Vapes, or e-cigarettes, contain a complicated mixture of carrier solvents, flavor chemicals, and nicotine carefully calculated to optimize the user experience. But the chemical interplay of these different components—particularly during the heating cycle of each puff—is poorly understood, and experts are increasingly concerned that an unknown raft of potentially harmful by-products is generated as the device is used. This risk is even more acute for high-puff-count pens, which concentrate these toxicants over a longer period and therefore increase the user’s exposure to these problematic compounds.
Glyoxal and methylglyoxal, which probably form by breakdown of a vape’s carrier solution, were both found in high concentrations in used vapes. Studies have shown methylglyoxal is even more toxic than diacetyl, the compound implicated in popcorn lung.
New research finds that as high-puff-count vapes run low, aldehyde breakdown products become concentrated in the remaining liquid (ACS Omega 2026, DOI: 10.1021/acsomega.5c13033). A comparison between the fluid composition of used and unused devices identifies a worrying increase in the toxic aldehydes methylglyoxal (MGO) and glyoxal.
Prue Talbot and her team at the University of California, Riverside, collected different brands and flavors of discarded and donated used vapes. They compared the composition of the residual fluid against new, unused products for each sample. Overall, the team analyzed 77 devices from 20 brands and detected almost 200 chemical components, including 9 aldehydes not listed by the manufacturers.
In particular, both MGO and glyoxal were present at notably higher concentrations in all the used devices. Talbot suggests that the compounds may be produced by thermal decomposition of the carrier solution during vaping. Previous studies have demonstrated that MGO is significantly more damaging to lung tissue than the known toxic ingredient diacetyl—the compound implicated in popcorn lung. The team further evidenced this with a simple lung cell assay: within just nine puffs of MGO, bronchial cells showed substantial cell rounding, a key marker of stress.
“If a vaper continues to the very end of that product, this is what they’re going to be inhaling. And that’s important, because it seems to be universal to all the products that they test,” says Donal O’Shea, an analytical chemist at the Royal College of Surgeons in Ireland who was not involved in the work.
But O’Shea says insight also lies in what the researchers didn’t detect. The aldehydes they found are formed by specific chemical pathways, and their presence in the residual solution can provide a window into some of the more volatile components that may already have been inhaled.
In the team’s initial analysis of the unused vapes, several brands and flavors contained the aldehyde acrolein, which is presumed to form after manufacture. But subsequent analysis of the used e-cigarettes revealed that in almost all cases, the concentration of acrolein had decreased or even vanished. “That particular aldehyde is very toxic, even in small quantities, and that should be alarming because it’s the one that they’re not seeing in the vaped samples,” O’Shea explains. “We know it’s present in the unvaped devices and being produced during vaping, so unfortunately it must be entering the lungs.”
For Havovi Chichger, a cell biologist at Anglia Ruskin University, these hidden aldehydes are an obvious target for further inhalation and whole organ studies. “The next step is to make this a bit more physiological,” she says. “It would be really good to understand what the vapor composition is, as what’s in the [liquid left in the] chamber is not the same as what the person will be inhaling, and that is what the lungs will see.”
