Many of these chemicals have never been tested on whether they’re safe to breathe in. And that makes vaping’s already unclear effects on health even murkier, because different flavors could be more or less dangerous. “Just because vanilla flavor or crème flavor is okay in your cookies doesn’t mean it’s okay when you heat it and then inhale it,” says Amanda Dickinson, a developmental biologist at Virginia Commonwealth University. “It seems that it’s a roll of the dice.”

A recent study coauthored by Dickinson investigated the effects of six different e-cig vapors on tadpoles, as a proxy test for how vaping while pregnant might affect human embryos. Some of the exposed tadpoles developed “clefts” in the bone behind the upper lip, somewhat similar to cleft palate in humans. These clefts only appeared in tadpoles exposed to two particular flavors out of six tested. When the researchers exposed tadpoles to nicotine-free versions of the same flavors, those tadpoles still developed clefts in the same ratios.

The flavors that correlated with the tadpoles’ facial defects weren’t “tobacco” and “menthol,” but rather flavors with fruit and cream overtones. These two flavors also had the most complex flavor descriptions: About 20 percent of tadpoles exposed to a flavor of “strawberry, almond, caramel, vanilla, biscuit, Vienna cream” and 70 percent of those exposed to a flavor of “cereal, berries, cream, citrus” developed clefts.

Dickinson and her colleagues pointed out that it may not be the fruity or creamy flavor additives per se, but the complexity of the flavor—or the number of chemical components in the vapor—that correlates with the cleft formation.

The tadpole study isn’t the first to note e-liquid flavors vary in toxicity. Robert Tarran, a cell biologist at University of North Carolina, has analyzed the composition of more than 100 e-liquids and tested their toxicity against human kidney cells in petri dishes by diluting the e-liquids and then slowly increasing their concentrations. The concentration needed to kill half the cells ranged from 5.997 percent for some flavors down to .002 percent for others, and could vary by as much as an order of magnitude between flavors mixed by the same vape shop. So far, Tarran hasn’t singled out any specific flavor ingredients as uniquely dangerous, but he has noticed a pattern. “The more chemicals there tended to be in the e-liquid, the more toxic it tended to be,” he says.

Tarran points out that these toxicity assays in kidney cells don’t necessarily predict overall health effects in humans, and that at this point, direct comparisons to the toxicity of conventional cigarettes aren’t feasible. It’s also possible that the effects of e-cigarettes in lungs could predispose people to a different set of ailments than tobacco smoke.