I'll admit that there are some agents that are lumped under the WMD listing that are no more than limited-effect blister toxins capable of producing little more than a nasty rash.
Mustard gas wasn't the main concern, though. Sarin, Cyclosarin, VX and Anthrax, of which hundreds of TONS are unaccounted for and when used properly, can cause MASSIVE casualties in a densely-populated area, hence their inclusion under UNSCOM verbage as "WMD".
