I see. So the levels should be relatively stable across time. Because what the company is saying in their defense is that initial measurements were anywhere from 30-80 and ended up at 150.
No, levels can change rapidly, that is another aspect of the danger of it. With 30 - 80 ppm no one should have been anywhere near that damn manhole.
There are safe ways of working in higher that normal H2S environments, lack of breathing apparatuses is NOT one.