Since it was first deployed in a 1921 experiment on volunteers from the Philadelphia Police Department, tear gas has been increasingly used to disperse crowds. That’s despite a ban for use in warfare by the Chemical Weapons Convention. Though the U.S. military has not officially used tear gas since 1975, local law enforcement is exempt from restrictions, said Stephen Semler, a Beirut, Lebanon-based researcher who has worked with Armament Research Services, a specialist intelligence consultancy.