One of the things that doesn't seem to get mentioned is the fact that chemical attacks are relatively ineffective unless they are done when conditions are just right.
I was my unit's assistant NBC NCO (went to Nuclear,Biological, and Chemical warfare school - 80 hour course). And there are specific meteorological conditions that are good for such attacks, and others that are not, and the prime conditions rarely occur, and probably less so in a desert.
Certainly Hussein gassed the Kurds. Of course, the Kurds lived in the mountains, and conditions are different there, and it is douobtful that the Kurdihs villagers had sophisticated detection and protection equipment. Using chemical weapons in open desert is just silly. Effective psychologically, but very few casualties would result.
The gassing in WWI was effective because 1. the theatre of operations was more conducive to their use and 2. like everything else in WWI, the attacks were huge - giant cylinders of gas would be opened when the prevailing winds were favorable, and usually in the early morning or evening, when atmospheric conditions were best. Not to mention the poor quality of protection for troops and, again, the incredible psychological effects.
I can't imagine how terrifying it must have been to look up and see a cloud of gas coming at you, then being able to make out the forms of machine gun toting storm troopers walking right in the cloud, coming at you...
Of course, the sarin gas attakcs in the Tokyo subway - an enclosed area - produced very few casualties.