Unlike standard cigarettes, the components of electronic cigarettes are not regulated and standardized, thus they vary widely between products. The characteristics of these e-cigarette elements, including their delivery systems, combustion apparatuses, and the composition of the nicotine solutions they contain may affect the levels of potentially hazardous substances in the vapor they produce.

– American Thoracic Society

Source: Potentially hazardous effects of mechanical and chemical characteristics of e-cigarettes I Medical Xpress