Wasn’t an irritant gas deployed in the Cu Chi tunnels? just a thought.
CS gas grenades were used on the caves of Cu Chi but with somewhat limited effect because of the way the miles of the caves were constructed. The were not straight line caves so the sharp turns greatly hindered the flow of the tear gas and even explosive grenades ranges.
I didn’t get to see the caves in person but I’ve seen enough photos to know that the Commies did a good job on them (took about 20 years to build a system that covered many square miles and possibly went under American base camps.
Never underestimate the ability of your enemy to kill you.