Propylene glycol can become formaldehyde — a carcinogen — when heated to vaping temperatures.
Salamanca, J. C., Meehan-Atrash, J., Vreeke, S., Escobedo, J. O., Peyton, D. H., & Strongin, R. M. (2018). E-cigarettes can emit formaldehyde at high levels under conditions that have been reported to be non-averse to users. Scientific reports, 8(1), 7559. doi:10.1038/s41598-018-25907-6
Formaldehyde and Cancer Risk was originally published by the National Cancer Institute. (n.d.).