The number of life-threatening lung illnesses linked to e-cigarettes has risen to 1,604 nationwide, according to the Centers for Disease Control and Prevention.
Most cases involve THC, the psychoactive ingredient in marijuana that produces a high. However, a minority of patients have reported vaping only nicotine.
Thursday, the CDC updated its count of confirmed and probable cases of what it now calls EVALI, short for "e-cigarette or vaping product use associated lung injury."
Meanwhile, state health departments tell NBC News they've either confirmed or are investigating more than 2,100 cases. Alaska remains the only state without any reported cases.
Two more vaping-related deaths were confirmed Thursday, one in Tennessee and another in Washington, DC. That brings the total number of deaths so far to 36. Other deaths are under investigation.
As the search for a definitive cause continues, a small study published last week in the American Journal of Clinical Pathology provided a closer look at the damage to lung tissue caused by vaping THC.
Researchers at the Cleveland Clinic analyzed lung tissue biopsies taken from eight patients who'd been treated either in Ohio or in other states. All are men, and all mainly vaped THC. One of those patients died.
A microscopic view of the tissue revealed two different patterns of lung injury. One is called organizing pneumonia, which is when tiny airways and air sacs become inflamed. Others had damaged alveoli, the tiny air sacs responsible for allowing oxygen into the blood and carbon dioxide out.
Some patients had a combination of the two.
"This is one more layer of evidence that this is a practice that is causing lung damage," Dr. Sanjay Mukhopadhyay, director of pulmonary pathology at the Cleveland Clinic, said. "That's the major advantage of having a microscopic study."
The study didn't look at which products or ingredients may have caused the damage, though one patient told his doctors that he'd switched to a product labeled "Dank Vapes" a few weeks before he was hospitalized.
The CDC previously reported that other patients diagnosed with EVALI also said they'd used "Dank Vapes," which are counterfeit.
Early on in the investigation, some doctors had diagnosed patients with a different form of pneumonia, called lipoid pneumonia. That occurs when inflammatory cells with abnormally high levels of fatty substances called lipids collect in the lungs.
The diagnosis made sense given that many vape devices tested contained high levels of vitamin E oil. However, that ingredient has not been detected in all of the devices, and public health officials have said that while vitamin E oil may be one clue, it does not explain all the cases.
Indeed, Mukhopadhyay's team did not find any evidence of lipoid pneumonia.
"We don't see under the microscope what you would expect to see if a lipid was causing the damage, but that doesn't mean necessarily that it's not," he told NBC News.
The Cleveland Clinic analysis also found similarities in symptoms among the patients: fever, night sweats, cough, nausea, vomiting, diarrhea and trouble breathing.
Those symptoms are all similar to another illness that's starting to show up nationwide: the flu.
Mukhopadhyay said he worries that some cases of the flu will get mistaken for vaping injuries, and vice versa. He also said it's possible those who vape may have worse outcomes if they end up with influenza.