"Propylene glycol and glycerin are the main base ingredients of the e-liquid. Exposure to propylene glycol can cause eye and respiratory irritation, and prolonged or repeated inhalation in industrial settings may affect the central nervous system, behavior, and the spleen.66 In its product safety materials, Dow Chemical Company states that "inhalation exposure to [propylene glycol] mists should be avoided,"67 and the American Chemistry Council warns against its use in theater fogs because of the potential for eye and respiratory irritation.68 When heated and vaporized, propylene glycol can form propylene oxide, an International Agency for Research on Cancer class 2B carcinogen,69 and glycerol forms acrolein, which can cause upper respiratory tract irritation.70,71
"Major injuries and illness have resulted from e-cigarette use,72 including explosions and fires.73,74 Less serious adverse events include throat and mouth irritation, cough, nausea, and vomiting.72"

Source

Rachel Grana, Neal Benowitz and Stanton A. Glantz, "Contemporary Reviews in Cardiovascular Medicine: E-Cigarettes: A Scientific Review," Circulation (Dallas, TX: American Heart Association, May 13, 2014). 2014;129:1972-1986. doi: 10.1161/CIRCULATIONAHA.114.007667, p. 1978.
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