As E-cigarettes - smoking devices that are promoted as a safe way to quit conventional cigarettes - gain in popularity, the nicotine liquid used to fill e-cigarettes may be lethal, HealthDay reported.

The key ingredients in e-cigarettes, "e-liquids," are extracted from tobacco and enhanced with flavorings, colorings and various chemicals, HealthDay reported.  The liquids are not regulated by the federal government.

According to the New York Times, at least a teaspoon of the liquid nicotine can kill a small child and less than a tablespoon, at high concentrations, can kill an adult.  They are potent neurotoxins that can cause vomiting, seizures and even death when small amounts are ingested or absorbed through the skin.

Experts say children are especially at a high risk for injury or death because they may be attracted to the nicotine liquids' bright colors "and appealing flavors such as bubble gum, chocolate and cherry," HealthDay reported.

"It's not a matter of if a child will be seriously poisoned or killed," Lee Cantrell, director of the San Diego division of the California Poison Control System and a professor of pharmacy at the University of California, San Francisco, told The New York Times. "It's a matter of when."

There has been a 300 percent increase in the number of accidental poisonings involving e-liquids from 2012 to 1,351 cases in 2013, Health Day reported, citing the national Poison Data System. Those figures are on track to double this year.

"This is one of the most potent naturally occurring toxins we have," Cantrell said of nicotine. Despite the serious and potentially deadly threat of e-liquids, they are now available legally in stores across the United States and online.

"It is sold all over the place. It is ubiquitous in society," she added.

According to the New York Times, the U.S. Food and Drug Administration said it plans to regulate e-cigarettes, but has not provided any details.