“We’ve found on the order of 25 or 26 different elements, including metals, in the e-cigarette aerosols,” says Prue Talbot, a professor of cell biology at the University of California, Riverside, and co-author of several of the studies. “Some of the metal particles are less than 100 nanometers in diameter, and those are a concern because they can penetrate deep into the lungs.”

Health advocates say they are troubled by a history of food and drug safety scandals in China, such as when manufacturers substituted diethylene glycol, an industrial solvent, for the sweetener glycerin when making toothpastes and cough medicine. That led to reports of more than 350 deaths in Panama, China and other countries in 2006 alone.

The risk of diethylene glycol showing up in e-cigarettes is real. In 2009, the F.D.A. issued a warning about the potential health risks associated with e-cigarettes, saying laboratory studies of some samples had found the presence of toxic chemicals, including diethylene glycol, which is used in antifreeze.

Pressure on Regulators

Eventually, analysts say, the F.D.A. could be compelled to certify e-cigarette factories and the manufacturing standards. But that could be months if not years away. The agency, however, is under pressure from public health advocates and medical experts.

“What if someone in China buys nicotine, solvents and flavorings, but the source of these ingredients is unknown and they’re manufactured with impurities?” says Maciej Goniewicz, a toxicologist at the Roswell Park Cancer Institute in Buffalo. “That could put consumers at risk.”

Keenly aware that tighter regulations are on the horizon, Shenzhen e-cigarette makers are beginning to establish overseas branches to make e-liquids — the substance that is heated, then turned into vapor and inhaled. The F.D.A. does not yet have standards for e-liquids, but many of the Chinese companies say they make them in labs in the United States that have passed F.D.A. quality-control standards.

“I can tell you that all of our e-liquid is manufactured, bottled and filled here in the United States,” the chief executive at Mistic e-cigarettes, John J. Wiesehan Jr., said in an email. “Our liquid never leaves the U.S. We get no e-liquid from China.”