If chemical weapons are used in Iraq, they will most likely not be a show stopper. There are two ways troops are taught to deal with chemical weapons. One is to suit up. The other is to move out of the area. If troops are on the move in armored vehicles, they are a hard target for chemical weapons, and the vehicles use an overpressure system to keep the chemical weapons outside. The overpressure systems (that keep the air pressure higher inside the vehicle than outside) have been much criticized for not working as advertised. But the failure of these systems is one of degree. Moreover, hitting moving groups of vehicles with sufficient chemicals to do damage is in itself difficult. There is a lot of experience with mustard and nerve gas against troops, but not against troops in vehicles. What little is known in this area indicates that non-lethal doses of nerve gas (like what might leak into a "gas proof" armored vehicle) makes drivers and other crewmen dizzy and disoriented, along with double vision. This will cut combat capability quite a bit. But nerve and mustard gas will not chase you around. After it arrives via artillery shell or rocket, the stuff tends to stay put. This is why troops are trained to "suit up and split". Iraqi artillery and rocket launchers are major targets for coalition artillery and aircraft. During the 1991 war, the Iraqis were shut down in this department and unable to fire many shells or rockets of any type. To do a lot of damage with chemical weapons you have to use a lot of it and the Iraqis are not liable to be able to do that. There is also the possibility of chemical mines (landmines that disperse chemicals rather than just explode), but these would be detected and cleared, destroyed or avoided like any other type of mine.