Government officials have warned people to avoid e-cigarettes after several people died and hundreds of otherwise healthy people ended up in emergency rooms across the country with lung damage that appears to be linked to vaping.

The illness has been dubbed “mysterious,” but new research may help explain what’s going on. A study exposing laboratory mice to the drug delivery liquid, known as “vape juice," found similar lung damage.

Researcher Farrah Kheradmand said she was surprised to discover that the vape juice was causing lung damage, even without any nicotine added. Kheradmand is a professor of pulmonary medicine at Baylor College of Medicine and senior author of the newly published study.

She and her team went to a vaping shop in Houston to get the exact ingredients in vape juice: propylene glycol and vegetable glycerin (PGVG).

“It has been manipulated to a point where vapers feel they get a very good hit in their lungs,” Kheradmand said. “It's the same kind of feeling as a cigarette smoke inhalation gives you — that you feel like something is really going down to your lungs.”

The problem is, the vape juice appears to be handicapping immune cells in the lungs. That’s what Kheradmand found in her lab mice.

“Their immune cells were … not working well,” she said. “Even a very small dose of influenza virus” killed most of the mice who had been breathing the vape juice, she said.

So why are these products considered safe?

The ingredients in vape juice have FDA approval for a completely different use, Kheradmand explained. Intensive care units use propylene glycol as a solvent to dissolve and deliver medication. But in these cases, the propylene glycol is being given intravenously.

“FDA has never, ever even tested or been asked to determine whether these things, these chemical solvents, are actually safe [for] inhalation, let alone chronic inhalation,” Kheradmand said.