Because water vapor is so harmful.

Via CNN:

Michael Bloomberg signed a bill Monday to regulate the use of electronic cigarettes — one of 22 bills he signed at City Hall that are the last he will sign as mayor of New York City, according to Evelyn Erskine, his deputy press secretary.

The legislation amends the Smoke-Free Air Act, which bans smoking in public places such as restaurants, bars, parks, beaches and places of employment. It now additionally prohibits the use of electronic cigarettes in all areas where smoking is prohibited.

E-cigarettes will still be permitted in areas where smoking is allowed, in addition to retail e-cigarette stores and vapor lounges.

Unlike a regular cigarette, there’s no tobacco burned and when users inhale, instead of smoke, there’s a steam-like vapor.

Some e-cigarette devices claim to help people kick their smoking habit, but Bloomberg said he’s not convinced and it’s just another way to inhale nicotine.

“It works about as well as patches, which is to say it really doesn’t work,” he said.