Rhiannon Griffith-Bowman smokes an e-Cigarette at Digital Ciggz on Jan. 28, 2015, in San Rafael, California. An independent study released this week says these electronic devices that vaporize liquids containing nicotine expose users to high levels of cancer-causing agents, including formaldehyde. Justin Sullivan/Getty Images

As millions of people embrace e-cigarettes as a healthier alternative to smoking tobacco, a study suggests these portable electronic vaporizing devices might not be as harmless as many believe. The Center for Environmental Health (CEH) released evidence this week suggesting most e-cigarettes release high levels of cancer-causing gases, and the organization said it’s taking legal action against e-cigarette-makers for not warning customers of this risk.

“CEH is concerned about the unregulated marketing of e-cigarettes, and especially sales to teens and young people, while little is known about the health hazards from inhaling e-cigarette smoke,” the Oakland, California, nonprofit group devoted to exposing toxin risks, said in its report.

E-cigarettes have been available in the U.S. and Europe for nearly a decade, and an estimated 20 million Americans currently have at least experimented with the devices, which use a battery to heat up so-called e-liquids, flavored solutions that usually contain nicotine, until they turn to vapor that is inhaled. Today the e-cigarette market is worth about $3 billion annually and could hit $10 billion by the end of 2017.

But contrary to some advertising claims that the vapor is harmless, the CEH found alarmingly high levels of formaldehyde and acetaldehyde, known carcinogens that increase cancer risk, getting sucked into “vapers' ” lungs. An independent laboratory analysis looked at 97 e-cigarette products from two dozen manufacturers and found most emit higher levels of these cancer-causing gases than allowed under California’s Proposition 65, the Safe Drinking Water and Toxic Enforcement Act of 1986.

The CEH study comes after similar results were announced in January in the New England Journal of Medicine. That study was criticized by e-cigarette manufacturers for allegedly using unrealistically high temperatures to heat the e-liquids that couldn’t be achieved by using the devices as instructed. But the CEH said its research was done through smoking machines designed to replicate how the devices are used, and they still found the high levels of carcinogens.

"This is especially troubling given the reckless marketing practices of the e-cigarette industry, which targets teens and young people, and deceives the public with unfounded health and safety claims," the report said.