After "Desert Storm" the UN destroyed almost all of the Shells containing "Mustard gas" ,Sarin and VX used in weapons have a short shelf life,even under IDEAL conditions of manufacture and storage their shelf life is around 10yrs ,if its put into weapons its several weeks.The sarin Saddam was producing was 40% pure meaning its Shelf life was much shorter,UNSCOM never found any production of VX,but even if Saddam was producing it, its Shelf-life would be very limited too.
There is ample evidence to prove that the UN Sanctions were working and Saddam wasn't able to acquire anything "new" after the Gulf War ,so even if we "Know" he had and used them prior to '88 .Any such weapons would pose more of a threat to the environment because of disposal methods, necessitated by their shelf-life,than him ever be able to use them.At the most he had a few mustard gas shells and after twelve years these would have been more dangerous to the "user" than the "target" .