This editorial has been updated to reflect news developments.

Health officials have identified one potential cause of the mysterious vaping-related illness that has sickened more than 450 people and claimed at least three lives: vitamin E acetate, an oil found in some marijuana-based vaping products . But there’s still a lot they don’t know. Are other adulterants also involved? Does a combination of vaping ingredients, or the use of a certain vaping device, increase the likelihood of falling ill?

Parents have been anxious, as many patients so far have been teenagers and young adults. Health officials are scrambling — the Centers for Disease Control and Prevention have advised those who are concerned to not use e-cigarette devices . And, as uncertainty persists, states and cities are taking matters into their own hands. On Wednesday , Michigan became the first state to ban flavored e-cigarettes. San Francisco has issued an even more comprehensive ban — of all e-cigarettes — and other communities have similar measures in the works.

O utright product bans are risky. If they give rise to a more dangerous black market, or drive vapers back to traditional cigarettes, the effects could be disastrously counterproductive. But the confusion and worry behind such actions are understandable, given how popular e-cigarettes have become and how little is known about their risks.

The first e-cigarettes entered the market more than a decade ago, with the promise that they would be less harmful than traditional cigarettes and could even help people quit smoking. But the Food and Drug Administration has yet to review any of these products to determine whether they offer a net benefit for public health. While e-cigarettes don’t contain the combustible tobacco that makes regular cigarettes so carcinogenic, they do contain several other chemicals whose long-term health effects are largely unknown.