A political compromise during the negotiation of the 1993 Chemical Weapons Convention created ambiguity at the heart of the treaty. The question of whether the use of toxic chemicals as weapons for law enforcement must be limited to riot control agents (“tear gas”) only, or whether other highly toxic chemicals could also be used, was left open to interpretation. This chapter traces the thirty-year struggle to close that loophole. These efforts eventually resulted in a legal clarification in 1 December 2021 confirming that the use of aerosolized central nervous system-acting chemicals is prohibited for law enforcement. The chapter explores key developments that led to this decision, with particular attention to the efforts of the ICRC and experts in academia, civil society organizations, and the scientific and medical communities, to highlight and reframe the problem. The loophole may have shrunk, but has it closed? The chapter highlights remaining ambiguities and draws lessons for broader disarmament and arms control efforts.