Use of e-cigarettes among middle and high schools students doubled from 2011 to 2012, according to a new government survey. The Centers for Disease Control and Prevention (CDC) found 10 percent of high school students had tried an e-cigarette last year, compared with 5 percent the previous year.

The CDC found 1.8 million middle and high school students said they tried e-cigarettes last year, The New York Times reports. “This is a serious concern because the overall impact of e-cigarette use on public health remains uncertain,” the researchers wrote in the CDC report. “In youths, concerns include the potential negative impact of nicotine on adolescent brain development, as well as the risk for nicotine addiction and initiation of the use of conventional cigarettes or other tobacco products.”

Most students who tried e-cigarettes also had smoked regular cigarettes, the survey found. Twenty percent of middle school students and 7 percent of high school students who said they tried e-cigarettes had not smoked a regular cigarette.

E-cigarettes are designed to deliver nicotine in the form of a vapor, which is inhaled by the user. They usually have a rechargeable, battery-operated heating element, a replaceable cartridge with nicotine or other chemicals and a device called an atomizer that converts the contents of the cartridge into a vapor when heated. E-cigarettes often are made to look like regular cigarettes.

In March, the CDC released a report that found one-fifth of adult smokers in the United States have used e-cigarettes. CDC researchers said e-cigarettes appear to have far fewer toxins than traditional cigarettes, but the impact of the devices on long-term health requires more research.

The Food and Drug Administration does not regulate e-cigarettes, but that is expected to change soon, the article notes.