Well unless you are sensitive to a particular thing (I am PG sensitive, although to be fair, I didn't DIE of it or anything and it's not even like an allergy, my system just reads it as "inhaling pins and needles" although it appeared no real harm done once I switched.
If you pay zero attention to battery safety and wind up with severely degraded or overdischarged cells, that could be harmful.
Most evidence seems to show that ecigs are far less harmful than smoking. I was ON THE WAY to a COPD diagnosis, until I switched to vaping. My docs have only noted health improvements.
I will say, I'm not out of the woods on cancer (most vapers aren't you have to be cigarette free fro a while) or other smoking related diseases, but at least vaping has halted the damage cigarettes were doing and giving me a chance to gain health points and etc. If I wind up with cancer of some sort, I will blame my several decades of heavy smoking, not ecigs. That's my own personal viewpoint.
Read the whole article, it's basically a statement of correlation vs causation, and how they can get mixed up, etc.
Anna