There is no immediate way to slow the stimulus. Tear gas is limited in open space, especially when it's windy, it's easy to blow away. So it's the best way to avoid it when you encounter tear gas. In addition, even if the choke is very uncomfortable, do not squat in situ, because the common tear substance CS heavier than air, will sink, squat down will only contact more CS.
After staying away from tear gas, clothes should be changed as much as possible, because the CS powder is powder form at room temperature, and the CS attached to the clothing will continue to irritate the skin. However, particular attention should be paid to the lack of exposure to chlorinated detergents which would render CS more toxic. Also, avoid touching your eyes, mouth and nose with your hands - yes, don't rub your eyes, and wash your hands before blowing your nose. Avoid CS on your hands and irritate the mucous membranes again.