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Mice exposed to the nicotine vapor were even more likely to develop pre-cancerous growths in their bladders, the results showed.
"The sensible conclusion is the chance that an e-cigarette would cause human cancer is probably pretty high," said lead researcher Moon-Shong Tang, a professor of environmental medicine with the New York University School of Medicine, in New York City.
But other medical experts said it's too soon to leap to that conclusion.
"I think we all know that e-cigarettes and vaping are probably not as safe as everyone believes, but I don't think you can conclude too much from a single animal study," said Dr. Fred Hirsch. He is executive director at the Center for Thoracic Oncology in the Tisch Cancer Institute at Mount Sinai, in New York City. "There needs to be much more substantial data."
And animal research doesn't always pan out in humans.
For the study, Tang and his colleagues exposed 45 laboratory mice to nicotine-containing vapor four hours a day for five days a week, over a period of 54 weeks.
A second group of 20 mice were exposed to e-cigarette vapor that did not contain nicotine, and a third group of 20 mice were exposed only to the filtered air of the laboratory.
By the end, about 23% of mice breathing nicotine-laced vapor developed lung tumors, compared with only about 6% of mice breathing filtered air. None of the mice breathing simple vapor developed tumors.
Additionally, 58% of mice breathing nicotine-laced vapor developed bladder urothelial hyperplasia -- lesions in the bladder that frequently turn cancerous, Tang said. By comparison, about 6% of mice breathing simple vapor and none of the mice breathing filtered air developed these lesions, the findings showed.
Gregory Conley, president of the American Vaping Association (AVA), scoffed at the results. The AVA is a nonprofit group that advocates for sensible regulation of vaping products.
Conley called the study and its results "laughable," saying that the mice were "exposed to massive levels of vaping aerosol." He added that, "anyone who uses notoriously unreliable mice studies to discourage adult smokers from switching to nicotine vaping products should be embarrassed."
However, Tang said this and other studies actually demonstrate that nicotine can be converted into a carcinogenic substance in the body -- called nitrosamine -- through a chemical process called nitrosation.
This means that nicotine, which is not considered cancer-causing, is "only one step away from becoming carcinogenic," Tang said.
Previous lab studies in mouse and human cells found that nicotine can indeed convert into nitrosamine while inside the body, Tang said.
This study carries those findings forward, showing that exposure to nicotine-laced vapor increases cancer risk in mice.
Dr. Wasif Saif, deputy physician-in-chief and medical director of the Northwell Health Cancer Institute in Lake Success, N.Y., said that nitrosamine is a reasonable suspect for the cancer found in these mice.
"We know all these products from nicotine and nicotine derivatives are carcinogens. They can cause inhibition of DNA repair in the tissues," Saif said.
While this shows that nicotine-containing e-cigarette products probably are promoting cancer, Tang added that the proof will only come in another decade or so.
"The jury is still out," Tang said. "In 10 years, we should know."
"The full effect of this cannot be clinically evaluated in human beings until a longer period of time has passed," Hirsch said. "We need to see much more substantial data, and of course at the end of the day it is clinical data that can tell us the real truth."
The new study was published online Oct. 7 in the Proceedings of the National Academy of Sciences.
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