In 2007, health officials in California revealed that numerous workers in local flavoring factories suffered from a rare, life-threatening lung condition called bronchiolitis obliterans, later nicknamed “popcorn lung.” The illness, which scars the air sacs of the lungs and makes breathing difficult, stemmed from exposure to diacetyl, a yellow chemical used to give microwave popcorn its buttery flavor. Now, that chemical is at the center of a new study revealing that it’s harmful in vape liquids as well.

"Why aren’t e-cig users receiving the same warnings?

In the study, published Friday in Scientific Reports, researchers from Harvard’s T.H. Chan School of Public Health show that diacetyl, together with a similar chemical called 2,3-pentanedione, can impair the function of the lungs when it’s inhaled. Flavoring chemicals are found in over 90 percent of commercially marketed flavored e-cigarettes, and of those chemicals, diacetyl, is the most common, the authors report. 2,3-pentanedione is used as a substitute in e-liquids, they add, likely because diacetyl is associated with popcorn lung. The European Union banned diacetyl in vape liquids in 2016.

Diacetyl, a compound used to impart a butter flavor, has been implicated in "popcorn lung" cases among flavor factory workers. Unsplash / Christian Wiediger

Though these chemicals are considered safe ingredients to ingest in food, diacetyl’s history strongly suggests that it’s not safe to breathe in, especially not in vape form. Workers in flavoring factories now receive warnings about the dangers of inhaling flavoring chemicals, said co-senior author Joseph Allen, Ph.D., who asked: “Why aren’t e-cig users receiving the same warnings?”

Allen and another co-senior author, Quan Lu, Ph.D., led a team who investigated what these chemicals do to the human lung. Rather than experiment on actual humans, they used normal human bronchial epithelial cells — the ones lining the lung — in a system closely mimicking a living human airway.

They saw that exposing their artificial airway to the chemicals for 24 hours significantly decreased the lung’s usual number of cilia, the finger-like protrusions that stick out from the surface of lung cells to sweep mucus and other dirt away from the lung and out through the mouth. Cilia, which can also be damaged by smoking, are often considered the lung’s first line of defense against large irritating particles, which can be coughed out. Normally, 50 to 75 percent of cells lining the airway have cilia.

The finger-like cilia of the lung epithelial cells sweep away particles to keep the lung clear. Wikipedia

Looking more closely at the genomes of these chemical-exposed cells, the team found that 163 genes were regulated differently after exposure to diacetyl; ditto for 568 genes after exposure to 2,3-pentanedione. Exposure to these chemicals via e-cigarettes for just 24 hours, the team concludes, changes the genes of cells in the airways, hampering their ability to sweep particles away.

That can’t be good, especially at the epidemic scale at which teens are using vapes. Originally marketed as a way to help people stop smoking — a claim that has some scientific support — vapes have been adopted as a new teen hobby. A lot of its immense popularity has been blamed on the fact that the e-liquids are so tasty and appealing to kids. Concerned San Francisco voters moved to ban flavored vape products in June 2018 for this reason, though the continued rise in popularity shows that few other cities have followed suit. Other studies showing the cell-harming effects of other e-liquid chemicals imparting cinnamon and butter aromas exist, though the evidence hasn’t been enough to stem vaping’s tide.

Complicating the research is the vaping industry itself, which has conducted its own studies on the usefulness of vape flavors for quitting smoking.

The greatest concern about vaping’s meteoric rise in popularity is that the science just can’t keep up. Scientists need to conduct studies to find out whether vaping is addictive, is a “gateway drug,” and has long-term effects, but between its introduction to society and its incredibly enthusiastic adoption, there hasn’t been enough time to find out.