A ban on electronic cigarettes went into effect Tuesday in New York restaurants, bars, parks, beaches and other public places.
A law was passed by the city council on December 19 and signed by former mayor Michael Bloomberg.
It extends an already strict ban on tobacco smoking in public places in the metropolis, where even some residential buildings don't allow tenants to light up.
In a further unprecedented move for a major US city, retailers as of May 18 will no longer be allowed to sell tobacco products or e-cigarettes to anyone under 21.
Restrictions on the use of the battery powered devices in most indoor public places in Chicago also went into force Tuesday.
In Los Angeles, meanwhile, lawmakers voted in March to ban e-cigarette use in public places where tobacco smoking is prohibited.
Marketed as aids to quit smoking, e-cigarettes allow users to inhale a nicotine-laced vapor. But experts say not enough is known about the effect of chemicals involved, both on smokers or those around them.
With regulation varying from state to state, federal US regulators last week proposed the first restrictions on the booming $2 billion e-cigarette market.
The new rules would bring e-cigarettes under many of the same rules that already apply to traditional cigarettes, including requiring sellers to enforce a minimum age restriction on those who wish to buy the products.
E-cigarettes are popular among young people: a December study by the US Centers for Disease Control and Prevention estimated that 10 percent of high school students had used them.