How to Detect and Fill Enclosed Spaces?

I am making a mod similar to Galacticraft, and I want to add a block similar to the Oxygen Sealer, so how would I would I be able to detect and fill an enclosed space with breathable air blocks?

Note: When you are in any of the space dimensions, it checks if you are wearing oxygen gear or are in a breathable air block, and if you aren't you'll start to take damage

How would I do this?

Any help would be greatly appreciated!