Propylene glycol is used in antifreeze, and also to create artificial smoke or fog in theatrical productions. The F.D.A. has classified it as an additive that is “generally recognized as safe” for use in food. But when asked whether inhaling it was safe, Dr. Richard D. Hurt, director of the Nicotine Dependence Center at the Mayo Clinic, said, “I don’t think so, but I’m not sure anyone knows for sure.”

Image Edwin Schwab, e-smoker. Credit... Christopher Capozziello for The New York Times

Of the e-cigarettes themselves, Dr. Hurt added: “We basically don’t know anything about them. They’ve never been tested for safety or efficacy to help people stop smoking.”

Public health officials also worry that the devices’ fruit flavors, novelty and ease of access may entice children.

“It looks like a cigarette and is marketed as a cigarette,” said Jonathan P. Winickoff, an associate professor at the Massachusetts General Hospital for Children and chairman of the American Academy of Pediatrics Tobacco Consortium. “There’s nothing that prevents youth from getting addicted to nicotine.”

Sales and use of electronic cigarettes are already illegal on safety grounds in Australia and Hong Kong, and some other countries regulate them as medicinal devices or forbid their advertising. So far the United States has focused only on stopping them at the border, although Senator Frank R. Lautenberg, Democrat of New Jersey, has asked the drug agency to take them off the market until they can be tested.

Distributors of electronic cigarettes fear that a bill making its way through Congress that would give the F.D.A. the authority to regulate tobacco could be used to put them out of business as well. The bill has passed the House and could be taken up by the Senate this week.