It is a popular conception that e-cigarettes like Juul are as harmless as smoke machines at prom dances, just benign alternatives to toxic, smelly cigarettes.

Yale scientists aren’t so sure.

In a new study published on July 30 in the American Journal of Preventative Medicine, researchers in Yale’s Zimmerman Lab connected a Juul to a makeshift puffing machine to identify which chemicals users breathe in when they vape using the product. What they found was surprising to some: The compounds measured in flavored e-liquid changed as they entered the body. Some of them, the researchers learned, were banned in cigarettes. To Hanno Erythropel, a postdoctoral associate at the Zimmerman Lab and first author of the study, the discovery shows how much more research needs to be done to understand how these chemicals — called acetals — can do damage.

“Why are we expecting that just because there are certain ingredients, it won’t react?” he said. “They can, and we need to understand better.”

After hooking the Juul to their custom contraption, the scientists used liquid nitrogen to freeze the puffs’ contents. Then, once thawed, the samples were analyzed for their chemical makeup.

In the flavor Crème Brulée, which is advertised at 5 percent nicotine content, much of the compound vanillin — an extract of the vanilla bean — was altered into an acetal once puffed, according to the study.

When inhaled, these acetals are stronger and potentially more irritating than what is found in the Juul pods themselves. This could pose a health problem for those with asthma or other respiratory problems, Erythropel said.

But, he added, more research is needed to understand what these compounds do in the human body — and if e-cigarettes are healthier than their tobacco counterparts.

“Our study doesn’t give a definitive answer. … We know now that there are reactions taking place but we don’t really know what the implications are,” Erythropel said.

For Deputy Director of the Yale Center for Health & Learning Games Kimberly Hieftje, whose team is developing a virtual reality game to help young people say no to e-cigarettes, the findings are not surprising. In fact, a game her team developed — Invite Only VR — includes a variety of scenarios in which the player must learn about the potential damage vaping can cause.

“I think research has shown that teens and youth really don’t have a true understanding of what they’re inhaling,” said Hieftje, who was not involved in the study. “They’re not really quite thinking about the harm that they’re doing to their body.”

As the Food and Drug Administration investigates Juul for its potentially harmful health effects and advertisements to young adults, Erythropel and Hieftje both said that they do not recommend smoking — e-cigarette or not.

“We can’t just go and sell these products that might form something new inside of them,” Erythropel said. “That seems crazy to me.”

The Juul e-cigarette was launched by PAX Labs in 2015.

Matt Kristoffersen | matthew.kristoffersen@yale.edu .