Study finds ‘significantly reduced’ toxicity from e-liquid aerosols compared with cigarette smoke

A new peer-reviewed study from Italian researchers has reported that aerosols from four popular nicotine-containing e-liquids caused dramatically less cytotoxicity and oxidative stress in human bronchial epithelial cells than smoke from a reference tobacco cigarette.

The paper, published last month in Molecular and Cellular Biochemistry, concludes that – under realistic vaping conditions – tested e-liquids showed “an excellent safety profile” in vitro.

The team, mainly based at the University of Catania, assessed four commercially available 20mg/ml nicotine e-liquids marketed in Italy, analysing their chemical purity and their biological effects on lung-derived H292 cells using an air-liquid interface (ALI) model designed to mimic real-world inhalation exposure.

Low contaminant levels and no detectable ROS

The researchers first screened the liquids for metals and microplastics. According to the paper, levels of arsenic, lead, mercury and cadmium were below detection limits, while trace elements such as aluminium and iron were present at concentrations well below WHO drinking-water guideline values. Microplastic counts ranged from 51 to 1,070 particles per gram – tiny fractions compared with levels found in bottled water.

In cell-free tests, the four flavoured e-liquids did not generate reactive oxygen species (ROS) at any puffing level tested. In contrast, the reference 1R6F cigarette generated “a large amount of ROS in a dose-dependent manner as early as 9 puffs.”

ROS are chemically reactive molecules containing oxygen that are generated as byproducts of metabolism or from environmental stressors. While they are crucial for cell signaling, an overproduction can lead to oxidative stress, which damages cellular components like proteins, lipids, DNA, and RNA.

Cigarette smoke caused severe cytotoxicity; e-liquids did not

Across multiple cytotoxicity assays – including Neutral Red Uptake, Annexin V apoptosis analysis and real-time cell analysis – cigarette smoke produced extensive cell death, while e-liquid aerosols exerted minimal impact.

In the NRU assay, exposure to five puffs of 1R6F smoke reduced cell viability to 32 per cent, whereas all four e-liquids maintained viability above 91 per cent after 24 hours. Sample D showed a small decrease, but still far below the toxic effect of smoke. “Substantial reduction of cell viability was observed for cells exposed to 5 puffs of 1R6F smoke,” the authors wrote, noting no significant loss of viability for samples A, B or C.

Annexin V analysis similarly found that only cigarette smoke drove large increases in apoptotic and dying cells, while “no differences in viable cells were shown” for any e-liquid compared with air controls.

Real-time cell analysis showed cigarette-exposed cells undergoing “complete cell death as early as 8 hours,” while cells exposed to e-liquids remained close to control levels for up to 48 hours.

Mitochondrial function preserved after vaping, but not after smoking

Using a JC-1 probe to monitor mitochondrial membrane potential, the study found that cigarette smoke caused a “strong and long-lasting reduction in mitochondrial function,” fully depolarising mitochondria by around 14 hours. In contrast, all e-liquids produced only mild, transient changes, with no significant difference from air controls across the 72-hour observation period.

Findings reinforce e-cigarettes’ reduced-risk profile

The paper concludes that the results “support the reduced harmful potential of e-cigs relative to tobacco cigarettes” in this in-vitro model. While the authors emphasise that these are short-term cellular findings and not clinical outcomes, they argue the data strengthen the evidence base for e-cigarettes as a harm-reduction tool.

“Under normal conditions of use and with the power settings recommended by manufacturers, the vaping products under investigation proved to be significantly less harmful to human cell systems compared to conventional cigarettes,” they wrote.

The researchers caution that cytotoxicity was tested at a single exposure level for the e-liquids, and that WHO drinking-water limits used as reference points for contaminants do not directly correspond to inhalation safety thresholds. They call for longer-term studies and additional assays to assess chronic effects.