With 90 per cent of the world's e- cigarettes being made in China, health experts have warned that poorly manufactured devices can vaporise heavy metals and carcinogens alongside the nicotine, harming the users.

This year, Chinese manufacturers are expected to ship more than 300 million e-cigarettes to the US and Europe, where they will reach the shelves of Walmart, 7-Eleven stores, gas station outlets and so-called vaping shops.

The devices have become increasingly popular, particularly among young adults, and yet hundreds of e- cigarette manufacturers in China operate with little oversight, The New York Times reported.

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

There have also been reports in the US of e-cigarettes that exploded after a lithium ion battery or electric charger overheated, causing burns.

"We need to understand what e-cigarettes are made of," says 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 by The Times of manufacturing operations in Shenzhen, a booming city in southern China, found that many factories were legitimate and made efforts at quality control, but that some were lower-end operations that either had no safety testing equipment or specialised in counterfeiting established brands, often with cheaper parts.

Chinese companies were the first to develop e-cigarettes, and that happened in a regulatory void. In the US, the Food and Drug Administration (FDA) 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," says Jackie Zhuang, deputy general manager of Huabao International, a Chinese tobacco flavouring company in Shanghai and an expert on China's e-cigarette market.

"I hope it will soon be well regulated."

In a five-square-mile area in the northwestern part of Shenzhen called Bao'an, in a district packed with industrial parks, there are believed to be more than 600 e-cigarette producers, and many more component suppliers selling bulk orders of tube casings, integrated circuit boards, heating coils and lithium ion batteries, the essential components of the e-cigarette.

If you are a manufacturer in Shenzhen and need 50,000 baked-metal casings, a local manufacturer can supply them for about USD 25,000 and have them delivered within hours.

Unlike the counterfeiters' shops, the largest Shenzhen e-cigarette manufacturing operations are relatively clean, with rows of workers seated on plastic stools along a fast- moving assembly line.

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.

The boom has made China the breeding ground for a new, and some would say innovative, product. And yet the Chinese government has played no role in the development of the industry or in regulating it.

Some Chinese companies, however, are trying to get ahead of the anticipated FDA rules. First Union is one of the biggest, operating several manufacturing complexes here in Shenzhen with about 6,000 employees. Its plants have glass- enclosed, dust-free rooms that the company says are as clean and sophisticated as pharmaceutical labs.

"We have the same quality-control standards as medical device makers," said Sunny Xu, the chairman at First Union.

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

Scientific studies hint at a host of problems related to poor manufacturing standards. A study published last year in the open access online journal 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," says Prue Talbot, a professor of cell biology at the University of California, and co-author of several of the studies.