Diacetyl, is one of the main chemicals people are worried about, most manufacturers have removed or extremely limited the amount used. These studies have a tendency to no be done in real world environments. It's kind of like the Saccharine scare for Tab cola. They basically loaded lab rats with enough of the chemical to put down a donkey and said it was unsafe. I do agree that some liquids made by sub par companies could be harmful but most of these liquids are not US made. Some of these so called US liquids are actually Chinese imports but are considered US since it was obtained here.
One other test comes to mind where they tested the vapor saying it contained a list of chemicals, what they didn't do though is tell people they were dry burning the liquid. They were running machines vaping liquid in such a way that no human could or would tolerate. I would say to anyone worried about the safety of vaping to not vape but to not smoke either. Vaping and smoking contains chemicals and neither should be considered safe but depending on where you live, neither should the air you breathe. There is a lot we don't know about with vaping but it does contain less chemicals than smoking. Short and sweet of it is, vaping may cause popcorn lung but smoking does cause cancer.
I personally don't se how we as humans have ever survived. We are told that the Sun, red meat, fish, bacon, hot dogs, bologna, salt, water, the air in our homes, the air outside, and just the weather itself will kill us. Lets not forget that the produce is contaminated as well. We are all human last I checked, which by the laws of science means we are beginning to die before we are born. It's like climate change or the end of the world sayers, if you say it every day eventually you'll be right, a broken watch is right 2 times a day.
We are born with cancer cells, most of our bodies expel them or kill them off naturally and some people's bodies don't. I could be diagnosed with cancer tomorrow and there could be a whole list of what MAY have caused it but the truth is, there are too many factors that are in play. One has to weigh the risks and benefits of everything in life, be as safe as one can rationally be, but in the end it's a crap shoot. I relate it to those Final Destination movies, if it's your time to go, it's going to happen one way or another and it can't be stopped, only prolonged (but at what cost).