Teen tobacco use hasn't declined at all since 2011 — and it looks like the rising popularity of e-cigarettes is to blame. A CDC report published today shows that in just four years, the rate of e-cig use among high school students increased tenfold — rising to 16 percent in 2015, from 1.5 percent in 2011. For middle school students, the rate of e-cig use rose to 5 percent, from 0.6 percent in 2011.

The rate of high school students who smoke regular cigarettes declined to 9 percent in 2015, from 16 percent in 2011, but e-cigs are more popular than ever. In 2015, the number of middle and high school students who said they were current users of e-cigarettes rose to 3 million, up from 2.5 million in 2014, the CDC says. That means that of the 4.7 million students who said they were current users of at least one tobacco product last year, more than half used e-cigs. So, even though teens aren't smoking conventional cigarettes as much as they once did, they're definitely still consuming nicotine.

"No form of youth tobacco use is safe."

"E-cigarettes are now the most commonly used tobacco product among youth, and use continues to climb," CDC Director Tom Frieden said in a statement. "No form of youth tobacco use is safe. Nicotine is an addictive drug and use during adolescence may cause lasting harm to brain development."

This isn't the first time the CDC has warned Americans about teens using these battery-powered devices, which vaporize liquid nicotine. Last year, the agency called the phenomenon "alarming." And this past January, the CDC announced that 18.3 million middle and high school students had been exposed to ads for e-cigs in 2014. Because e-cigs come in many different flavors — like vanilla and chocolate — they're particularly appealing to teens. Such flavors are banned in regular cigarettes. To curb the rise of teen vaping, the CDC wants to impose strict e-cig regulations and ban their sale to minors (right now the only e-cigs that are regulated are those that are marketed for therapeutic purposes). But getting these regulations approved has taken a long time. Even though they were first proposed in April 2014, they're still being finalized by the US government.