More cases of severe lung damage and breathing problems linked to vaping have been reported across the country, with doctors and health officials now investigating cases in at least eight states: California, Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio and Wisconsin.

Doctors tell NBC News that most patients are otherwise healthy teenagers or young adults who come to the hospital with symptoms of a bad respiratory infection: shortness of breath, chest pain, fatigue and coughing.

Their symptoms quickly become worse, and many need to go on ventilators.

“We go through the process of eliminating infections and other things that could be causing inflammation in the lungs,” said Dr. Humberto Choi, a pulmonologist and critical care specialist at the Cleveland Clinic.

“By elimination, it comes down to a history of vaping.”

All of the patients reported vaping either nicotine, marijuana or both before their symptoms appeared. But doctors have not yet been able to pinpoint any specific device, brand or ingredient that would tie all of the cases together.

Choi has treated at least two such patients in his intensive care unit in Ohio this week alone.

Earlier this week, Illinois reported six cases, and Minnesota reported four.

On Thursday, the Wisconsin Department of Health Services updated its confirmed case count to 15, up from 12 earlier this week. Officials there are investigating an additional 15 cases.

And suspected cases are popping up in other states as well, possibly because awareness is spreading among physicians.

“We were aware of reports in other states, so we proactively sent a message to Iowa doctors and health professionals to be on the lookout,” said Dr. Caitlin Pedati, state epidemiologist and public health medical director for the Iowa Department of Public Health.

Since then, one case has emerged in that state that’s now under investigation.

The California Department of Public Health tells NBC News it's looking into 16 possible cases, and Indiana health officials are investigating six.

“We need to start collecting these cases and see what they have in common and figure out what kind of patterns we can find,” Choi said.

The Centers for Disease Control and Prevention says it’s working with several of the Midwestern states, as well as California.

These are serious chemicals that are lung toxic.

Doctors in Michigan tell NBC News they’ve had an increasing number of cases recently, and some are quite severe. Some patients have experienced bleeding in their lungs and have needed life support.

“Over these last few months, we’ve probably seen about a dozen in the hospital,” said Dr. Shelley Schmidt, a pulmonary critical care specialist with Spectrum Health in Grand Rapids, Michigan.

Similar to cases described by other doctors, patients arrived at the hospital with what looked like pneumonia or a bad viral infection, and were treated as such.

“But then they don’t get better, or they get worse despite therapy,” Schmidt said. “And then you realize you’re dealing with something that’s more than just a garden-variety infection.”

“As their lungs worsen and become more inflamed, it becomes clear that it’s an inhalation injury.”

It can take months to recover, Schmidt said.

Her patients also reported vaping both nicotine and THC, the psychoactive ingredient in marijuana.

“This is not harmless water vapor,” Schmidt said. “These are serious chemicals that are lung toxic.”

Pedati, who is also a pediatrician, said it's critical to discourage vaping among youth.

"We don't know yet what the long-term effects of vaping in young people will be."

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