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– In a grimy workshop, among boiling vats of chemicals, factory workers are busy turning stainless steel rods into slender tube casings, a crucial component of electronic cigarettes. Not long ago, Skorite Electronics was a tiny firm struggling to produce pen parts. Today, it is part of an enormous — and virtually unregulated — supply chain centered here that produces about 90 percent of the world’s e-cigarettes.

This year, Chinese manufacturers are expected to ship more than 300 million e-cigarettes to the United States and Europe, where they will reach shelves of Wal-Marts, gas station outlets and vaping shops.

The devices have become increasingly popular, and yet hundreds of e-cigarette manufacturers in China operate with little oversight. Experts say flawed or sloppy manufacturing could account for some of the heavy metals, carcinogens and other dangerous compounds, such as lead, tin and zinc, that have been detected in some e-cigarettes.

One study found e-cigarette vapor that contained hazardous nickel and chromium at four times the level they appear in traditional cigarette smoke; another found that half the e-cigs malfunctioned and some released vapor tainted with silicon fibers.

There have also been reports in the United States of e-cigarettes that exploded after a lithium ion battery or electric charger overheated.

“We need to understand what e-cigarettes are made of,” said Avrum Spira, a lung specialist at the Boston University School of Medicine, “and the manufacturing process is a critical part of that understanding.”

A review of manufacturing operations in Shenzhen found that many factories were legitimate and made efforts at quality control, but some were lower-end operations that either had no safety testing equipment or specialized in counterfeiting.

Little quality control

The e-cigarette industry in China has developed differently from other industries, like toys, apparel and smartphones, where global brands outsource their manufacturing here but monitor and enforce quality control standards. Chinese companies were the first to develop e-cigarettes, and that happened in a regulatory void. In the United States, the Food and Drug Administration has just begun to move toward regulating e-cigarettes, working on rules that would force global producers, in China and elsewhere, to provide the agency with a list of ingredients and details about the manufacturing process.

But analysts say setting those rules and new manufacturing guidelines could take years. In the meantime, Chinese factories are quickening the pace, hoping to build profits and market share before regulatory scrutiny arrives and most likely forces many e-cigarette makers to close.

“This is really a chaotic industry,” said Jackie Zhuang, deputy general manager of Huabao International, a tobacco flavoring company in Shanghai. “I hope it will soon be well regulated.”

The largest Shenzhen e-cigarette manufacturing operations are relatively clean, with rows of workers seated on plastic stools along a fast-moving assembly line. Smaller ones are much sloppier.

In 2004, a Chinese pharmacist named Han Li helped develop the e-cigarette, which was then sold through his company, Beijing Ruyan. Other manufacturers soon followed, and by 2009, as e-cigarettes became more popular in the United States and Europe, more factories opened.

Global tobacco giants that have entered the e-cigarette market are also manufacturing in China, and they insist they are using stringent controls.

Altria, formerly Philip Morris, sells the e-cig brand MarkTen. In a statement, Altria said MarkTen is made in China “by an established manufacturer of e-cigarettes, which is following Nu Mark’s design specifications and quality control requirements” with “detailed quality-control measures.”

Low-skilled workers

Smaller manufacturers, though, are more representative of the ethos here. Tiny start-up factories buy components from suppliers, set up assembly lines and hire low-skilled migrant workers to snap, stamp, glue and solder the components together.

Scientific studies hint at a host of problems related to poor manufacturing standards. A study published last year in PLoS One found the presence of tin particles and other metals in e-cigarette vapors and said they appeared to come from the “solder joints” of e-cigarette devices.

Another study of nearly two dozen e-cigarettes bought in the United States found large amounts of nickel and chromium, which probably came from the heating element, another suggestion that poorly manufactured e-cigarettes may allow the metals to enter into the e-liquids.

“We’ve found on the order of 25 or 26 different elements, including metals, in the e-cigarette aerosols,” said 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.”

Eventually, analysts say, the FDA could be compelled to certify e-cigarette factories and the manufacturing standards. But that could be months if not years away.