More than 16 million children in 10 states and the District of Columbia have legal access to electronic cigarettes, according to a federal study released on Thursday.

The underage use of e-cigarettes, which are metal tubes that heat liquid into an inhalable vapor, concerns health officials because they contain nicotine, which can be addictive and harm adolescent brain development.

Critics also cite a lack of data on the health effects of their long-term use, while proponents call them a healthier alternative to traditional cigarettes.

“We’ve made a lot of progress addressing youth access to these products,” said Brian King, a senior scientific adviser with the U.S. Centers for Disease Control and Prevention’s Office on Smoking and Health.

Since 2010, a total of 40 states have banned e-cigarette sales to those under 18. Four of those states also ban sales of the product to 18-year-olds.

Even so, e-cigarette use among high school students tripled from 2011 to 2013, with 4.5 percent of students using the products last year, according to a CDC survey released in November.

The CDC cites increased advertising of e-cigarettes as a possible cause of their mushrooming popularity among young people. E-cigarette ads can be seen on TV, a practice banned for traditional cigarettes since the 1970s, King said.

The devices are unregulated by the federal government, although the U.S. Food and Drug Administration in April proposed rules that would ban the sale of e-cigarettes to anyone under 18.

The states allowing sales of e-cigarettes to people under 18 include Texas, Michigan and Nevada, the CDC said.