As the FDA considers how to regulate e-cigarettes, a recent study finds very tentative reason for concern: When it comes to promoting cancer development in certain types of lung cells, it may not matter whether you're smoking the real thing or the nicotine-laced vapor in an electronic cigarette.

How the test-tube study worked: Researchers grew human lung cells, which had been altered to have mutations related to cancer risk, in two types of liquids.

One had been exposed to e-cigarette vapor, the other to tobacco smoke. Both sets of cells changed—in ways associated with cancer, the New York Times reports.

There are a number of caveats, from the researchers themselves: no actual smokers were involved, and no connection between e-cigarettes and cancer was proven. And the study hasn't yet been published; its results were aired at a January meeting of lung cancer researchers, and received little attention at the time.

But lead researcher Dr. Steven Dubinett stressed what it does show: how in the dark we are about the long-term health consequences of using e-cigarettes and their ingredients.

As far as ingredients go, e-cigarettes generally contain three: nicotine, flavoring, and propylene glycol. Scientific American notes that propylene glycol, the synthetic liquid used to help products stay moist, is "generally recognized as safe." But it's typically eaten (in everything from soda to salad dressing), not inhaled—so it's not clear whether inhaling it is truly safe.

(As for liquid nicotine, just a teaspoon could kill.)

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