So some of you may recall my aversion to PG and VG ejuices due to concerns about degradation byproducts and the large volume of these solvents being boiled and inhales in ejuice preparations. Unfortunately, it appears that my concerns are starting to be more clearly born out in the scholarly research. "Aldehyde emissions increased by more than 60% after the device was reused several times, likely due to the buildup of polymerization byproducts that degraded upon heating. These findings suggest that thermal degradation byproducts are formed during vapor generation. Glycidol and acrolein were primarily produced by glycerin degradation. Acetol and 2-propen-1-ol were produced mostly from PG, while other compounds (e.g., formaldehyde) originated from both. Because emissions originate from reaction of the most common e-liquid constituents (solvents), harmful emissions are expected to be ubiquitous when e-cigarette vapor is present." http://pubs.acs.org/doi/abs/10.1021/acs.est.6b01741 here's the link for the scholarly article I'm citing above. What this tells us is that even the more 'health-friendly' sounding Vegetable Glycerin, when vaporized in ecig tanks releases 'powerful' respiratory irritant (acrolein), the 'probable carcinogen' Glycidol and more. PG also shares a number of dangerous degradation byproducts. Here's a diagram from the article showing us the chemical pathway that leads to some such byproducts: It also appears that there is a buildup of dangerous byproducts emitted by the same cart after extended reuse over time vs first use. As is reported above, the nasties increase the more you use the same cart. It is of course your decision as to what you are happy with doing with your body, but I share this for anybody at all concerned about limiting exposure to toxic materials.