WASHINGTON — Federal health officials have identified vitamin E acetate in the lung fluids of 29 people sickened in the outbreak of dangerous vaping-related lung injuries. The discovery is a “breakthrough” that points to the oil as a likely culprit in the outbreak that has sickened more than 2,000 people and killed at least 39, a top official said Friday.

“These findings provide direct evidence of vitamin E acetate at the primary site of injury within the lungs,” said Anne Schuchat, principal deputy director at the Centers for Disease Control and Prevention. The latest information points to growing evidence of vitamin E acetate as “a very strong culprit of concern,” she said in a briefing with reporters.

The findings announced Friday do not rule out other possible compounds or ingredients that may be causing the lung injuries. But Schuchat described the lab results as a “breakthrough” in the investigation. CDC tested for a wide range of substances that might be found in patients’ lung fluids, including plant oils and petroleum distillates, such as mineral oil.

But what they did not find was just as important. “No other potential toxins were detected,” Schuchat said.

CDC officials found vitamin E acetate, an oil derived from the vitamin, in all 29 samples of lung fluid collected from patients who had fallen ill or died from lung injuries. THC, the psychoactive ingredient in marijuana, was also found in 23 patients, including three who said they had not used THC products. Nicotine was detected in 16 of 26 patients. Most patients who have fallen ill in the outbreak have vaped THC, officials have said.

Vitamin E acetate, an oil derived from the vitamin, has already been identified in previous testing by the Food and Drug Administration and state laboratories in vape products that contain THC. New York state’s Wadsworth Center lab was the first to discover it about two months ago in samples from sick patients. Of 595 vaping product samples linked to patients that have been tested by FDA, 70% contained THC. Half of those THC-containing products also had vitamin E acetate, with concentrations as high as 88%, the FDA said Friday.

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Many of the THC-containing products were obtained on the illicit market, officials have said. Vitamin E acetate has been used in recent months as a cutting agent or additive on the cannabis black market to stretch the amount of THC in vape cartridges, officials and industry experts have said. Vitamin E acetate is a popular additive because it is colorless and odorless, has similar viscosity to THC oil and is much cheaper.

The findings are significant because for the first time, scientists have been able to connect results from product testing with clinical specimens from patients, she said. The 29 patients are from 10 states, representing a diverse geographical area, making the findings “much more robust” than if all the patients were from a single location. Most of the patients were male, with a median age of 23. Two of the patients died.

“They help us better understand the potential compounds” that may contribute to the injuries, Schuchat said. “They tell us what entered the lungs of some of these patients.”

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Vitamin E acetate is found in many foods and in cosmetics, especially skin care products. It’s not known to cause harm when swallowed or applied to the skin, Schuchat said. But when it is heated and inhaled, it may interfere with normal lung function. Its properties could be associated with the kinds of respiratory symptoms that many patients have reported: cough, shortness of breath and chest pain, officials and experts have said.

Public health experts welcomed the news, but cautioned that more testing needs to be completed. “While this is a big step in helping us understand what may be causing these injuries, these findings do not rule out the potential for other compounds or ingredients as contributing factors,” said Scott Becker, executive director of the Association of Public Health Laboratories. “There may be more than one cause of the outbreak.”

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The findings, which were also detailed in a CDC report released Friday, also reinforced health official warnings against using e-cigarette or vaping products that contain THC, especially those bought off the street.

CDC is maintaining its recommendation that consumers consider refraining from using all vaping and e-cigarette products, including those containing nicotine. That’s because a small proportion of patients continue to report exclusive use of nicotine-containing products, Schuchat said.

Chemistry professor Michelle Francl has described vitamin E acetate as basically grease. Its molecular structure means that “you have to heat it up pretty hot” for it to vaporize, said Francl, a professor at Bryn Mawr College. The substance’s boiling point is 363 degrees Fahrenheit, which is well above the 212 degrees F boiling point for water.

Once the oil is heated enough to vaporize, it can potentially decompose, and “now you’re breathing in who-knows-what,” Francl has said.

But officials still need to test for the substance in other people who vaped and who did not experience these injuries, Schuchat said. Officials also want to test a broader number of lung fluid samples from patients in different locations, she said.

Animal studies also need to be undertaken to better understand how vitamin E acetate might cause the harm in lungs, she said.

For the lung fluid testing, CDC scientists had drawn up a priority list of chemicals that had the potential to be toxins. They included vitamin E acetate; plant oils; petroleum-based oils such as mineral oil; and terpenes, organic compounds that give fragrant aromas to plants and flowers, said Jim Pirkle, director of the laboratory science division at CDC’s National Center for Environmental Health. Pirkle is overseeing much of CDC’s outbreak-related lab testing.

“We haven’t found any of those,” except for vitamin E acetate, Pirkle said. Scientists no longer have on the priority list a “top five” of potential toxins “that say we need to dive into next,” he said. But the samples account for only a small portion of the number of people sickened, and more testing will continue.

Finding vitamin E acetate in 29 out of 29 samples of lung fluid is “a very strong signal,” he said in a later interview, adding that it is “pretty much unheard of.”

Vitamin E acetate is a sticky substance, like honey, Pirkle said. “When it goes into the lung, it does hang around.” The oil can disrupt the natural ability of the lungs to stay inflated, officials said.

THC, on the other hand, “is not something you would expect to be hanging around in the lung fluid,” Pirkle said. It is more likely to be found in urine, he said. That could account for why it was not found in some samples. It’s also possible that the vitamin E acetate was part of different kind of vaping liquid, officials said.

Video: As illnesses and deaths linked to vaping continue to rise, public officials have begun a crackdown on e-cigarettes. Despite the negative headlines, the e-cigarette market is expected to double by 2024. (Luis Velarde, Jorge Ribas / The Washington Post)