The U.S. surgeon general says e-cigarettes are an emerging public health threat to young people.
In a new report, Dr. Vivek Murthy says they aren't harmless, and too many teens are using them.
E-cigarettes turn liquid nicotine to vapor, without the tar from tobacco.
Vaping was first pushed as being safer, but there's no scientific agreement on that.
Dr. Murthy says nicotine is bad for a developing brain, no matter how it's exposed.