Federal regulators are asking for public input on chemicals found in electronic cigarettes and e-liquid.
On Friday, the Food and Drug Administration announced that the agency is seeking comments on a proposal that would add 19 chemicals to the already established list of harmful ingredients found in tobacco products.
"As our oversight and scientific knowledge of tobacco products has evolved, so too should our requirements for manufacturers and importers to provide information about the chemicals or chemical compounds in their products that cause or could cause harm to users and nonusers," acting FDA Commissioner Ned Sharpless said in a statement.
This includes compounds created when e-liquid is heated, forming a vapor that users breathe into their lungs.
The move comes just days after a Yale analysis identified chemicals called acetals in some flavors of a specific brand of e-cigarette: Juul.
Those chemicals, the researchers said, may be especially irritating to the lungs and cause damage when inhaled. However, the study did not test for these effects in humans or animals.
A spokesperson for Juul dismissed the Yale team's lab research, writing in a statement, "The researchers' hypothetical exposure analysis failed to take into account real world conditions, including realistic human exposure to vapor products like Juul."
One of the chemicals that the FDA is considering adding to the list is diacetyl, which gives products a butter-like flavor.
"We’re seeing a growing number of studies highlighting that e-liquid flavors are particularly problematic," said Thomas Ylioja, a tobacco cessation expert at National Jewish Health in Denver.
The FDA considers artificial flavors used in food as "generally recognized as safe," but less is known about the long-term effects of inhaling aerosols containing these flavors.
"What hasn’t been done are tests to see whether or not they remain safe for consumption when you heat them up to a temperature an electronic cigarette is using," Ylioja said.
E-cigarette manufacturers do not have to list all of their ingredients on product labels, but are required to give the FDA information about the levels of the "harmful and potentially harmful constituents" found in their products.