Well, look at it like this, then: you are harming yourself about 1% compared with smoking - if that. I reckon that should fix your worries.
The things that elevate risk are the following, so you can avoid/mitigate:
- A genetic predisposition to vascular disease. This means a family history of early death from stroke or similar. If you have this, then you are in an unusual position, and need to pay careful attention to your nicotine intake.
- Using very large quantities of flavouring. About the only unknown in the equation now is the question of inhaling flavours for decades: will some of them have a long-term impact on health? We don't know. Most of them will most likely prove harmless; but some will have issues. We already know that one is highly toxic to inhale (diacetyl or butter popcorn flavour). However, all these kinds of things are dose-dependent: the effect depends entirely on how much you consume.
- Using a glycerine-based e-liquid if you have severely compromised lungs as a result of smoking (diseases such as emphysema or stage 4 COPD). This is one case where a PG liquid is a better choice. Actually Snus is a better choice.
- Don't stack batteries in a non-electronic APV. You can't buy a new face in Tesco's.
Flavours are the unknown quantity, right now. To be absolutely safe, you'd vape unflavoured base. That is probably going to be a step too far for most people, though. As a more practical measure you could simply avoid any ketone-based flavouring (stuff related to diacetyl), cinnamon, capsaicin (chilli extract) and vanilla (certain types). There are probably a few more, but all this is all purely opinion at this stage - and opinions vary. On the other hand there are some flavours that are unlikely to ever prove harmful for inhalation (people have been using them for about a hundred years for various forms of inhalation, already), but I'm not going to stick my neck out and name anything on a public forum
It's all good after that.
