The Wisconsin health officials also said that patients reported using open-tank systems and devices with interchangeable cartridges. Open systems allow users to concoct their own brews of vaping liquids, if they want.

Dr. Emily Chapman, chief medical officer for the Children’s Minnesota hospital system, said that in the last month or so, it had treated four cases of acute, severe lung damage — including respiratory failure — in teenagers who had been vaping. Although the hospitals’ patients have each shown improvement, Dr. Chapman said, it’s not clear if they will fully recover.

She also noted that investigating a teenager’s vaping patterns can be tricky.

“The risk here is that if people are presenting to hospital emergency rooms, or urgent cares, they either may not think of vaping as something that is threatening and may not include it in their history,” Dr. Chapman said. “Or if asked directly, they may not be comfortable sharing that.”

Dr. Chapman also said she was concerned about how much of the public believes that vaping is safe.

“The truth of the matter is, we have so little experience with vaping, relative to the experience we have with cigarettes and cigars. Recall how long it took us to figure out that cigarettes were linked to lung cancer. There is so much we don’t know.”

One recent study from Yale and Duke identified chemicals called acetals in some Juul e-cigarette liquids. Those chemicals, the researchers said, may be especially irritating to the lungs and can cause damage when inhaled.

Juul disagreed with the study’s conclusions.

“The researchers’ hypothetical exposure analysis failed to take into account real world conditions, including realistic human exposure to vapor products like Juul,” said Lindsay Andrews, a Juul spokeswoman.

Juul pods contain high levels of nicotine, which can cause addiction and health problems. Some experts worry about the effects of nicotine on a developing teenager’s brain, and some studies have suggested that nicotine ingestion can also affect the heart and arteries.