Some studies have found vapes to contain lead, nickel, tin, silver, formaldehyde, manganese, toluene, and other chemicals linked to cancer and central nervous system problems.
Williams, M., Villarreal, A., Bozhilov, K., Lin, S., & Talbot, P. (n.d.). Metal and silicate particles including nanoparticles are present in electronic cigarette cartomizer fluid and aerosol. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/23526962
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