Los Angeles is to ban e-cigarettes in public places where tobacco smoking is prohibited.
The LA City Council voted by 14-0 to outlaw their use in indoor workplaces, outdoor dining areas, parks, recreational areas, beaches, bars and nightclubs where lighting up is banned.
"Vaping" lounges and stores will be exempted, in line with cigar and hookah lounges where tobacco smoking is allowed, as will using e-cigarettes for "theatrical purposes".
The battery-powered devices, marketed as aids to quit smoking, allow users to inhale a nicotine-laced vapour, but experts say not enough is known about the effect of the chemicals involved, on smokers or those around them.
"Safer does not mean safe," said the LA County's public health director Dr Jonathan Fielding. "Although they are less harmful than traditional cigarettes, some e-cigarettes contain some health risks."
He said e-cigarettes had grown into a "US$1.5 billion industry that has caught the attention of big tobacco which historically has had scant regard for public health".
Last December, New York City extended its strict smoking ban to e-cigarettes, barring them from bars, restaurants, parks, beaches and other public places.
The industry has exploded in the United States, doubling turnover in one year to US$1 billion-US$1.7 billion at the end of last year, according to financial group Wells Fargo. Sales grew ninefold between 2010 and 2012, studies suggest.
Regulation varies from state to state in the US, but they are often banned on planes or trains.
In Europe several countries have banned e-cigarettes wherever tobacco smoking is banned, while others outlaw sales to children.
E-cigarettes are banned in several Latin American countries, while in Asia there is relatively little interest in countries where tobacco is cheap.