Electronic cigarettes may not be harmless but they are clearly much less harmful than cigarettes
Public Health England (PHE) produced a review of e-cigs in the middle of 2015 which came to this conclusion:
The role and impact of electronic cigarettes has been one of the great debates in public health in recent years and we commissioned this independent review of the latest evidence to ensure that practitioners, policy makers and, most importantly of all, the public have the best evidence available.
Many people think the risks of e-cigarettes are the same as smoking tobacco and this report clarifies the truth of this.
In a nutshell, best estimates show e-cigarettes are 95% less harmful to your health than normal cigarettes, and when supported by a smoking cessation service, help most smokers to quit tobacco altogether.
From a public health perspective where the goal is minimising harm, this is a major and significant conclusion. E-cigs help people quit real cigarettes and they are inherently safer (though whether you should take the back-of-an-envelope 95% number seriously is a subject of much contention.)
This is unsurprising from a chemical point of view. Nicotine is the active ingredient of cigarettes, it provides the key physiological effect, and it is somewhat addictive. But it isn't what causes the harm: that would be the thousands of other chemicals associated with it when tobacco is burned. This include polycyclic aromatic compounds and tars which are both known to be potent carcinogens. This is well known, Wikipedia-level knowledge. E-cigs don't contain most of those harmful ingredients and the mechanism of inhalation doesn't create them (some are produced by burning).
Some would argue that nicotine itself is a poison. After all plants produce it to deter animals and insects from eating them. But then, so is caffeine.
So it looks as though chemical intuition would support the PHE conclusion.
But a recent study (mentioned in the question) suggests e-cigs are carcinogenic. This headline-friendly study was rebutted in The Guardian by Linda Bauld, a professor of health policy (my emphasis):
...the key issue for this current study of e-cigarettes is not whether extensive and prolonged exposure to e-liquid vapour (of a duration and intensity that wouldn’t occur in human use) changes human cells, but rather what the e-liquid was compared to, and what this can tell us about the relative harm of tobacco smoking compared with e-cigarette use. The authors claim their study shows that e-cigarettes are no safer than tobacco, and experienced science editors in newspapers were quick to reproduce these claims without careful scrutiny of the original article.
In reality this study tells us little or nothing about the safety of e-cigarettes compared to smoking...
In only one small part of the article, not covered in the press release and not picked up by the media, do the authors mention that they also exposed some cells to tobacco smoke... Yet the authors could not directly compare the cigarette and e-cigarette treated cells, because the cigarette treated samples all died within 24 hours. Cigarette smoke was so toxic that the cells did not survive beyond this short period, whereas the e-cigarette cell lines were topped up with e-liquid every three days, and the testing continued for several weeks.
...Indeed an alternative headline for the press release, as a colleague from a cancer charity has already pointed out, could have been ‘cells can survive for 8 weeks in e-cig liquid but only 24 hours in cigarette extract’. In other words, if we compare e-cigarette vapour with fresh air we find the presence of some toxicants, as previous research has done. But it seems from the results of this study that if we compare e-cigarettes with tobacco smoke, e-cigarettes are safer.
If we look at the comparisons actually being done in tests, even the ones that are used as evidence to cast doubt on the safety of e-cigs, the actual conclusion we reach is that they are much safer than real cigarettes.
Supplement: why there is a controversy
But not everyone agrees. The PHE report was heavily criticised and understanding the debate provides useful insight into why there is a great deal of fear, uncertainty and doubt around its conclusion but also why it looks like a good judgement call on the current science.
The response to the PHE report was led by this article in the BMJ. Its key criticisms were:
Public Health England’s endorsement of the safety and efficacy of e-cigarettes is based on uncertain evidence
The quality of evidence that e-cigarettes help smokers to quit is weak
Recent evidence questions the conclusion that e-cigarettes are not a gateway to smoking
Until better evidence is available public health strategies should follow the precautionary principle
The tone of the article, especially that last point, suggests that the right standard is not harm reduction but absolute proof that e-cigs are completely safe. The critique also made the strong claim that some of the evidence relied on by PHE was based on studies containing serious conflicts of interest (CoI). In responding to this critique, David Nutt, a renowned expert on harms for legal and illegal drugs said (my emphasis):
The justification both commentaries use for this claim is not the data we reported but the fact that a few of the experts in our group of twelve had declared potential conflicts of interests (CoIs) in relation to tobacco harm reduction products. Also it was implied that the funder of the MCDA project, EuroSwiss Health, was paid by the tobacco industry. These claims were publicly refuted by those named straight after the Lancet editorial [where the results were previously discussed], so why did the BMJ article continue in the same vein?
We suggest that in repeating these allegations the BMJ authors were following a sadly all-to-common path for those who disagree with research conclusions to use CoI claims to undermine confidence in the findings rather than to critically analyse the data. A charitable view could be that they in fact can find no fault with the data (other than they dislike the conclusions), so have no alternative but to attack the scientists who generated it.
Another response pointed out that the critique of the PHE report was ideological:
McKee seems to start with the view (in not just this piece but in many others) that we must crusade against anything associated with the evil tobacco industry. If the industry likes it, we should oppose it. Other players in the debate (including that of Public Health England) adopt a more pragmatic position that the focus of tobacco policy should be harm reduction.
You don't actually need to look at the quality of evidence to know how it will be interpreted, you just need to know which camp the author is in.
The ideological position of fighting a crusade seems to lead to position where every weakness in a piece of evidence is interpreted as industry spin, conflict of interest or a damning fundamental flaw. All things not proved to be harmless should be opposed even if they clearly reduce harm dramatically compared to the alternative. The slightest hint of e-cigs being a gateway to tobacco use is proof that they are even if the clear weight of evidence suggest this effect is insignificant.
But ideology does not make a good basis for rational policy assessment. And it encourages exactly the same unscientific tactics as the evil tobacco industry adopted to try to deflect the evidence of harm when it first appeared. McKee et. al. demand absolute proof of safety and seek every possible piece of uncertainty and doubt on the existing studies. But their standard is a poor basis for health policy (harm reduction not absolute harmlessness is surely a better position) and exacerbating every flaw in studies is exactly how the evil industry tried to minimise the evidence of harm. Just because you are on the side of the angels doesn't mean this tactic is a sensible scientific way to build public health policy.
These two responses do a good job of summarising the debate. A key point is the issue that absolute safety is a distraction and better safety than cigarettes is more reasonable (the evidence is pretty clear on this point: e-cigs are much safer).
Critics are not showing that e-cigs are as bad as cigarettes; they are arguing that they are not absolutely safe. This alone explains why there is so much FUD on the topic.