A growing number of people are smoking marijuana out of e-cigarettes, NBC New York reports. Marijuana in liquid and wax forms used in e-cigarettes and vapor pens does not create an odor. Because the devices don’t produce a flame, a person smoking marijuana in an e-cigarette can take a puff and then quickly put it in a pocket.

Local law enforcement officials and drug counselors are concerned about the trend, particularly in minors. Last month, the Centers for Disease Control and Prevention (CDC) published a survey that showed use of e-cigarettes among middle and high schools students doubled from 2011 to 2012. The CDC found 10 percent of high school students had tried an e-cigarette last year, compared with 5 percent the previous year. According to the survey, 1.8 million middle and high school students said they tried e-cigarettes last year.

Detective Lt. Kevin Smith, who heads the Narcotics Unit for the Nassau County Police Department in New York, said an officer arresting someone on a drug charge who has an e-cigarette is now directed to test the device for illegal drugs.

New York Assemblywoman Linda Rosenthal introduced a bill last year which made it illegal to sell e-cigarettes to minors. New York Governor Cuomo signed the bill in September 2012. “Once you try electronic cigarettes, you can become hooked to them, move on to cigarettes and then move on to other drugs,” Rosenthal said.