As the debate over the safety of e-cigarettes continues, more evidence suggests certain ingredients could be harmful to users' health.

A new study from Yale University found that the ingredients in pods from popular vape brand Juul react at room temperature to create chemicals that aren't listed on packaging.

This is likely because the reactions are an unintended byproduct of using the vapes.

This is the first study to find these specific chemicals, called flavor aldehyde acetals, in e-liquid pods used to fuel e-cigarettes.

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As the debate over the safety of e-cigarettes continues, more evidence is accumulating to suggest that the smoking alternative could have health consequences.

In a recent study published in the American Journal of Preventative Medicine, Yale University researchers looked at the contents of Juul's nicotine e-liquid "pods," which fuel the popular e-cigarettes. They found that certain ingredients can react to create throat-irritating chemicals that aren't on the pod packaging, likely because these reactions were unintended.

Specifically, the researchers found acetals in Juul's creme brûlee pods.

Acetals are a type of chemical that can cause skin, eye, nose, and throat irritation. They're formed when alcohols and flavor aldehydes — chemical compounds used to make scents or flavors in perfumes, food, and e-liquid pods — in the pods reacted with each other.

Researchers also looked at Juul's cool cucumber, fruit medley, and cool mint flavors, but only found acetals in the creme brûlee pod flavor. They believe it's possible the chemicals could also be present in other pod varieties, however.

Researchers don't know whether there are more serious health consequences of vaping acetals

Critics of Juul and other manufactures claim their fruity flavors are marketed towards children. Getty

All the researchers know so far is the unintended acetals created within the pods lead to throat irritation. It's unclear whether that irritation signals a bigger health risk, lead researcher Hanno Erythropel told INSIDER.

To test acetal exposure, the researchers used a machine with a built-in pump that pulls air and then traps the vapor in order to simulate a person using a vape.

Once in the machine, "we cryogenically freeze the vapor and we can unfreeze later it and determine what's in it," Erythropel said. "So we can see what's in the liquid but also what's in the vapor that people are exposed to, because it has to get to the vapor to get to the user."

Read more: San Francisco's most popular e-cigarette brand Juul may have pushed rival brands to compete in a 'nicotine arms race.' Now all e-cigs are banned in the city.

In a statement to INSIDER, Juul wrote that the researchers "failed to take into account real world conditions, including realistic human exposure to vapor products like Juul." Juul also said the machine the researchers used to mimic vaping overestimates the amount of vapor a user would inhale, and creates an inaccurate depiction of the actual health risks associated with vaping Juul pod e-liquids.

Erythropel believes more research must be done to determine the health risks that could arise from vaping these chemicals.

"When manufacturers prep these e-liquids, we think they didn't give much thought to the reactions that would occur," he said.

He added that it's possible other vape manufacturers besides Juul have the same issues with their e-liquids.

"I can see situations where other manufacturers have stronger e-liquids where you could maybe get more of these compounds, but we don't know if higher doses are worse necessarily," Erythropel said. "We only have this irritation measure so far."