The e-cig technology, which emerged over the last few years but whose health advantages over cigarettes are unclear, is already banned in Argentina, Thailand, Singapore, and 24 other nations and is subject to age restrictions in many places. U.S. federal law permits the sale of e-cigarettes, but states and municipalities are creating their own restrictions. The Public Health Law Center keeps a map and database here.

The U.S. Food and Drug Administration is studying how to discourage e-cigarette use among younger people. The U.S. Centers for Disease Control and Prevention says that e-cigarettes may help existing cigarette users to reduce their exposure to some of the harmful products of burning tobacco, since e-cigarettes just heat the tobacco. However, it also states that “scientists still have a lot to learn about whether e-cigarettes are effective for quitting smoking” and that children, pregnant women, and non-smokers should not use e-cigarettes.

Lam cited the “health of our citizens, particularly children and teenagers” in announcing her plan to ban e-cigs in the Hong Kong. The city’s Council on Smoking and Health, which backs measures to control tobacco, says the devices could “renormalize smoking behavior and serve as a gateway to youth smoking.” The agency says that 37% of people who experiment with e-cigs are between ages 15 and 29. That said, Hong Kong already has one of the lowest smoking rates in the world; about 10% of its population smokes, Bloomberg reports.