Hi Sachiaiko, you'll find multiple articles and papers saying that metallic nanoparticles may be a problem, and an equal number that say there isn't a problem.
Medics are divided on the issue although the split does not appear to be among experts in this field: the specialists in rare lung diseases or workplace air quality issues who are the only people with a valid opinion on this topic - the split appears to be more along 'party lines' according to if the medics are generally supportive of vaping or not. In other words, the anti-vaping medics say there could be an issue, but the specialists have not identified any problem (and they are the only people worth listening to here), and the pro-vaping medics say that if the specialists can't find an issue then it's probably not worth worrying about.
Since all the links on this topic we have fall into one of those camps, I just gave you the short version. You can easily find these materials by searching the web or PubMed, using search terms such as: ecigs, e-cigarettes or vaping; together with: nanoparticles, metallic particles, air quality, particulate matter, vapor quality, secondhand vapor, etc. By: Burstyn, Polosa, Phillips, Etter, Siegel, Bates. (These are all pro-vaping medics and advocates, but I cannot even think of an anti-vaping medic who is not also associated with propaganda and even lies; and if someone is prepared to lie in order to suit their agenda then it tends to put the validity of everything they say in doubt).
In that camp you will find Goniewicz, Benowitz and many others. They have certainly done valuable research, but a proportion of it clearly has a commercial agenda, so it's hard to work out what is valid or not. You can disregard work by researchers such as Talbot, who are paid by pharmaceutical corporations to attack vaping (as pharma depends for a significant portion of its income on smoking-related disease).
You can completely disregard any statements by or research published on behalf of the FDA or CDC, since - at least in the vaping-related area - their materials have more blatant lies per square inch than all other fields of medicine and public health put together. They appear to work exclusively for the pharmaceutical industry, not the public.
So the short version is:
- There is no valid issue currently identified by experts in the precise topics under discussion.
- This does not mean that something may not turn up later, after decades of experience (we only have 8 years' experience of vaping to date, and only about 50 million user-years, and neither of these is enough to definitively state there are no problems).
- Vaping is not completely safe - it can't be, since we are only built to inhale air along with airborne particulate matter encountered in a forest; it is just a lot safer than smoking.
- You can decide yourself what the safety improvement is, since the experts can't agree; start at around 1,000 times safer than smoking and you probably won't be far out [by more than one order of magnitude anyway

]. This still means someone somewhere will die from something that is vape-related. We all take the bet it won't be us, as the odds for smokers are a whole lot worse.
There is only one proven safe and effective alternative to smoking, and that is Swedish Snus. It is proven (not simply demonstrated or posited) that the health outcomes for smokers who switch to Snus and those who totally quit are the same. That is to say, it doesn't matter if you totally quit smoking and nicotine, or instead go on to become a Snus user, then your health issues will statistically be the same (meaning on average there is no difference). For more info see the work of Foulds, Rodu, Phillips, PN Lee, Lee & Hamling, and analysis by Bates and others.
Obviously, we are all betting (or hoping) that vaping turns out to be as safe as Snusing. Neither are safe; someone somewhere will experience a medical condition as a result; but we already know, as far as Snus is concerned, that the effects are statistically invisible.