Smoking cigarettes is responsible for the vast majority of lung cancers in the world. This terrifying prospect has swayed many smokers to switch to e-cigs, which are thought to be benign in comparison to cigarettes. Vaping, however, is relatively new and there is still much we don’t know about the long-term health consequences. A new study on mice suggests that vaping may also cause lung cancer, for instance.

Researchers at New York University performed an experiment in which they exposed mice to two types of vapor. One contained nicotine, emulating the kind of vapor inhaled when using e-cigarettes, the other was a vapor that contained two additives commonly found in e-juice, but no nicotine.

Nine out of the 40 mice exposed to the nicotine vapor developed adenocarcinoma, which is the most common type of lung cancer — that’s an incidence rate of 22.5%. Meanwhile, none of the 18 mice exposed to the e-juice vapor developed lung cancer. One of the 18 mice who weren’t exposed to any type of vapor (the control group) got cancer.

There’s one big caveat to this study — the mice were engulfed in vapor for four hours a day for five days a week. This doesn’t come anywhere close to how e-cigs are actually used in the real world, which caused the study to garner a lot of criticism.

Yet, the study’s methods aren’t necessarily flawed. They show that, albeit in a very extreme case, that there is a very strong connection between nicotine vapor exposure and lung cancer. The findings suggest that e-cigs might cause cancer in some situations — at least it’s an area of research worth pursuing very seriously.

The number of vapers has been increasing rapidly — from about seven million in 2011 to 41 million in 2018, globally. The global market is now estimated to be worth $19.3bn – up from $6.9bn just five years ago.

“Tobacco smoke is among the most dangerous environmental agents to which humans are routinely exposed, but the potential of E-cig smoke as a threat to human health is not yet fully understood,” says Moon-shong Tang, a professor at NYU’s Departments of Environmental Medicine, Medicine, and Pathology. “Our study results in mice were not meant to be compared to human disease, but instead argue that E-cig smoke must be more thoroughly studied before it is deemed safe or marketed that way.”

The new study builds upon the team’s previous work, carried out in 2018, which exposed mice and human cells to e-cig vapor. The researchers reported that e-cig vapor induced DNA mutations linked to lung cancer.

“Our results support the argument that the nicotine-derived DNA adducts are likely the main causes for carcinogenesis in mice exposed to E-cig smoke,” says study author Herbert Lepor, MD, the Martin Spatz Chair of Urology at NYU Langone Health. “Our next step in this line of work will be to expand the number of mice studied, to shorten and prolong E-cigarette exposure time, and to further investigate the genetic changes caused by E-cigarette smoke.”

The findings appeared in the Proceedings of the National Academy of Sciences.