How harmful - not yet known. Compared to cigarettes though - at least 95% safer according to PHE and for most of us, ex-smokers, that's the beautiful thing
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We still get nic but we don't get the problems that are known killers with cigarettes.
One of the biggest ones for me is the lack of DNA mutations observed in lab tests with vapour vs smoke. Zero mutations observed in tests where cells where exposed to very high levels of vapour for a long amount of time. Cell mutations lead to cancer and it is this side effect of smoking that always had me the most worried. Of course as a smoker of 27 years I could still get hit with this as once a cell's DNA is damaged then this can lead to it turning cancerous at any point - but at least with vaping I am not introducing more cell damage and increasing the likelihood of cancer in the future. Have a look at this from 2016:
https://www.planetofthevapes.co.uk/...using-dna-mutations-absent-in-vape-tests.html
In terms of the unknowns of vaping - how exactly vaping can be damaging us - here is a good on on the effects on the lungs -
https://www.planetofthevapes.co.uk/...2-09_polosa-s-no-negative-harms-findings.html
Popcorn lung - not much chance of this being an issue - and certainly no finding of it so far and some people have been vaping for quite a while now. The industry is also working more to remove the components that are thought might cause this so I expect the risks for this will decrease for vapers over time.
There may be things that we haven't considered so far that may be damaging us long-term but we'll see. If you are risk averse then don't vape. Or go outside. Or have gas or electricity in your house. Or eat food past its sell by date. Or breathe unfiltered air. In fact we might as well all kill ourselves now just in case something happens to us in the future