The first thing you should do when conducting research, reading a non-fiction book or reading an editorial is to "know" the author.
"I know smoking is bad for my health, but I'm going to die of something eventually,so why not pick my poison?"
I've never met the person who can go through a day without at least one good rationalization.
I had a conversation with a group of folks where the topic was: At what age did the doctor start telling you, "At your age..." or "If you were a younger man/woman...but at this point in your life, I wouldn't bother having this test or that procedure."
I started vaping to save money, and if it reduced the hacking, that'd be fine. To my surprise, vaping tobacco extracts are as flavorful as I had expected smoking to be when I had my first cigarette and my first bowl. Smoking was a compulsion, a craving, vaping is enjoyable.
Is vaping safe? Common sense tells you probably not. Is it safer than smoking? Maybe, who knows.
If we understood how cells turned cancerous, we might know how to prevent it. We do know that there is a relationshio between cetain chemicals and increased likelyhood of cancer. Nitrosamines are one of those chemicals. PGA contains nitrosamines, PGA nitrosamines have been connected to increased incidences of digestive tract cancers. Although PG and VG contains no nitrosamines, the safe limit for intake is considered 1g/ml. How does that relate to vaping 5ml of juice a day?
The real danger of tobacco is in the smoke. Vaping eliminates the smoke from burning so it is safer. Really? Never heard of cancer of the mouth, gums, throats etc. from smokeless tobacco? There was a study awaiting peer review in 2012 that found a connection between the nitrosamines in smokeless tobaccos and cancer (interesting that I've never found it published.)
I smoked an ounce of tobacco a day, an ounce of tobacco will produce enough extract to last me a month of vaping, so. I'm now using 1/100 of the tobacco vaping than I did when smoking. It must be healthier. Really? Maybe the extract is so concentrated that it contains a thousand times more of the carcinogenic substances like nitrosamines than when smoking.
I personally make, or only purchase, extracts that are made with filter, CO2, or Steam processes. Seen any lab reports that show any of those processes result in no presence of nitrosamines? Supposedly calcium carbonate and activated charcoal filtration may reduce nitrosamines, but they would likely result in elimination of flavor.
So you eliminated all of the nitrosamines. What about the radiation levels? Tobacco has shown to have high radiation levels due to cultivation processes that create radiation containing dusts that adhere to the plant. These dusts wii wash off most edible plants, but because of the sticky coating of tobacco leaves due to the resin in the plant trichomes, it remains on tobacco. This dust born radiation is inhaled and builds up in the lungs. You got a filter or process that will eliminate particles as small as .01 microns?
Bottom line is that bodily intake of anything in excess or that is not necessary for survival is usually unhealthy. Risk vs reward. We easily accept the reward, When the risks are unknown or when known, are contrary to our head -strong desire for reward, we revert to rationalization.
In this case, we really have no idea what the risks truly are. It's unlikely we will learn them before any damage is done.
As things currently stand, if I were a younger man with any common sense, I'd quit vaping or I would stop worrying about it.
No offense meant.
... but I will continue to research and if I find any more real studies I will follow up. 
I also know we are all filtering much finer than cheesecloth, so who really knows? 
