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The smokeless tobacco industry that began with low-voltage cigarette look-alikes has evolved to include customizable, high-wattage machines capable of generating enormous clouds of vapor ― and potentially toxic substances.

As the technology continues to change, researchers are finding more evidence that the way vaping devices and e-liquids interact could harm consumers. High-powered devices may overheat vaping liquids to produce toxic chemicals, tobacco experts warn, and the aerosol that is inhaled may be contaminated with dangerous metals from the device.

Although researchers say they still don’t have enough data to know whether vaping devices are less dangerous than cigarettes, Stanton Glantz, a professor of medicine and the director of the Center for Tobacco Control Research and Education at the University of California-San Francisco, said the scientific evidence convinced him that vaping is far from a harmless substitute.

“Nobody knows what’s in any of these products,” said Glantz. “What you’re actually exposing yourself to is not in any way, shape or form standardized.”

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Many consumers credit vaping and electronic cigarette products with helping them kick their cigarette habit. But as the technology has changed, so have e-liquids. Formulas today can deliver the same amount or more nicotine than a cigarette in the same number of puffs, researchers say.

Liquids and devices are being scrutinized by the Centers for Disease Control and Prevention to identify what is causing the outbreak of vape-related illnesses. As of Oct. 15, the CDC has identified nearly 1,500 lung injuries related to vaping. Thirty-three people have died, according to the agency.

Investigators suspect many of the injuries are related to bootleg cartridges laced with THC, the psychoactive ingredient in marijuana. This point is frequently played up by the vaping industry and its advocates to defend products created and sold by reputable businesses.

However, the ingredients and materials that make up these products are often a mystery, even when they are made by legitimate manufacturers. Researchers like Thomas Eissenberg, a professor of health psychology and co-director of the Center for the Study of Tobacco Products at Virginia Commonwealth University in Richmond, said published cases of e-cigarette-related illnesses preceded the latest outbreak.

“Clearly, there’s something strange about these … cases popping up all at once,” said Eissenberg. “But I suspect we’ve been seeing numerous cases of these kinds of diseases ever since e-cigarettes were first sold.”

“Cigalikes” represent the earliest form of e-cigarettes. The disposable gadgets were typically prefilled and often resembled cigarettes. They also contained modest amounts of nicotine and were not very effective in delivering that to consumers.

Then, vape pens entered the market. They were refillable and packed more power than the cigalikes to heat up the e-liquid. The pens also tended to contain more nicotine.

The third generation of devices ― mods ― proved to be a technological and cultural leap for vaping, said Ana María Rule, an assistant professor who researches e-cigarettes at Johns Hopkins University in Baltimore. Consumers could personalize their gadget by altering the battery, heating elements and e-liquid. The device inspired the creation of groups like “cloud-chasers,” vape users who compete at events around the country to make the largest cloud of aerosol.

“They completely changed the scope and the whole culture of vaping,” Rule said.

This type of vaping means consumers are inhaling larger volumes of the aerosolized chemicals, and that might mean more toxic chemicals, researchers say.

Then Juul hit the market in 2015. The rechargeable device resembles a flash drive, could not be customized and has generally lower voltage than its mod predecessor.

But what Juul lacked in technological innovation, it made up for it in its e-liquid. Each pod contains a concentration of nicotine equal to a pack of cigarettes, which concerns public health officials and researchers because nicotine is addictive. The company cut the nicotine with benzoic acid to reduce irritation, making it more palatable. It ultimately became a runaway success ― particularly among youth.

Yet, nicotine is far from the only harmful substance a consumer could be inhaling. Studies have found that overheating an e-liquid could cause “thermal degradation,” a process where the ingredients start breaking down. In some cases, this dismantling can create toxic chemicals like formaldehyde, a cancer-causing agent, said Eissenberg.

Overheated or burnt e-liquid can taste bitter, he said, but typically consumers have no way to detect a problem.

“It’s important to realize it doesn’t always taste bad when that thermal degradation has occurred,” Eissenberg said.

Aerosol from e-cigarettes can also be laced with harmful metals that cause pulmonary and neurological health problems. One study co-authored by Rule collected e-liquid samples from different parts of 56 e-cigarette devices owned by daily users. The findings showed that the fluid sitting in the device and inhaled by the consumer had markedly higher levels of metal than the liquid in the refillable dispenser.

The authors suggest that the device’s heating element could be introducing toxic metal substances like chromium, nickel and lead into the e-liquid.

Despite the worrisome findings, tobacco researchers say the verdict is still out on the danger of e-cigarettes versus traditional cigarettes. There’s simply not enough data, they say.

However, the lack of evidence hasn’t stopped states from reacting to the cases of vaping-related illnesses by cracking down on the industry. Massachusetts temporarily has banned the sale of all vaping products in the state, but the ban is being challenged in court. Selling flavored e-cigarette and vaping products has been temporarily halted in a handful of other states and local jursidictions, too.

Some vaping advocates view these state actions as fearmongering. Gregory Conley, president of the nonprofit American Vaping Association, emphasized that the vast majority of sickened patients reported using bootleg cartridges that contained THC.

“The U.S. is in the middle of a moral panic right now, and good public health policy rarely flows out of moral panics,” Conley said.

Conley acknowledged that stricter regulations could have helped keep Juul out of the hands of teens. The Food and Drug Administration, however, did not regulate e-cigarettes until 2016. A court order requires the companies that had products on the market as of August 2016 to turn in applications by May 2020 for FDA approval.

As the agency rolls out those regulations, researchers stress that e-cigarette companies still have wide latitude to sell their product without federal oversight.

E-cigarette and vaping products are “not regulated in terms of quality control, market control, anything right now,” said Bonnie Halpern-Felsher, a professor of pediatrics who researches tobacco use prevention at Stanford University in California. “It is completely wide open.”