ROCHESTER, Minn. — Overturning the prevailing view of the mechanism behind the vaping-related serious lung injury that has local and national health officials on high alert, Mayo Clinic on Wednesday, Oct. 2, released findings from the largest tissue sample study undertaken yet on patients with the illness.

The new research suggests that chemical injury, not oil accumulation, lies at the root of the vaping illness.

The study, led by Mayo Clinic Scottsdale lung pathology researcher Dr. Brandon Larsen and teams at four Mayo sites, was published as a correspondence in the New England Journal of Medicine. It reported the results of lung biopsies from 17 patients with confirmed or probable cases of the illness. Most of the patients were adult males, 12 had reported using THC or other non-nicotine vaping products in prepackaged pods or tank vaporizers, and two of them died.

"Until we actually look at biopsies we're kind of guessing as to what's actually going on when these patients get so sick," said Dr. Jennifer Boland, an anatomic pathologist at Mayo Clinic Rochester and co-author of the study. "People thought it might have been lipid in the vapor that was actually accumulating in the lung .... we really didn't actually see that pattern. What were seeing is injury to the small airways and to the surrounding lung. That would indicate a chemical type of injury that is happening when the vapor is inhaled into the lungs."

In early September, a NEJM correspondence based on imaging of 19 cases listed lipoid pneumonia among four patterns believed to be associated with the illness, albeit noting the absence of computed tomography evidence of oil in the lungs. As recently as last week FDA commissioner Dr. Ned Sharpless told a congressional panel that vitamin E acetate, an oil found in illicit THC products, was emerging as an agent of concern among the 150 patient-submitted products under review by the agency. "A significant fraction of the products, like maybe half of them, are contaminated with vitamin E acetate," Sharpless said. Inhaling oil causes lipoid pneumonia.

But the Mayo study reported finding no large droplets of oil in the lung tissues sampled, ruling out a characteristic of lipoid pneumonia. The tissue changes they found better resembled "the type of changes that are characteristic of toxic reactions to medications or noxious chemical fumes, suggesting a similar mechanism of injury," the authors wrote. Boland says the finding does not rule out any particular agent in the vaping products, only the mechanism by which patients were sickened.

Though the results may prove useful in the search for the agent or combination of agents responsible for the illness, the majority of cases reviewed by the Centers for Disease Control and Prevention reported having used illicit THC products, liquids potentially containing numerous chemicals, dilution agents and adulterants.

"Based on pathologic findings alone," Boland says, "I don't think we can speculate on what the exact offending agent might be. I think more research is going to be necessary to figure it out."

The illness has sickened 805 Americans, and Minnesota reported 59 confirmed or probable cases as of Wednesday afternoon. Twelve patients have died nationwide, and one in Minnesota.