In as an extensive job as I can do, researching this whilst not being a chemist, this is what I have gathered. Diketones when used, can be considered what we call "safe" at a level of 10ppm and below. Though there is no real safe way to ingest/ inhale something considered toxic, these are the levels that OSHA has come up with for 'safe' exposure. Unfortunately, it is hard to calculate between something like a solid particulate floating in the air versus something being vaporized as well as differentiate in the time of exposure as you are not constantly vaping for 8 hours on end.
This is a good read, there are a lot of level for exposure to different chemicals, even those that are not diketones. Many are listed at the bottom in a table, I think if enough people parse through it, we can come to fairly accurate conclusion on what we consider to be reasonable level for putting in our eliquids.
Lmao, no, not me but I think I just found a new best friend!