many the ritual of smoking is ingrained. So much so that it is almost instinctual to light the first smoke of the day. Rituals and habits are generally interchanged phrases used, in general a “habit” is something (generally an action) that someone does, at specific times; such as the coffee and smoke-break at work, or the beer and smoke. A ritual is a series of motions or actions performed according to a prescribed order.

The ritual of smoking is the hand-to-mouth action, the inhale and exhale. The habit of smoking is lighting up at regular times, or being triggered by some external influence. Vaping, as a general rule doesn’t break the ritual of smoking, but it does offer a way of breaking the habit of smoking.

It is touted as “fact” that ‘many smokers wish to give up’, which does make me wonder if smokers are actually understood by those that wish they would actually ‘give up’. I suspect not, though it is fair to say some do understand.

That same lack of understanding of smokers is more obvious when it comes to vaping. As both myself and Jessica Harding noticed recently there is definite divide in knowledge on this subject with some folks in the wrong places (i.e. having far too much policy making power) having no significant knowledge on the subject, with others (like us vapers) having the lions share of knowledge and very little in the way of power. How then, do we communicate this knowledge?

Through our experiences. Sometimes it can be difficult to get a point across using facts, figures and data. Not to mention that the type of information quickly becomes a mind-numbing experience (I speak from experience on that). The trick is balancing the information in easy-to-understand snippets. This is where I sought help from others as everyone has a differing view. I asked some very simple questions of other vapers, what follows here are my thoughts, and those of Tom Pruen, Shannon, Lorien Jollye, Alan Beard, Jessica Harding and Abi Cottrill. Their comments are theirs alone and do not represent any organisation that may be or are affiliated with.

• Why do you think vaping is misunderstood?
• Are flavours an essential part of why vaping works?
• What does vaping mean to you ?

Why do you think vaping is misunderstood?

For me, vaping is misunderstood for a variety of reasons. Most of these centre around the need for those making the decisions to pigeon-hole things, the square-peg, round-hole scenario. Vaping isn’t smoking, but it is an alternative smoking-like behavior so it doesn’t fit into “not-smoking” or “smoking” in the minds of some. This is the most widespread mis-understanding of vaping, but it is linked to several others. Tom Pruen nails this in his answer:

“I think the mis-perception of vaping is the result of a whole host of interconnected reasons – most of them, in theory easily to resolve with more information:

A fundamental inability (and in many cases unwillingness) to look at vaping as a separate activity than smoking. The “it’s just another kind of smoking” attitude is particularly notable in the US, where even theoretically science driven bodies like the CDC class vaping as tobacco use. It also seems to be the prevailing though in the Welsh Senedd, sadly.

There is also an unhealthy dose of “not invented here” – if the first ecig had been produced by one of the pharmaceutical giants, the global perception would be completely different (although it probably would have disappeared without trace, given the restrictions placed on design and innovation by medicines regulations).

The biggest and most lasting factor I think, is also the one that itself is most misunderstood. Vaping appeals to people – people actually want to use these products, and are quite happy to abandon cigs in favour of vaping. This is fundamentally scary to many in Public Health. PH interventions are almost always a case of forcing people to act against their preference, for their own good. Something so popular that people seek it out for themselves and become evangelical about is not an intervention, and runs counter to the current “persuade with a big stick” thinking in PH. It’s much more suited to the nudge approach we hear so much about, but see so little of.” — Tom Pruen

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Smoking has been a “social extreme” for a really long time. Smokers are easy targets for bullying because “they’re nicotine addicts, they stink, the smoke is annoying, etc.” Vaping is the disruption to that line of thought, but non-smokers can’t see that disruption (or choose not to). Am I vaping an e-liquid with nicotine? “Oh, you’re still an addict.” If there’s no nicotine? “You’re just trading one addiction for another.” If someone sees my vapor cloud from 30 feet away, they frequently begin to have an uncontrollable coughing fit (even though it’s quite obvious that it’s a fake fit). I stopped smoking cigarettes; what more do these people want from me? — Shannon Sparkles

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I believe there are at least 2 reasons from totally different sources:

Wilful ignorance from opponents in Tobacco control, Public Health, Pharma and Tobacco Industries, who endeavour to distribute disinformation about vaping for a variety of vested interests. General population perception and antipathy has resulted from a vacuum of reliable information in the mass media, highlighted by ASH, the Public Health England report should have been the game changer in the UK, but even this was subjected to misleading initial press reports, and following two journal entries in the Lancet and the BMJ further undermining the intended clear message. Scaremongering propaganda is of course not unique to the UK,we at least have been spared the excesses of the Spanish press negativity as exemplified with the false and highly damaging lipid pneumonia scare story that has to a large extent destroyed the market in Spain. — Alan Beard

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I haven’t been vaping that long, but I’ve spent quite a lot of it pondering these issues and talking to new vapers.

