A new federal report points to an oily chemical known as vitamin E acetate as the likely culprit behind more than 2 000 cases of severe lung illness among vapers.
After taking fluid samples from the lungs of 29 vapers who were hospitalised for the illness in 10 states, scientists from the US Centers for Disease Control and Prevention spotted the chemical in all of the samples.
"This is the first reported identification of a potential toxicant of concern (vitamin E acetate) in biologic specimens obained from [these] patients," the researchers said in the November 8 online edition of the CDC's Morbidity and Mortality Weekly Report.
"These findings provide direct evidence of vitamin E acetate at the primary site of injury."
But the CDC researchers added that more study is still needed.
"Based on these data from 29 patients, it appears that vitamin E acetate is associated with [severe lung illness]; however, it is possible that more than one compound or ingredient could be a cause of lung injury, and evidence is not yet sufficient to rule out contribution of other toxicants," according to Benjamin Blount and his colleagues at the CDC.
Vitamin E acetate is derived from vitamin E, which is found in vegetable oils, nuts and seeds, and leafy green veggies. It is available as a dietary supplement and skin treatment.
But when vaped and inhaled, this oil can harm lung cells, one respiratory expert has said.
Can prove fatal
"My understanding of vitamin E acetate, the oil, is that it needs to be heated to a very high temperature in order to be transformed into a vapour," said Patricia Folan, who directs the Center for Tobacco Control at Northwell Health, in Great Neck, New York.
"However, when an individual inhales the vapour into their lungs, the temperature in their lungs is lower, causing the substance to return to its oil state," she added. "This in turn causes shortness of breath, lung damage and the respiratory illness being seen in [these] individuals."
Dr Teresa Murray Amato is chair of emergency medicine at Long Island Jewish Forest Hills, in New York City. She noted that, "once inhaled, oil can set off an inflammatory response that can lead to severe lung injury.
"Acute respiratory distress syndrome (ARDS) is one of the dreaded complications as it can lead to the need for intubation - placing a breathing tube - and being placed on a ventilator to assist in the respiratory effort." In the most severe cases, ARDS can prove fatal, she added.
More than 2 000 affected
As of November 7, the number of Americans stricken with severe lung illness tied to vaping had reached 2 051, the CDC reported.
That's a rise from the 1 888 case total from a week ago.
Cases have been reported in every state except Alaska, the agency noted.
The related death toll has also risen by two over the past week, to 39 fatalities, spread across 24 states and the District of Columbia. Deaths have involved patients ranging from the ages of 17 to 75, with a median age of 53.
The CDC has noted that 86% of cases involved products that contained THC, the psychoactive ingredient in marijuana.
Young men are being especially affected, with 70% of patients being male and 79% under the age of 35.
In light of the most recent findings on vitamin E acetate, the CDC continues to recommend that people refrain from vaping.
'Have to take care of our kids'
Meanwhile, on Friday, speaking outside the White House, US President Donald Trump said his administration plans to raise the US federal legal age to buy e-cigarettes from 18 to 21.
That's just one plank in the administration's new plan to curb youth vaping, Trump told reporters.
"We have to take care of our kids, most importantly, so we're going to have an age limit of 21 or so," the Associated Press reported.
The White House's proposed move is not the first in the United States: About a third of states have already raised their sales age to 21. But federal law still puts the age at 18.
Back in September, President Trump announced that the White House would propose a ban on all flavoured e-cigarette products as well. Flavoured vapes are thought to be especially attractive to kids and teens.
However, the absence of such an announcement on Friday has some experts worried that Trump might not come through with that move, the AP said.
Trump's statement to reporters was somewhat vague.
"We're talking about the age, we're talking about flavours, we're also talking about keeping people working -- there are some pretty good aspects," he said.
And in a separate White House event on Friday, Trump adviser Joe Grogan seemed to offer a softer stance on vaping.
He called e-cigarettes are "a viable alternative to combustible cigarettes," the AP reported. "We really want to make sure we're data-driven on this and striking the right balance between adult choice and protecting kids," Grogan said.
Kids are being harmed, however: The latest federal data shows 1 in every 4 high schoolers now vapes.