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E-cigarettes are reportedly catching on among smokers as a substitute for smoking traditional ones. An electronic cigarette is a battery-operated device that is believed to stem smoking.

They first were marketed in China 15 years ago. Now it is the most popular mode of smoking among teenagers in the US.

But there is a growing body of research that suggests that vaping may be chancy, even fatal.

A growing number of Americans say that vaping e-cigarettes is at least as harmful as smoking traditional cigarettes, according to a Reuters/Ipsos poll. The poll has some substance, as seen from a mysterious outbreak of vaping illnesses in the US. More than 2,000 Americans who vape have got sick since March, many of them teen and young adults, and at least 40 people have died. The bulk of the cases occurred in August and September but new cases are still being reported.

Agency officials cautioned they cannot rule out all other toxic substances.

Now American health officials announced a breakthrough on Friday, reporting they have a "very strong culprit."

The same chemical compound was found in fluid taken from the lungs of 29 patients across the country, the Centers for Disease Control and Prevention said. The compound - vitamin E acetate - was previously found in liquid from electronic cigarettes and other vaping devices used by many of those who got sick.

But this is the first time they've found a common suspect in the damaged lungs of patients, officials said.

"We are in a better place in terms of having one very strong culprit," said the CDC's Dr Anne Schuchat.

Agency officials cautioned they cannot rule out all other toxic substances, and it may take animal studies to clearly show vitamin E acetate causes the lung damage that's been seen.

Vitamin E acetate has only recently been used as a thickener in vaping fluid, particularly in black market vape cartridges. While vitamin E is safe as a vitamin pill or to use on the skin, inhaling oily droplets of it can be harmful. It's sticky and stays in the lungs - the CDC's Dr Jim Pirkle likened it to honey.

E-cigarettes and other vaping devices heat a liquid into an inhalable vapour.

Many who got sick said they had vaped liquids that contain THC, the high-inducing part of marijuana, with many saying they got them from friends or bought them on the black market.

E-cigarettes and other vaping devices heat a liquid into an inhalable vapour. Most products contained nicotine, but THC vaping has been growing more common.

Pirkle said thickeners like vitamin E acetate probably would not be routinely added to nicotine liquids, which need to be more watery for vaping.

Symptoms of the vaping illness include trouble breathing, chest pain, fatigue and vomiting. Imaging tests show lung injuries and doctors can't find infections or other causes.

About two months ago, New York drew attention to vitamin E acetate when the state's public health lab discovered it in samples of vaping products from sick patients. In some instances, it made up more than half of the liquid in the cartridges.