An analysis of 145 different electronic-cigarette flavoring products reveals that many e-cigarette users may be exposed to a potentially harmful chemical. In a research letter published online in the peer-reviewed journal Thorax, a research team led by Maciej Goniewicz, PhD, PharmD, of Roswell Park Cancer Institute (RPCI) reports that high levels of the respiratory irritant benzaldehyde were detected in the vapor from most of the flavored nicotine products they studied, with the highest concentrations in vapor from cherry-flavored products.

http://www.sciencedaily.com/releases/2016/01/160129091641.htm

It’s worth pointing out that custom vaping solutions, produced at the point of sale using unknown ingredients from China (as most electronic products do) may have a higher risk of problems due to unknown combinations and quantities of ingredients.