The reason vaping is misunderstood is that it looks like smoking. People have been conditioned to recoil when a visible exhalation takes place. I’ve even seen it on a cold winters morning. The fact that people breathe out and exhale the smell of their lunch is something we are conditioned to politely ignore, a visible exhalation is another matter. — Abi Cottrill

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I think it’s because there’s an assumption that e-cigs are for smoking cessation and so there’s an expectation that vaping should be boring and uncomfortable and users should stop doing it once they have stopped smoking cigarettes – as with smoking cessation treatments.  This is why dual users are so frowned upon in some quarters: they have their cake and eat it.    Also, thanks to public health campaigns which precede the internet (I remember the Nick-o-teen campaign) nicotine is wrongly perceived to be very harmful.  

We’ve been conditioned to think that smoking is evil and that nicotine (the one element which is common to both vaping and smoking) is the worst part of smoking.   If snus was widely used in the UK then perceptions might be very different, as the concept of tobacco harm reduction would not be so novel and society would realise that nicotine in small doses isn’t actually bad for you.  

The mindset is slowly shifting in the UK but I think that widespread acceptance of vaping can only happen when a critical mass of people witnesses the positive effects of vaping at first hand.  This is unlikely to happen now as from May 2016 Article 20 of The Tobacco Products Directive will drastically reduce the numbers of new vapers.  In the meantime there’s a well co-ordinated media disinformation campaign which is striving to set hearts and minds against vaping.

Vaping has actually been incredibly successful when you consider what has been stacked up against it.  The tragedy is that legislators all over the world are scrambling to impose regulations which will prevent future generations from vaping. — Jessica Harding

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I wish there were a simple answer to that. Unfortunately, I think this is a case of the perfect storm of vested interests and ideology. From the public perspective, there is an element of ‘they are just smokers’ and that somehow we don’t deserve an easy way out. Years of smoker demonisation has meant that vaping has slid into people’s hatred of the sight of smoke disturbingly easy. Then you have researchers whose focus is entirely on the funding they can gain rather than the quality of the work they are going to produce with it.

After that, a media who have no sense of public responsibility or even morals, these appalling studies picked up without question and replicated the world over. Add in to this the financial interests of tobacco and pharmaceutical industries with their vast lobbying power and suddenly legislation that will damage vaping (and benefit tobacco cigarettes) popping up all over the world.

And then to support all this, are those in PH and TC who..I don’t know what to even say about them. For me, they are the worst. All sense of helping the public and protecting our health has gone out of the window and now they are causing real harm. Genuine harm that will result of years of death, pain and suffering. — Lorien Jollye

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Are flavours an essential part of why vaping works?

Flavours. As noted above, flavours are part of everyday life. Our sense of enjoyment can be linked to our flavour preferences. We have the sense of sight, smell, touch, hearing and taste. Each one paints an aspect of a much larger picture for our brains to process, triggering a variety of responses. No two individuals have the exact same response to sensory input and taste is even more divisive. As Tom notes, Marmite is the most common divider in the taste area, you either love it or hate it (I love it).

But it is far more than just divided opinion on taste and sensory input, taste brings a level of enjoyment that simply cannot be attained with tobacco smoking or via the “recommended” therapies. It allows us, the vaper to enjoy the sensations of smoking, the ritualistic motions, but also have the additional sensation of a unique flavour. It’s like comparing drinking water to flavoured water. Both have the same sensation, the difference is one is tasty while the other is, well bland and boring.

Absolutely! There is nothing quite so subjective as a ‘nice’ flavour, Marmite being the classic example. Once people realise that they are no longer stuck with minor variations on the same thing (with the exception of menthol, there isn’t much to distinguish the flavour of cigarettes) they are free to pick something they actually like. Not only does this increase the appeal of vaping directly – it makes it harder to go back. Even among my colleagues at work, there is considerable variation – we have one menthol (and menthol only!), one tobacco (but it has to have lots of biscuity flavour notes), one fruit (often with custard). And then there’s my flavour preference, which is normally RY4 – except when it’s vanilla custard or something similar. That’s an awful lot of variation within a small number of people!  –Tom Pruen

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I believe flavours are incredibly important when considering why vaping works. As smokers, we become accustomed (and perhaps even welcoming) to the taste and smell of cigarette smoke. In order to break that connection, a variety of flavours other than tobacco are essential. In my 19 months as a vaper, I’ve purposely avoided tobacco flavoured e-liquid. While I understand that everyone is different and has their own preference, vaping tropical fruit and banana nut bread has ensured my success and satisfaction from day one. — Shannon Sparkles

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My reasoning for flavours been an essential part of why vaping works, is that it is a reinforcement of the move away from the taste of tobacco. From my earliest days I deliberately avoided tobacco flavours to make the transition away simpler, so chose mint, menthol, fruit, dessert flavours and probably over a near 3 year period have bought upwards of 200 different flavours. My pattern of use involves a constant rotation throughout the day to preserve the attractiveness of the chosen flavour, so sometimes 5-6 different flavours are used daily.

