PREFACE: so i had this idea for awhile, but i really don't have any modding skills whatsoever, so i just posted this here. Tanks! Also, if anyone acctually decides to make this, dont give me any credit if u dont want to and pls make it for 1.12.2.
SUGGESTION: So here is the idea for this mod: Basically, the air above a certain y level is poisonous(configurable), and belowground is safe. You need a gas mask to go above to the surface. Your gas mask has an air meter, and when you go aboveground, it shows up. if it runs out, you have to craft/replace the filter with a new one. There are two different types of filters: one small one, which is cheaper to craft but has less air, and a bigger one, which is harder to craft and gives you more air.
When you take damage with a gas mask on, there is a chance it will crack, and you will lose air faster while your gas mask is cracked. you can repair your gas mask using glass or tape(if you wanna add more new items or not)
And that's it! I think it should be configuable, like which biomes/dimensions it affects, how long the filters give you to breathe, etc.