that's basically just co2, since (iirc) the panic from not being able to breathe comes from high co2 levels the blood, rather than low oxygen levels in the blood. this is also why simple asphyxiants like nitrogen gas are still very dangerous, because you might not notice that you're not getting any oxygen, especially since your ability to think is probably also impaired because of said low oxygen.

so if you wanna make yourself anxious for funsies, then you can just breathe a bunch of co2 (but also don't because you might die, and even if you don't, elevated co2 levels are still pretty bad for you)

I think that just means it needs an oxygen concentration of 20%.