Health officials have identified a possible culprit behind the spate of vaping-related illnesses that have sickened thousands: vitamin E acetate.

The chemical — used as an additive or thickening agent in some vaping products — turned up in every sample of lung fluid collected from 29 patients with vaping-related illness, the Centers for Disease Control and Prevention reported Friday. While vitamin E acetate is used in supplements and skin creams and does not seem to cause harm when swallowed or used topically, previous research suggests that inhaling vitamin E acetate might impair people’s lung function.

The finding marks a significant development in an investigation that has left federal and local health officials scrambling to find a cause as the number of vaping illnesses recorded nationally has continued to climb. Health officials said that the findings need to be confirmed, including through animal studies, and that it’s too soon to rule out other possible causes.

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“These new findings are significant because for the first time, we have detected a potential toxin of concern — vitamin E acetate — in biologic samples,” said Anne Schuchat, principal deputy director of the CDC. But, she added, “there’s more work to do.”

There have been 2,051 confirmed and probable cases of the illnesses, dubbed EVALI, across 49 states, Washington, D.C., and the U.S. Virgin Islands. There have been 40 deaths tied to the outbreak, which is believed to have started earlier this year.

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“The trend in cases appears to be downward, but we understand that some states are still hard hit and this continues to be a very active investigation,” Schuchat said.

The latest samples were collected through a process in which fluid is pushed into the lungs and then collected for analysis. Lab testing found THC, the ingredient that gives marijuana users a high, in 23 of 28 patient samples — including those from three patients who said they hadn’t vaped THC products. The CDC said the lack of THC in five of those samples does not definitively indicate the patients didn’t use the drug, because THC can be difficult to detect in samples taken from lungs.

The testing also detected nicotine in 16 of 26 samples. (When there wasn’t enough liquid to run every analysis, labs prioritized tests for cannabinoids and vitamin E acetate.) Researchers also looked for other potentially harmful additives during the testing — such as plant oils — but didn’t find notable levels of any in the patient samples.

It isn’t clear how widespread the use of vitamin E acetate is in e-cigarette and vaping products. Schuchat said the substance might be unintentionally introduced to vaping products or intentionally added to dilute vaping liquids and cut down on the amount of THC added to a product.

Health officials continue to urge the public to avoid vaping and in particular, to avoid products that contain THC or that were purchased from informal sources.