The US Food and Drug Administration is proposing to add 19 chemicals to its list of harmful and potentially harmful constituents (HPHCs) in tobacco products. The 19 chemicals are specific to electronic nicotine delivery systems, such as e-cigarettes and e-liquids. The substances include diacetyl, a buttery-tasting flavoring agent; ethylene glycol, sometimes used as a solvent in e-liquids; and glycidol, a thermal by-product of glycerol found in e-cigarette vapor. The agency’s initial list of HPHCs, established in 2012, contains 93 chemicals found in tobacco products or tobacco smoke. Researchers have linked the chemicals to cancer, addiction, and cardiovascular, respiratory, and reproductive diseases. The FDA did not have the authority to regulate e-cigarettes when that list was established. “As our oversight and scientific knowledge of tobacco products has evolved, so too should our requirements for manufacturers and importers,” FDA acting commissioner Ned Sharpless says in a statement. Manufacturers and importers must provide the FDA with the levels of each HPHC in tobacco products. The FDA is accepting comments on its proposal to add 19 chemicals to the HPHC list until Oct. 4.