Wouldn't sustaining stun damage every turn kinda defeat the purpose of the gas mask since it's meant to protect you from the smoke that causes stun damage every turn?
I don't think it is feasable to have an item apply damage over time any ways. I'm not sure how it is done in this mod, but you can reduce the soldier's stamina if they are wearing the gas mask, which would represent the difficulty to breathe. Reducing TUs (to reduce movement capabilities due to having a harder time breathing, but not impacting firing weapons) and/or Reactions (the mask impairs vision) also work to help balance it.
