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E-Cigarette Vapor Linked to Lung Cancer in Mice

A new study offers yet more evidence that vaping isn't the completely safe alternative to smoking it was once thought to be.

Ed Cara, Gizmodo, Oct 7, 2019

The researchers claim to have found evidence, in mice, that e-cigarette vapor is capable of causing certain kinds of cancer. But there's still a long way to go before we can know if the same is true in people and how large of a cancer risk it could pose.

There've been hints that e-cigarette vapor, much like tobacco smoke, could be carcinogenic. In an earlier study by the same team of researchers, all based at New York University's School of Medicine, they found that vaping could damage DNA in the bladder and lung cells of both mice and people enough to increase their risk of turning cancerous. And other scientists have found the presence of chemicals known to be carcinogenic in e-cigarette vapor, especially from flavored products. But the new study, published in the journal PNAS, is perhaps the first to so directly tie e-cigarettes to cancer.

For their study, the researchers experimented with mice for little over a year. Over a period of 54 weeks, they exposed groups of mice to three different conditions, each lasting four hours a day, five days a week. One group of mice spent time in a chamber filled with e-cigarette vapor created by a machine that mimicked a typical vaping product, meaning that nicotine was heated and aerosolized from a liquid containing the solvents propylene glycol and vegetable glycerin. Another group was exposed to vaporized air just containing the solvents, and the third simply spent their time breathing filtered air.

By the time the experiment ended, nine of the 40 mice (22.5 percent) exposed to typical e-cigarette vapor developed lung cancer, while only one of the mice in either control group did the same. More than half of the e-cigarette vapor group also developed an enlarged bladder (a condition called hyperplasia), a risk factor for bladder cancer, compared to a single mouse who did the same across both control groups.

According to lead author Moon-Shong Tang, a molecular biologist at NYU, the team also found that certain cancer-causing compounds, called nitrosamines, are formed in the bodies of mice when exposed to vapor filled with nicotine. These nitrosamines are known carcinogens in both mice and humans.

"So, the probability is very high that e-cigarette vapor is a human carcinogen," Tang told Gizmodo via email.

The team's findings, scary as they are, do carry some important limitations. Mice aren't people, of course, but the type of mouse used by Tang and his team is more susceptible to cancer-causing chemicals over its one-year life span than other mice. These mice are often used in cancer research because it usually takes a long time to spot whether any particular thing can cause cancer (that said, they aren't genetically predisposed to lung cancer). So while this study might show that e-cigarette vapor can be cancerous, it can't predict how carcinogenic it could be in humans.

There's a similar gap when it comes to figuring out how much of a cancer risk e-cigarettes are compared to traditional tobacco smoking. At this point, Tang said, there isn't any sense of how relatively carcinogenic vaping might be when stacked up against smoking. And the authors wrote that their results shouldn't lead the public to "equate the risk" of vaping to smoking.

At the same time, some public health experts and organizations have been vocal in pronouncing e-cigarettes to be around 95 percent less harmful than tobacco cigarettes. Much of that calculation has weighed on evidence that nicotine itself isn't carcinogenic, or that vaping barely exposes people to its cancer-causing byproducts, the nitrosamines. But if Tang and his team are really onto something here, then that 95 percent figure might lie on shaky ground.

In any case, the debate is unlikely to be settled anytime soon, even as research in both mice and humans continues.

"It takes two decades or more for a life-time smoker to develop lung cancer," Tang noted. "If tobacco smoke-induced lung carcinogenesis is a paradigm for e-cig carcinogenicity, then it will take at least another decade to have e-cig-related human lung cancer to show up."

In other words, it's going to take a long time studying people who vape to truly know how much risk there is-and that's leaving aside the more immediate, potentially fatal dangers of using unregulated vaping products.

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