The lung damage seen in some people who have become ill after vaping nicotine or marijuana products resembles a chemical burn, doctors from the Mayo Clinic reported today.

Their findings are based on samples of lung tissue from 17 patients around the country whose biopsy specimens were sent to Mayo to be examined under the microscope by experts in lung pathology. Two samples came from patients who died.

“All 17 of our cases show a pattern of injury in the lung that looks like a toxic chemical exposure, a toxic chemical fume exposure, or a chemical burn injury,” said Dr. Brandon T. Larsen, a surgical pathologist at the Mayo Clinic in Scottsdale, Arizona. “To be honest, they look like the kind of change you would expect to see in an unfortunate worker in an industrial accident where a big barrel of toxic chemicals spills, and that person is exposed to toxic fumes and there is a chemical burn in the airways.”

The findings were published today in The New England Journal of Medicine.

More than 800 cases of lung illness in 46 states have been linked to vaping, and 16 people have died. The majority have vaped THC, the psychoactive ingredient in marijuana, but some say they have vaped only nicotine.

Medical investigators have been unable to identify exactly what is causing the lung damage, or even how many harmful substances are involved. They do not know whether the source is the liquids being vaped, or a toxin released from the materials used to make vaping devices. It is also unclear whether some devices used in vaping may be defective.

Last Friday, the Centers for Disease Control and Prevention identified several names of THC-related products in prefilled cartridges that they said patients had reported using before becoming ill. Dank Vapes, Moon Rocks, Off White and TKO were among those listed by health officials who interviewed patients in Wisconsin and Illinois.

But officials said that Dank Vapes was not an actual brand, but just a label and packaging that anyone selling THC vaping liquid could buy and stick on a product. The extensive use of prefilled THC cartridges suggests they might play an important role in the outbreak.

Nebraska, Virginia and New Jersey reported deaths this week, bringing the total to 16.