In Ecigs102, I pointed out that flavors have a whopping number of ingredients, and can be dangerous. So you kinda need to keep an eye on emerging issues.
Here's one that was news to me – cinnamaldehyde, a compound found in cinnamon and cola flavors. In fact, it may be the component of cola flavors that clouds plastic, as mentioned in Ecigs102. The article I read, in Science News, March 5, 2016, said these compounds suppressed the ability of immune cells in the lungs and bloodstream. I cross-checked this on the Internet and found this thread on the ECF, that reports some people having really dangerous reactions to cinnamon, up to and including visits to the emergency room.
Gut feel: mixing cinnamon flavors is not a good idea.
The Science News article reported several other "scary" things. But I poked into those and didn't find the arguments very compelling.
1. "Lung and nasal tissue of smokers – and especially vapers – may be more susceptible to any kind of infection."
Well, I'd research that more if I were concerned. But I'm not very prone to infections. And I believe smoking depresses your lungs for a half hour after a cigarette.
2. Ecig vapors increased plaque buildup in arteries. (And thus harm your cardiovascular health.)
The mechanism of this is supposedly formaldehyde and acetaldehyde in the vapor. The thing is, you only get that from using an e-cig incorrectly, too high voltage or getting a dry puff (which tastes awful – that's you, tasting this bad stuff). Moral of the story: if it tastes yucky, don't vape it. I think we knew that.
One of the reasons I read up on this is because of the latest fad of temperature-control devices. I saw somewhere that it was inspired by avoiding formaldehyde. But, apparently not.
There was also a long passage about the researchers exposing pregnant mice to ecig vapors. Especially with zero-nicotine vapors, this seemed to make the babies hyper and possibly caused brain changes. But these were the same researchers who generated vapors containing all that formaldehyde.
Anyway, I could be wrong. But the main take-home message I got from the scary article was, don't vape cinnamon. Or cola, which contains cinnamon.