Each flavor compound used in synthetic flavorings is a known entity with known inhalation safety standards. The vast majority are most likely fine for
e-liquids. There are some however that have a bad track record, custard note diketones are just one type, but they seem to be the most dangerous. Meat flavorings can also cause problems, but those are not so popular in
vaping.
Whole food extracted flavors, as I said, can have a whole host of biomolecules that the lungs are not good at expelling, and that can be very prone to thermal decomposition.
And decomposition of ANY organic compound, flavors, PG, VG, will create toxic aldehydes and possibly acrolein. And free radicals, which are also toxic. Partial combustion products, generally detected as dry-puff when their concentrations are high enough. If the coil gets too hot, stuff breaks down, and generally into things you don't want to inhale.
BTW, butyric acid is a carboxylic acid, not an ester. "Esters" is a term often used in brewing (incorrectly from a strict chemistry standpoint) to describe certain flavor compounds, including diacetyl, which is also not an ester. Some of these compounds are esters, but many strictly speaking are not, but its an old term in brewing and not likely to change soon.
