The issue focused on electronic cigarettes and vaping devices that vaporize liquids containing nicotine or propylene glycol. Electronic cigarettes contain chemicals similar to tobacco smoke but produce no combustion byproducts. Nicotine is chemically addictive and smokers typically vape electronic cigarettes instead of combusting tobacco cigarettes. Propylene glycol is used as an additive in electronic cigarettes and inhalers to mask the odor and flavor of nicotine. But critics argue that electronic cigarettes cause respiratory irritation and allergies. In December 2014, California imposed strict limits on electronic cigarette advertising and flavors