The danger of VX, in particular, lies in direct exposure to the chemical agent persisting where it was dispersed, and not through its evaporating and being distributed as a vapor (i.e. it is not a "vapor hazard"). VX is considered an area denial weapon due to these physical and biochemical characteristics. As a chemical weapon, it is categorized as a weapon of mass destruction by the United Nations and is banned by the Chemical Weapons Convention of 1993, where production and stockpiling of VX exceeding per year is outlawed. The only exception is for "research, medical or pharmaceutical purposes outside a single small-scale facility in aggregate quantities not exceeding per year per facility".