If I remember correctly, chlorine gas was the first poison gas used in WW1.
It’s easily produced and may be available to the Isis creeps from water purification plants.
Definitely low tech and not a complex modern toxic.
One time I accidentally mixed two common drain cleaners in a garage laundry sink, to clear a blockage. Acrid fumes started pouring out. I quickly opened all the doors and windows and got my family out of the house at some distance, and waited for some time for the cloud to dissipate. I learned a lesson, to check the ingredients labels before using drain cleaners! Chlorine gas is nasty!