As smokeless tobacco devices gain in popularity, more people are using e-cigarettes and vapor pens to imbibe marijuana in public, with little fear of getting caught.

With no odorant smoke, the devices emit little sign of what’s inside, experts and users say. “I was on the train from New York to Baltimore and I enjoyed the pen the whole way there and back with no one noticing,” a marijuana user told NBC New York Friday, on condition of anonymity. “I was absolutely thinking, ‘This is not bad at all.’”

Production of smokeless marijuana products, such as waxes typically used in vapor pens, has increased in states now allowing legal consumption of marijuana, either for medicinal or recreational purposes, experts say. Those products can then be used illegally by others in other state jurisdictions, making liquid and wax forms of marijuana particularly attractive to users.

Bobby Black, senior editor of High Times magazine, described a growing trend among marijuana users in smokeless products. "Marijuana is still widely smoked in its flower form, but this is a new way," Black said. "The younger generation is embracing it the same way rock and roll was around and then it was replaced by heavy metal."

The trend worries some health officials, who say minors may begin to consume drugs through smokeless delivery devices, such as e-cigarettes and vapor pens. Soon following the introduction of such products on the U.S. market, the percentage of American teenagers using nicotine and other drugs through such devices more than doubled, from 2011 to 2012. The U.S. Centers for Disease Control and Prevention (CDC) says that 1.8 million adolescent and teenage Americans tried e-cigarettes last year. “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.”

Now, health officials worry minors may begin using the devices to consume marijuana and other drugs.

"I care a lot about it," Detective Lt. Kevin Smith, who heads the Narcotics Unit for the Nassau County Police Department, told reporters. "For young people, marijuana is a gateway. The next thing you know they’re doing acid, molly, even heroin. I don’t like it that people are giving it a pass."

Smith said cops now routinely test smokeless tobacco devices for illegal drugs, when arresting a suspect on a drug charge. Although the sale of e-cigarettes to minors is illegal in New York and New Jersey, most states, and the federal government, lack such prohibitions — making illegal drug use that much easier, Smith said.