The Organisation for the Prohibition of Chemical Weapons (OPCW) says that 90% of declared chemical weapons—about 63,000 metric tons—have been destroyed worldwide. “This is a major milestone that shows we are well on the way to ridding the world of chemical weapons,” says Ahmet Üzümcü, director general of OPCW. The organization monitors compliance with the Chemical Weapons Convention, a 1997 international treaty that banned the production, stockpiling, and use of chemical arms. The destroyed materials include stocks of unitary weapons, such as sulfur mustard, and precursor chemicals for producing deadly nerve agents such as sarin. The process to eliminate Cold War-era stockpiles in Russia and the U.S., which have the “biggest arsenals,” is well under way and should be complete by the end of 2020 and 2023, respectively, according to OPCW.