While vaping has been linked to dangerous medical conditions, smokers typically turn to e-cigarettes as a means to avoid cigarette-driven diseases. Of all the dangerous long-term effects of smoking cigarettes, the risk of cancer is likely the most infamous. Cigarette smoking is linked to between 80 and 90 percent of lung cancer deaths in the United States.

However, early-stage research suggests e-cigarettes may have closer ties to cancer than we thought — even though this kind of research field has been contentious in the past.

"Our results show that e-cigs are carcinogenic in mice."

Findings presented in Proceedings of the National Academy of Sciences on Monday, showed that mice who were surrounded by the nicotine-laden haze from e-cigarettes developed lung tumors. While this study has several important caveats, these authors are very clear that their study highlights a connection between e-cigarettes and cancer risk that raises new questions about just how safe e-cigarettes actually are.

E-Cigarettes and lung tumors

In this experiment, lead study author Moon-Shong Tang, Ph.D., a professor at New York University’s Department of Environmental Medicine, and a team of scientists investigated what happens to mice exposed to two types of vapor: nicotine-laden vapor from a e-cigarette and a vapor that contained two additives used in e-juice: propylene glycol (PG) and vegetable glycerin (VG), but no nicotine. Then, they compared those mice to a control group.

In mice, e-cigarettes caused a type of DNA damage that Tang and the team suggest is linked to lung tumor susceptibility. Unsplash / Itay Kabalo

These mice were literally surrounded by vapor for four hours per day for five days each week. This isn’t how a person would use an e-cigarette, which is an issue that this team has been criticized for in the past.

Of the 40 mice exposed to the nicotine vapor, nine of them (22.5 percent) developed adenocarcinoma, the most common form of lung cancer. None of the 18 mice who were exposed to the PG or VG got lung cancer, and only one of the 18 mice in the control group got cancer.

“Our results show that e-cigs are carcinogenic in mice,” Tang tells Inverse.

However, others have historically pushed back against Tang’s findings.

E-cigarettes and cancer

E-cigarettes differ from traditional cigarettes largely because they don’t actually burn anything, but vaporize a liquid instead. Sidestepping the burning process means that e-cigarettes don’t release the 70-some dangerous chemicals, like carbon monoxide and benzene, that come from burning a tobacco product.

That’s one of several reasons that e-cigarettes have been marketed as a safer alternative to smoking. But in February 2018, Tang and co-authors published a different study in Proceedings of the National Academy of Sciences also suggesting that e-cigarettes pose a different kind of risk that could contribute to cancer.

In that experiment, which evaluated mice and human cells, Tang showed that mice exposed to e-cigarette vapor ended up with DNA damage or the formation of DNA adducts, which are mutations in DNA that are often correlated with cigarette smoking. Tang explains that those two adducts “increase the susceptibility to mutations and tumorigenic transformation in human cells.” In other words, they open the door for the cells to become cancerous over time.

However, when this paper was published several scientists took issue with the methodology. The mice were exposed to vapor in a way that wasn’t at all similar to the way a human would vape, and they didn’t expose any mice to tobacco smoke, so it was impossible to make that comparison to cigarettes directly. One of those commenters reported ties to the vape industry, but not all did.

In response to those comments, Tang and his team doubled down on their findings:

“Our results unambiguously demonstrate that ECS can induce DNA damage in the lung, heart, and bladder, as well as inhibit DNA repair in mouse lung,” they wrote in response to a critique in 2018.

In this most recent study, the team built upon their 2018 results showing that once that door is open, tumors can form in mouse lungs.

In a statement, the study co-author Herbert Lepor, M.D., explains that they do think it’s the DNA damage that’s to blame for the process:

“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,” he said.

As for humans, Tang explains that “Our results show e-cig[s] induce the same types of effects in mice as nicotine does in human cells.” That suggests that the carcinogenic effects of e-cigarettes that he saw in mice could also play out in humans too.

The human connection is far from confirmed, and this research has been fraught in the past. But for now, it raises concerns about just how dangerous e-cigarettes could be in the long term in a way that scientists are just now beginning to understand.

Abstract:

Electronic-cigarettes (E-cigs) are marketed as a safe alternative to tobacco to deliver the stimulant nicotine, and their use is gaining in popularity, particularly among the younger population. We recently showed that mice exposed to short-term (12 wk) E-cig smoke (ECS) sustained extensive DNA damage in lungs, heart, and bladder mucosa and diminished DNA repair in lungs. Nicotine and its nitrosation product, nicotine-derived nitrosamine ketone, cause the same deleterious effects in human lung epithelial and bladder urothelial cells. These findings raise the possibility that ECS is a lung and bladder carcinogen in addition to nicotine. Given the fact that E-cig use has become popular in the past decade, epidemiological data on the relationship between ECS and human cancer may not be known for a decade to come. In this study, the carcinogenicity of ECS was tested in mice. We found that mice exposed to ECS for 54 wk developed lung adenocarcinomas (9 of 40 mice, 22.5%) and bladder urothelial hyperplasia (23 of 40 mice, 57.5%). These lesions were extremely rare in mice exposed to vehicle control or filtered air. Current observations that ECS in- duces lung adenocarcinomas and bladder urothelial hyperplasia, combined with our previous findings that ECS induces DNA damage in the lungs and bladder and inhibits DNA repair in lung tissues, implicate ECS as a lung and potential bladder carcinogen in mice. While it is well established that tobacco smoke poses a huge threat to human health, whether ECS poses any threat to humans is not yet known and warrants careful investigation.