Manufacturers say the mist is harmless, and most scientists agree that regular smokers who switch to e-cigarettes are lowering their health risk substantially.
The devices, though, aren't heavily regulated. And experts say consumers can't yet be sure whether they are safe either for users or people exposed to second-hand vapor puffs.
Like regular cigarettes, the nicotine in e-cigarettes is also highly addictive. People who use them may be unable to quit, even if they want to. That has raised concerns that a new generation of young people could gravitate toward e-cigarettes and wind up hooked for life or even switch to tobacco cigarettes.
The U.S. Food and Drug Administration has said it intends to regulate e-cigarettes as tobacco products but has yet to issue any rules, leaving manufacturers free to advertise while regular cigarette ads are banned.
Several states, including New Jersey, Arkansas, Utah and North Dakota, have already expanded their indoor smoking bans to include e-cigarettes. Other bans have been proposed in several big cities. About half of the states restrict sales to minors.