Just saw this on Japanese news. Apparently tests on sulfide were 15 X the legal limit, although only two of the workers reportedly died of inhalation ? Not sure that is possible or the beginning stages of an investigation.
If the concentration was over 20 ppm it wouldnt take very long, but it would be survivable with immediate medical care. Above that and you might as well not even bother. At 50 - 100 ppm, anyone going in without scuba gear will just add to the total. More than that and they will have no idea it happened because their death will be one or two breaths. As I do not know what the Japanese regulations are (industry standard is alarm and evacuate at 10 ppm).