Need to Breathe!
This mod makes sure the air in the overworld is unbreathable. So just don't put this mod in your modpack or you will be in big trouble. This mod is meant for modpack makers who have a specific story to tell. This mod adds a purifier that runs on RF and coal and which will purify the area in a closed area. If the area is not closed it will not be as effective. There is also a helmet which reduces poison intake 30% (by default) but doesn't completely eliminate it.
This mod is still beta. By default the mod works in the nether but you can also configure it to work in the overworld or any other dimension. The mod also integrates well with The Lost Cities in which case it will (by default) make sure the biosphere domes contain fresh air.
There is already a lot that you can configure. Including the effects the player gets based on poison levels. In addition passive mobs are also affected (but by default not killed) and hostile mobs are boosted (get regeneration)
Modpacks
You are free to use this mod in your modpacks.