I once took an H2S safety training class. The first rule of working in an area with known H2S is if you see one of your colleagues suddenly drop to the floor for no apparent reason get the hell out of the area and whatever you do, don't stop to render aid to the fallen. After you have reached a safe well ventilated place, you should put on your safety gear and then head back to rescue the incapacitated.

There are too many stories like this where a whole bunch of people die trying to rescue the first victim so it makes total sense but it is kind of weird working in an environment where you know your colleagues will not help you (at least at first) if something goes wrong.

Btw: H2S doesn't really dull your sense of smell because of prolonged exposure. It only becomes odorless at deadly concentrations. So it turns out you don't know if you cannot smell it because it isn't present or it is just about to kill you.