To restrict this flavour choice would most certainly negatively impact me,and most likely a number of potential new switchers who I would recommend to start with a number of different flavours.

Over time I have learned how to mix my own juices by DIY, for a number of reasons not purely down to the vast cost savings, more for self-preservation if over-regulation occurs. However this is of little use or comfort to a current smoker whose choices and options could end up limited by some arbitrary ruling by clueless regulators(spurred on by Tobacco Control). — Alan Beard

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Flavour and therefore odour is absolutely key to the early transition to vaping. I would still be a smoker it there had not been a full range of flavours available. I have recently thought that I might switch to an unflavoured mix for more discrete vaping, but without flavour I could not have stopped smoking. It was absolutely key to dropping my morning cigarette. I looked for a while before finding just the right flavour to work post-toothpaste. — Abi Cottrill

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Yes, for me they are.  Not for everyone though.  I think flavours are really important but not crucial:  the people who only use one flavour or can’t taste anything wouldn’t use an inhalator instead; the appeal of vaping isn’t simply down to flavours. — Jessica Harding

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For me flavours are fundamental. Whether it is a custard, apple, tobacco or leather shoe in-sole flavours are a huge part of what keeps people vaping. There is the fun element, the discovering new flavours as your taste buds come back but I think there is something more than this which makes them indispensable. Re-association. I often say dissociation but really I think they are the same thing. We had a habit, which a very simple taste but a multitude of triggers and associations. When you take up vaping, you keep a lot of those associations and you can respond to the same triggers (unlike when you are going cold turkey or using NRT when you avoid every trigger with something close to fear) which means the habit essentially stays the same.

However, you can take one element and change your association completely and that is the flavour. Before long, if you are purely using ecigs, all those embedded behaviours become linked to a different flavour and for many that flavour is preferable. So you positively re-associate the inhale and exhale, cementing vaping as a new habit. You dissociate from the old smoking habit.

The effect of this is likely protective. If you pick up a cigarette and try it everything seems wrong, you don’t recognise it anymore. But it takes time to get this point and sadly I think many people are going to have this taken away from them before this is able to happen. — Lorien Jollye

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What does vaping mean to you ?

Vaping, as a lifestyle choice is surprisingly freeing. I can pick and choose what I want from it and when I want it. I started vaping as an alternative to smoking, I really had no idea what to expect from it, nor what I would actually gain from it. I hadn’t planned on stopping smoking, but when I realised the sensations were the same, and the variability in the devices and flavours could be tailored to me it was effectively a “no-brainer” to switch completely. I’ve also been blessed with meeting some truly interesting and fabulous like-minded characters in the community, something that, as a smoker wouldn’t be, and hadn’t been possible.

Sure, I could go to a pub or club and mingle with the smokers outside (if they decided to go out at all), but there isn’t a sense of community or fellowship in that. It felt like a gathering of the damned even though I, and those with me, enjoyed smoking. With vaping, everyone I’ve met is unique with different hobbies, interests and backgrounds but each and every one of them all share a commonality. They all seem to be kind and generous folks (there’s always going to be outliers), they all enjoy vaping and the socialised aspects that come with it. The variety of shows, meets and events that go on throughout the year. Never really had that as a smoker!

Being a bit of an early adopter (way back in March ’09), vaping was always a smoking alternative to me – getting the bits about smoking I enjoyed, without the same risk of an early death. It’s been interesting to see vaping evolve into something quite distinct, and ironically  (given my first point about vaping being misunderstood) one which I find quite unsettling, in some regards. Cloud chasing is probably the best example of this – and one that makes public places bans more likely. It’s fair to say I’m a bit conflicted about it! — Tom Pruen

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Vaping has completely changed my life. When I smoked cigarettes, I felt hopeless and ashamed every time I failed a quit attempt. The switch to vaping was completely seamless, and I never dreamed it could really be that simple. It gave me a new sense of empowerment that I’d never had before, and the confidence in my in my decision was only reinforced by how much better I physically felt almost immediately. The most important thing that vaping has done for me is give me a wonderful group of people I can happily call family. They have guided me from day one, always having an answer for me or providing advice. They have stood by my side and defended me when things on Twitter got ugly, which is a frequent occurrence. My vape family understands that I have a past full of ridiculous mistakes, but they love me and accept me for who I am regardless. I know I was destined to be a vaper because I honestly can’t imagine my life without any of the members of my vape family, and I love every one of them in their own unique way. — Shannon Sparkles

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This is a very important wide-ranging question that is very difficult to summarise in a few sentences.

