Hm, he may be referring to diacetyl/AP/diketones. Some flavorings have certain chemicals in them which are known to be harmful. As it is, it's pretty hard to identify which juices may or may not contain this.
I'm a bit paranoid about the entire situation, so I stay away from any juices that are known to have diketones in them, and I only
vape juices that have been lab tested. There are a LOT of threads on the ECF about diketones ("the great diketone debate" is a hot topic). In the end, it's up to you to make the decision.
I quit smoking with the intention of maximizing harm reduction. This means eliminating any flavorings that are known to be harmful. For this reason, I vape mostly unflavored/DIY my own
juice with very small amounts of flavorings from reliable, tested, and trustworthy companies.
For the record, some known juices that are particularly/alarmingly rife with diketones are
five pawns juices (their entire line is pretty bad), Suicide Bunny, Cuttwood, and Nicoticket (Sure Bert in particular). Some of these companies are better about disclosing their diketone-inclusion than others. Personally, I vote on staying away from any juices that have diketones. Nothing against their companies, I just don't think we know enough about the potential harm we could be doing by inhaling this stuff which, in the past, has been proven to be very harmful to human lungs.