Heating glycerin above 290*C makes it decompose into acrolein, a highly & toxic corrosive gas. I wonder if this is what you're inhaling. Look up acrolein's wikipedia entry.
At 280C it
begins to decompose to acrolein and water. This is just below the BP. Most will vaporise unchanged. The % decomposed would vary widely depending on many factors, but is probably <1% taking an average for
vaping. Possibly a lot less given that the VG is in a mixture (notwithstanding a certain amount of fractional distillation).
How much is released in the vapor is unknown. Certainly a lot of it polymerises or oxidises onto the coil where it degrades do a carbonized gunk. Smell can detect around 2ppm, and this is the level where it begins to be harmful.
If there is VG in the juice, there will be some acrolein in the vapor. How much though needs more investigation. It is nothing to panic about. Vaping VG is likely 100 times safer than burning tobacco (but it is not totally safe).
PG seems to be heat stable but some people are allergic to it. I find that the thickness of VG helps create a better juice in that it doesn't run where it shouldn't. Whether PEG offers a third way is unknown; there
might be a heat degradation issue with PEG.
