Most people who died from vaping-related injuries used products containing THC, the psychoactive ingredient in marijuana, federal health officials said Friday, offering another data point tying the outbreak of lung illnesses to products made with that compound.

Based on data available from 860 of the 1,604 patients who have fallen ill with the disease, about 85 percent reported using THC-containing products, compared to about 10 percent who reported exclusively vaping nicotine-containing products, officials said. Many sick patients said they bought THC vape products on the black market, and those have come under increased scrutiny.

“The data do continue to point towards THC-containing products as the source of individuals’ injury,” said Anne Schuchat, principal deputy director at the Centers for Disease Control and Prevention, which is leading the investigation. Officials don’t know what about the products are harmful, “but we’re seeing THC as a marker for products that are risky,” she said.

It is also becoming clearer that the surge in cases in recent months is not the result of better recognition of an existing disease, but “something riskier that is in much more frequent use,” she said. Schuchat cited the use of cutting agents that are added to THC-containing products to increase profit, and the increased availability of online videos that may have “skyrocketed” do-it-yourself instructions.

One substance that has turned up in many product samples is vitamin E oil, known as vitamin E acetate. Experts in the legal marijuana industry have said it has been added to THC oil used to fill vape cartridges. Vitamin E acetate, which is sold legally, is commonly used as a nutritional supplement and in skin-care products. It’s not harmful when ingested or applied to the skin. But health officials warn it could be hazardous when inhaled, potentially causing the sorts of symptoms many patients have reported: cough, shortness of breath and chest pain.

The Food and Drug Administration, which is testing more than 900 samples of products and devices, is also investigating the supply chain of potential illicit vaping products. FDA is working with U.S. Customs and Border Protection officials at international mail facilities to trace how products entered the U.S. marketplace, said Mitch Zeller, director of FDA’s Center for Tobacco Products. He declined to provide specifics.

As of Oct. 22, the CDC and local officials have confirmed 35 deaths among at least 1,604 cases of vaping-related injuries in every state except Alaska as well as in the District of Columbia and the U.S. Virgin Islands, according to the CDC.

The median age of patients who have died is 45, and 23 for those who survived.

For patients who have died, officials and clinicians have not always been able to obtain a full history of what they used and for how long. Schuchat said CDC has the exposure history for about 19, roughly half, of the patients who died.

While cases are still being reported, the latest data suggests cases may be leveling off or declining. But Schuchat cautioned it was too early to know whether that is what is happening. State officials may be pursuing investigations less intensively, or delayed in reporting data, or enforcement actions may be affecting the supply chain in parts of the country, she said.

“It’s also possible that warnings about THC are having an effect,” she said. As part of what she described as a “call to action” to the nation’s youth, the CDC is hoping young people listen to the voices of those who have experienced injuries first-hand and are quitting vaping, she said.

CDC officials are continuing to recommend that people refrain from using all e-cigarette and vaping products although the majority of people who have fallen ill used products containing THC and no specific component or substance has yet been determined as the cause.

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