What are the known potential harmful effects of 4th generation e-cigarette use related to propylene glycol/vegetable glycerol in absence of nicotine or THC?
"Acute vaping of propylene glycol/glycerol aerosol at high wattage with or without nicotine induces airway epithelial injury and sustained decrement in transcutaneous oxygen tension in young tobacco smokers. Intense vaping conditions also transiently impair arterial oxygen tension in heavy smokers" (Chaumont, Martin, et al. “Fourth Generation e-Cigarette Vaping Induces Transient Lung Inflammation and Gas Exchange Disturbances: Results from Two Randomized Clinical Trials.” American Journal of Physiology-Lung Cellular and Molecular Physiology, vol. 316, no. 5, 2019, doi:10.1152/ajplung.00492.2018.)