E-cigarettes heat up nicotine (extracted from tobacco), flavorings, and other chemicals to create an aerosol that is inhaled. Common tobacco cigarettes contain 7,000 chemicals, many of which are toxic. Vaping has risks, regardless of what you vape. Starting to use e-cigarettes, or switching from cigarettes to e-cigarettes, increases the risk of devastating health effects.
The safest option, according to the American Cancer Society, is to completely avoid vaping and smoking. Needless to say, the popular practice of dropping some essential oil or steam into these humidifiers isn't good either. Second-hand exposure to e-cigarette vapor is said to be less toxic than second-hand exposure to cigarette smoke. While second-hand vapor may not affect the lungs in the same way as vaping, it's best to avoid it if possible.
More research is needed to understand the long-term health effects of passive exposure to e-cigarette vapor. When vaping, a device (usually a vaporizer or a mod, an improved vaporizer that may look like a USB memory stick) heats a liquid (called vaping juice or electronic liquid) until it becomes a vapor that is inhaled.