Researchers announced they have discovered that e-cigarette vapor containing nicotine causes lung cancer in mice.

Nine of 40 or 22.5 percent of mice exposed to nicotine-containing e-cigarette smoke for a year and two weeks developed tumor cells in the lungs, according to a study published Monday, Oct. 7 in the journal Proceedings of the National Academy of Sciences (PNAS). None of 20 mice exposed to vapor without nicotine developed cancer. The researchers also discovered that e-cigarette smoke caused pathological bladder cell changes in over 57% mice exposed.

The study is believed to be the first animal experiment to find that nicotine alone can cause lung cancer.

'We wanted to see if e-cigarette vapor would cause cancer in mice," said lead author and molecular biologist Moon-shong Tang, professor of Environmental Medicine at New York University School of Medicine. "We found lung cancer in mice and also pathological changes in the bladder."

The  study divided 85 mice into three groups, with 45 being exposed to e-cigarette vapor for four hours a day, five days a week for 54 weeks. Another 20 mice were exposed to the same vapor without nicotine, and final group of 20 mice were exposed to ambient air conditions. Adenocarcinoma of the lung was detected in nine mice exposed to nicotine vapors, none in the group exposed to non-nicotine vapor, and one in the control group.

Tang's team had already reported results in 2018 showing that e-cigarettes caused DNA damage in the lung, heart and bladder of mice after twelve weeks. "That's why we followed up the experiment," he said, "to see if (nicotine vapor) will cause cancer or not."

E-cigarettes, which contain nicotine but not tobacco, have been  marketed as a safer alternative to cigarette smoking, and their use has exploded among young people and adults as cigarette smoking has declined. According to a  recent Minnesota Student Survey, 26 percent or one-in-four 11th graders have vaped in the last 30 days, and 76 percent believe there is either no, sleight or moderate risk to using e-cigarettes.

Tobacco is a known carcinogen due to the effects on tobacco leaves of curing and combustion, which creates cancerous chemicals known as aldehydes and nitrosamines. Until now, health officials have highlighted the dangers of e-cigarette use on the developing brain and warned that the vaping of nicotine is highly addictive, but because nitrosamines have not been found in the bodily fluids of persons vaping nicotine, the possibility of getting lung cancer from vaping commercially-available e-cigarettes has not been depicted as part of their risk profile.

Tang's research suggests cancer-causing nitrosamines are created within cells and undetectable to blood, saliva and urine testing.

"The belief that nicotine in general is not carcinogenic is based on one study done many years ago in Europe," said Tang. "They exposed rats to aerosolized nicotine for two years, and they didn't find cancer formations in the lung, so they concluded that nicotine is not carcinogenic." Tang says that study was too small, however, and used air to disperse nicotine, resulting in particles too large to reach the inner sacs of the lungs.

"With e-cigarette vapor, the particle size is very small," he says, "and can easily penetrate all the way down to alveola air sacs in the lungs," which is where his team found cancer in mice.

Among the study's limitations were that the mice were exposed to vaping smoke whole body, and that the study was small. Tang also says the mechanism by which nicotine is carcinogenic to mice may be ultimately prove less carcinogenic than tobacco, and that it remains unknown if vaping is likely to affect humans in the same way as mice. Epidemiology studies will not answer that question for years to come.

"It takes two decades to develop lung cancer for a lifetime smoker," he says. "E-cigarette vapor became popular about 8 years ago. It will take another decade to find out if lung cancer emerges in humans." Tang plans to study if mice develop cancer earlier than 54 weeks, and if more mice develop cancer after longer periods.

Though he does not know if regulators should warn users that e-cigarette use is carcinogenic to humans, Tang believes he knows enough to make a general warning about vaping.

"Its definitely not harmless," he says. "Its harmful."