A new study out Monday is yet more evidence that vaping isn’t the completely safe alternative to smoking it was once thought to be. 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.

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