To be as safe as possible, avoid Diketones. Specifically diacetyl, acetyl-propionyl and acetoin. These are usually found in custard flavors (creamy desert type stuff) and some fruits. Most vendors have gotten rid of diacetyl, but many still use AP and acetoin. Some vendors are trying to simulate the custards without using these ingredients, with varying results. The good news is that it's easy to avoid these compounds today. Most flavor manufacturers will clearly state on their website whether a certain flavor contains any of the diketones. The problem is that most e-juice makers wont do the same, so if you buy your e-juice then you will either have to avoid custard/desert flavors or find an ejuice vendor that is responsible (like Mountain Oak Vapor or another AEMSA certified line of juice). But since this is the DIY forum, I suspect you are into DIY'ing. If so, then it's easy to avoid the diketones by merely reading the flavor manufacturer's website (TFA is really good about labeling everything that contains diketones).
Secondly, watch out for the temperature of your coil. The safest way is to get a DNA-40 or some other temp controlled device, but they are expensive. Personally, I like to stay around 4 volts (no matter what resistance). A "hot" vape concerns me, so I try and keep it cool. The reason for this is that PG/VG can break down (thermally degrade) into harmful compounds when heated beyond the boiling point. If you use a mod and a RBA, then it's all guesswork right now. There is no guideline as to what wattage/voltage/resistance combinations are safe. That is unless you have a TP device that you can set to around 420 degrees.
Several studies have been done on the chemical composition of vapor. Most of these studies agree than when the coil gets too hot, bad stuff is emitted. Again, this is no surprise as PG/VG are very well studied solvents that have been used in many scientific, medical, and food applications for many decades. We always knew that this overheating could be a problem, but the question has always been "how hot do our coils really get?"
The problems with the aforementioned studies are:
1) They didn't use real humans to calibrate the heat (i.e. no humans to provide input on how the vapor tastes at high voltages). Therefore, we don't know if anyone would vape at the voltages they pushed to the coils in the lab. Most people suspect that they overheated the coil and if a human would have been on the other end, he/she would have gagged. I think this is a good assumption, at least for the devices they were testing.
2) The studies have mostly been done with cigalikes (1st gen devices) and ego twist style vape pens (2nd gen devices). To date there have been no studies done on the higher powered RBA type devices that us "enthusiasts" use. The studies on the ego type devices all seem to be in agreement that the "bad chemicals" like the aldehydes aren't released at all under 4v (using a standard CE4 clearomizer at around 2 ohms). It isn't until you get above 4.5v that these chemicals are present in any significant degree. The NEJM "formaldehyde" study only found significant amounts at the higher voltages (which is in agreement with other studies I have seen). Therefore, if you use a standard starter kit device (ego and a cheap clearomizer), it is probably smart to keep it under 4v. This shouldn't be a problem, as the vapor will taste burnt at voltages above this.
So that's where we are now; working on eliminating diketones and keeping our temps under control. That's not to say that those two things will be the "end" of the safety concerns, but they are the biggest things on the radar right now. Basically these are the KNOWN safety concerns and it's best to eliminate the knowns before you go looking for the unknowns.
Yeah, it doesn't work that way.