• On a very simplistic and obvious level, vaping has literally transformed my life. 45 years a smoker of 40/day, the health and cost benefits are enormous.
• I was fortunate enough to retire reasonably comfortably about 12 years ago, my retirement was enjoyed in a motor home roaming around Europe and sometimes playing golf. Vaping has given me a powerful activity to focus upon and to a large extent has re-energised my existence, the sense of total injustice watching the ENVI committee meetings in early 2013 definitely was a great motivator in this process.
• A very strong sense of community between the activists exists and many new friends have been found in this common cause, but it is quite striking to me that on very many issues not necessarily vape related much common ground and interests are shared

–Alan Beard

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Vaping allows me to enjoy nicotine in a relatively safe way. I didn’t want to quit smoking because for me the benefits outweighed the obvious dangers. Vaping gives me the best of both. — Abi Cottrill

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Vaping has been very liberating for me.   I smoked for 26 years.  I tried and failed to stop many times.  I tried will power, reading and re-reading Allan Carr (Easy way to stop smoking, Easy way for women to stop smoking etc) and various NRT’s in conjunction with advice from a NHS Smoking Cessation nurse.  The failed attempts left me feeling worthless and very depressed – quit or die is essentially a very morbid idea.

Vaping addresses the best bits of smoking.   Public health campaigns have always ignored the fact that some aspects of smoking are enjoyable – possibly because the people behind them don’t get it.  With vaping you do get the best of both worlds.  Yes there is a risk but it’s a calculated one.

I don’t miss smoking now, because I know can smoke if I want to; it doesn’t have to be all or nothing.  Ironically I don’t smoke at all now because I prefer vaping.  I’d like to smoke sometimes to show solidarity with smokers – I think the current demonisation of smokers is outrageous.

Writing this has made me aware that for me vaping works because it isn’t quitting: I’m happily continuing a habit which I started in my teens but in a less harmful way. — Jessica Harding

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I am not sure what vaping means to me. I have not smoked for 3 years as of tomorrow (21st November), I don’t really feel much about that cos it has been so easy but at the same time I am amazed by it. My husband, my dad, both ex-smokers now and both as likely to have still been smoking as I would have been.

I think I am so jaded by everything that has happened it is difficult to imagine what I would have felt like had I not been involved. Maybe, if I am honest, what vaping has done for me is expose a system that has made me feel sick to my core. I love the community and I adore the people I have met and work with, but what I found behind the mask of public health, tobacco control, government and industry has shocked me to my core. I feel ill.

It is so complex and so outlandish that I try not to explain to someone asking me for the first time cos I sound like a tin hat wearing loon. But I am not, I know I am not. All of us are in the same boat. The last 3 years have been the most stressful of my life and why? Because I chose to fight for smokers to have the same option as I did.

All that said, I meet and hear of so many people who DO feel like I should. Elated, over the moon, surprised and empowered that they independently found a solution, some deliberately and some by accident. I take a lot from them, I really do. I look forward to the day I can stop crying over what is happening and just enjoy it. — Lorien Jollye

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Since putting the question out to a few people, I’ve had a lot of responses back including the results of the surveys designed by Kevin (VapingIT) and Randy Willis (VapedUpGaming). These surveys included the comments from all vapers that participated in the surveys and demonstrate exactly how flavours are important to them and why vaping works.

Have something to add?

The majority of this post goes back to basics, coupled with the viewpoints of some notable figures within the vaping community. As with any post, I encourage and welcome comments.

Finally, I would like to proffer my sincerest thanks and deepest heartfelt appreciation to those that have taken the time to provide me with the answers to three seemingly innocuous questions. I really wasn’t sure what I would get in response and, with the exception of some teeny tiny alterations, those responses are included in this post verbatim.

References

1 – SIDS Initial Assessment Profile – http://www.inchem.org/documents/sids/sids/57-55-6.pdf

2 – SIDS Initial Assessment Profile (Glycerol) – http://www.inchem.org/documents/sids/sids/56815.pdf