Firstly, the flavors we use for DIY are neither USP nor safe for inhalation. They are generally flavors just as the ones used for food, drinks and baking, which we (ab)use for DIY, and the vaping-branded flavors are the same but just often more concentrated, and e.g. oils are often not in the ingredients and the other bad stuff i.e. Acetoin, Diacetyl, and Acetylpropionyl are often not there, or listed, or supplied also as free-of-these versions as a dx or v2 version.
The best you can do, is what everyone else does, which is avoid Dia/AP/A and oils and use reputable brands for vaping, which often have sorted the bad stuff out from the sortiment, or have e.g. dx or v2 flavors, which is free from Dia/AP/A and where the original included it.
There's haven't been extensive testing done on the inhalation of food-flavors done yet, so we all just take a risk with it, and avoid the currently found bad stuff... A well-known and much-regarded professional chemist around here(Kurt Kistler) has previous stated that the risk of inhaling these food flavors in the long run can be everything from nothing at all or mild irritation, to cronic respiratory system issues.
Lastly, something being USP grade doesn't mean it's safe for inhalation, btw. Something safe for inhalation, has in addition to USP grade a license stating it's OK to inhale without ill-effects, and that's available as a drug. However, e.g. VG/PG, when pharma-grade i.e. USP and/or EP, is just fine for inhalation, since that has been tested extensivelly without ill-effect throughout decades...
Yes, something can be both pharma and food-grade. We want pharma-grade, if the product has an USP monograph, as that's always the cleanest, although there are many cases where food-grade is just as pure a product.
Don't stress so much over it, and just buy name-brand flavors of either e.g. TFA/TPA, capella, inawera, hangsen, flavorah, molinberry etc and you should be fine...
I recommend checking this youtube video out, which features Kurt answering many DIY questions about safety and flavors etc.
