Shouldn’t require any kind of special monitor for H2S: it’s characteristic odor is rotten eggs...
Apparently it can cause the sense of smell to flake out so that if you are in an area where you smell it and stop smelling it, you need to leave post haste.
It may not be an indicator that you are away from the gas, but an indicator that the gas is poisoning you.
I tried to find out more in a google search but haven’t had any luck yet.
Here….
https://hsi.com/blog/how-to-protect-against-the-dangers-of-hydrogen-sulfide-h2s
Although hydrogen sulfide can be detected by its rotten egg scent, odor should not be used to confirm its presence since we lose our ability to distinguish an odor after prolonged exposure, and if the concentration is high enough we won’t be able to smell hydrogen sulfide at all.