Original: As chemical weapons , they are classified as weapons of mass destruction by the United Nations according to UN Resolution 687 ( passed in April 1991 ) and their production and stockpiling was outlawed by the Chemical Weapons Convention of 1993 ; the Chemical Weapons Convention officially took effect on April 29 , 1997 .
Simplified: Since they are chemical weapons , they are classified as weapons of mass destruction by the United Nations according to UN Resolution 687 .