Chemical warfare as "poison gas" ..not effective. The clones are walking around inside dropsuits in which they have climate control inside them in order to enter "hostile" planet environments like one with say sulphuric acid atmosphere so putting it into this perspective some mustard gas would hardly even have an itch on these suits. Chemical warfare therefore has no meaningful effect and something modern needs to be developed..

Enter microwave emitters and EMP weapons. A powerful microwave emitter could easily heat up an area, would cause PG disruptions and damage. Same goes for EMP cannon which would completely shut down the dropsuits, vehicles and weapons utilizing electric parts etc.

Dust dropsuits entirely cover the skin of the wearer. They also may or may not have either self-contained breathing apparatuses or some kind of advanced filtration. Being able to survive in outer space is a totally different thing from being able to be safe from chemical warfare. A rubberized hazmat suit will keep you safe from chemical attacks, but you would die if you ever wore it in space. Considering the wide variety of planets Dust mercs are expected to fight on, it would follow that their dropsuits are able to withstand hostile and toxic environments.

Anyway, the realism of it isn't important. What matters is the gameplay that is being advocated.