And the great e-cigarette debate continues.

While several studies have shown e-cigarettes emit levels of nickel and chromium four times higher than tobacco smoke, ultimately tripling smoking rates, there are some standing by the claim e-cigs are less harmful than their tobacco version. Researchers from the University of Pennsylvania’s Annenberg School for Communication are the latest to weigh in the topic — but their focus is on advertising. In 2014, e-cig advertising totaled over a billion dollars in 2014 and it’s expected to grow at a 50 percent rate over the next four years.

After spending a year scouring the Internet for examples of e-cig ads — ads that strictly showed people inhaling or holding e-cigs, which is known as vaping — they selected 16 ads to show 900 former, intermittent, and regular smokers. Smokers were randomly assigned to one of three conditions: watching ads, watching audio-only ads, or taking a written survey. And afterwards, smokers in each condition took a survey to determine their smoking urges and cravings, their confidence to remain abstinent, and whether they actually smoked during the experiment.

The results showed regular and former smokers viewing ads with vaping craved tobacco more than regular and former smokers viewing ads without vaping. Former smokers in particular felt less confident they could refrain from smoking after viewing ads with vaping.

As for smoking during the experiment, 35 percent of daily smokers admitted to lighting up while watching ads with vaping versus 22 percent of daily smokers viewing ads without vaping and 23 percent of daily smokers not viewing any ads.

“Given the sophistication of cigarette marketing in the past and the exponential increase in advertising dollars allotted to e-cigarette promotion in the past year, it should be expected that advertisements for these products created by big tobacco companies will maximize smoking cues in their advertisements,” researchers wrote. “...if not regulated, individuals will be exposed to much more e-cigarette advertising on a daily basis.”

Researchers added visual cues were what prompted daily smokers to act on their cravings. These visual cues include ashtrays, matches, lighters, and smoke. So the next step is for researchers to determine if it's a cigarette look-a-like producing the smoking cue, or if all types of visual cues produce the same effect. Study co-author Dr. Joseph N. Cappella said this is important, because “the jury is still out about the efficacy of e-cigarettes to reduce tobacco use and tobacco smoking.”

"If it turns out to be the case that e-cigarettes are a good vehicle for reducing tobacco addiction, then we may not want to stand in the way of advertising e-cigarettes," he added. "But it doesn’t mean we can’t carry out that advertising without the vaping cues in order to not have deleterious consequences for former smokers...and current smokers in terms of their cravings for tobacco cigarettes.”