There’s some (ok, a lot of) controversy in the vaping world over the use of diketones (diacetyl, acetyl propionyl and acetoin) in e liquids.
While some argue that they should not be used due to diacetyl’s connection with bronchiolitis obliterans (aka “popcorn lung”), others argue that cigarettes contain far more diketones than e liquids do, and we don’t see smokers coming down with popcorn lung so it can’t be that bad.
In the end its a personal decision that we as vapers have to make for ourselves. Here are a few links to kickstart your research on the subject if you’re still on the fence.