I would implore you to think about what is being stated by Yammy.
He is saying that the "ectoplasm" is so thin within the atmosphere, that it is hard for him to breathe. What thing would a being who has lugs, and uses those lugs to breathe a particular substance that allows them to continue on living, be called? It cannot be this hard for you to understand that it would be Air...
Like, it's the reason why characters throughout the series, when they're tired, start breathing in and out to replenish themselves.
It's the reason why particular techniques even work in the first place, like fire-based abilities.
Hell, it's even directly stated to
exist in Zangetsu's spiritual world.
Are you genuinely going to hold the position that Reishi constitutes literal atoms and sub-atomic particles like Sunlight or Electricity, but for some arbitrary reason, it wouldn't constitute one of the most prominent and important molecule for continued survival on planet Earth?
The double standard to just wank Bleach characters the most unsupported ways possible never ceases to amaze me...