Researchers may have discovered a key to unlocking a medical mystery: why hundreds of Americans around the country have been seriously sickened by vaping marijuana products. At least two have died.

The culprit may be an oil derived from vitamin E.

“Investigators at the U.S. Food and Drug Administration found the oil in cannabis products in samples collected from patients who fell ill across the United States,” reports the Washington Post, citing officials who took part in a telephone briefing with the FDA on Wednesday.

Contaminant found in vaping products is linked to deadly lung illnesses, according to state and federal lab tests https://t.co/Bjg9JXM0bx — The Washington Post (@washingtonpost) September 5, 2019

(Vaping is the popular use of e-cigarettes, which heat nicotine, marijuana-based substances or other drugs until they vaporize and can be inhaled.)

The same oil — called vitamin E acetate — was found by researchers in New York state in nearly all cannabis samples from patients who fell ill in recent weeks, a state health department spokeswoman told the Post.

Vitamin E is found naturally in some foods, including canola oil, olive oil and almonds. The oil is a common nutritional supplement and topical skin treatment, not known to be harmful.

But Vitamin E acetate has a molecular structure that could make it hazardous when inhaled, the Post says, leading to serious breathing problems.

“Its oil-like properties could be associated with the kinds of respiratory symptoms that many patients have reported: cough, shortness of breath and chest pain,” officials told the newspaper.

“Many of those who have fallen ill say they have vaped products containing marijuana, but some also used traditional nicotine e-cigarettes. Many report using both,” the Post says.

Although FDA lab tests “found nothing unusual in nicotine products that had been collected from sick patients,” the newspaper says, authorities are not ruling out the possibility of adulterants in nicotine vaping products.

As of late August, more than 200 suspected cases of the vaping-related lung ailment were reported in 25 states. More are being reported every week and are under investigation by the Centers for Disease Control and Prevention.

“Vitamin E acetate is basically grease,” Michelle Francl, a chemistry professor at Bryn Mawr College, told the Post, noting that “you have to heat it up pretty hot” for it to vaporize. That may cause the oil to decompose, meaning that “now you’re breathing in who-knows-what.”

Then, when the vapor cools down in the lungs, it becomes oil again, coating the inside of the vaper’s lungs. And, say experts, unlike the digestive tract, “which can break down and get rid of foreign substances, the lungs aren’t designed to handle anything except gases.”

