How carcinogenic e-cigarette use is for humans "may not be known for a decade to come," but the study is the first to definitively link vaping nicotine to cancer. Funded by the National Institutes of Health, the study was published Monday in the Proceedings of the National Academy of Sciences.

"It's foreseeable that if you smoke e-cigarettes, all kinds of disease comes out" over time, Moon-Shong Tang, the study's lead researcher, said in an interview. "Long term, some cancer will come out, probably. E-cigarettes are bad news."

E-cigarette vapor causes lung cancer and potentially bladder cancer in mice, damaging their DNA and leading researchers at New York University to conclude that vaping is likely "very harmful" to humans as well.

A February study by the University of Southern California found that e-cigarette users developed some of the same molecular changes in oral tissue that cause cancer in cigarette smokers, according to the study published in the International Journal of Molecular Sciences.

In the NYU study, researchers found that e-cigarette vapor caused DNA damage in the lungs and bladder and "inhibits DNA repair in lung tissues." Out of 40 mice exposed to e-cigarette vapor with nicotine over 54 weeks, 22.5% developed lung cancer and 57.5% developed precancerous lesions on the bladder.

None of the 20 mice exposed to e-cigarette smoke without nicotine developed cancer over the four years they studied the mice, researchers said.

That's "statistically very significant," said Tang, who's a professor at the NYU School of Medicine.

Tang said his results heighten the need for more research about the relationship between e-cigarette use and cancer in humans. Because the market is still relatively young, he said it might be another decade before its impact on humans is more thoroughly understood. Based on his findings in mice, Tang said he doesn't think the research will show e-cigarette use is safe for human consumption.

The amount of smoke the mice were exposed to was similar to what a human would inhale if they vaped regularly for about three to six years, Tang estimated.

"If they use e-cigarettes regularly, that's probably similar," he said. Much like combustible cigarettes, Tang said his findings suggest that secondhand vaping fumes also pose a risk to other people within close proximity.

There were limitations to the study. The mice did not inhale the vapor as deeply as a human would, for instance. It also was conducted in a small number of mice that were more likely to develop cancer over their lifetime, researchers noted.

However, the data comes at a time of increased scrutiny of e-cigarettes as underage use rises and U.S. health officials trace an outbreak of a deadly lung disease back to vaping, mostly THC, the active compound in marijuana. Some of the more than 1,000 victims who have fallen ill have reported using only nicotine, leading doctors to say they can't rule anything out.

Flavored e-cigarettes have fueled what government regulators are calling a teen vaping epidemic. The Food and Drug Administration is currently finalizing its guidance to remove all nontobacco flavors of e-cigarettes, including mint and menthol, from the market to deter underage usage. Some state and local governments are starting the removal process, too.

Market leader Juul, which didn't respond to a request for comment, is under investigation for marketing their products as a safer alternative to smoking and as a way that adults can wean themselves off of cigarettes. Some research does back up those claims. The Federal Trade Commission also opened a probe in August of the industry's marketing practices, seeking information from Juul and five other companies.

However, Tang noted there's a difference between being safer than cigarettes and safe in general.

"Young kids think it's safer," Tang said. "But it will cause cancer in mice."

Tony Abboud, executive director of the Vapor Technology Association, disputed the findings. He cited a 2015 study by Public Health England that found e-cigarettes to be "at least 95% safer" than traditional cigarettes.

"As with all individual studies there are enormous limitations such as the minute sample size that the authors in this specific study point out," Abboud said in an emailed statement. "The larger body of scientific evidence must be considered and more research should be done, but today's study in PNAS does not stand for the proposition that the headline suggests."

He also cited a 2016 report by British doctor's group the Royal College of Physicians that said the harm caused by e-cigarettes equated to about 5% "of the burden caused by tobacco smoking."

Linda Cuthbertson, spokeswoman for the Royal College of Physicians, said "elements of our reports and statements have been used in isolation." The report cited by Abboud said e-cigarettes, while less harmful than tobacco cigarettes, may still be more hazardous than other forms of nicotine replacement since the industry isn't uniformly regulated and manufacturing varies, she noted.

The same report said "some of the carcinogens, oxidants and other toxins present in tobacco smoke have also been detected in e-cigarette vapour, raising the possibility that long-term use of e-cigarettes may increase the risks of lung cancer, COPD, cardiovascular and other smoking-related diseases," although it's likely substantially less of a risk than in traditional smoking.

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