Breathing hydrogen sulfide gas at low concentrations can be fatal. Which combination is listed as fatal?

• A. 0.01%, 20
• B. 0.1%, 30
• C. 0.5%, 20
• D. 1.0%, 30

Answer: (B)

The main idea here is that toxicity from hydrogen sulfide depends on both how much is in the air and how long you breathe it. Even at low concentrations, a long enough exposure can be deadly because H2S interferes with the body's ability to use oxygen, leading to rapid breathing trouble, loss of consciousness, and potentially death.

At a concentration of about 0.1% (roughly 1000 parts per million), exposure for about 30 minutes is within the range that can be fatal for some individuals, which is why that combination is listed as fatal. The other options pair lower concentrations with the same or shorter times or higher concentrations with shorter times in ways that aren’t identified as fatal in the reference data used for this question, despite the high danger of those concentrations. This question is testing the recognition that a mid-range, sustained exposure to hydrogen sulfide can be fatal, whereas the other pairings don’t match the fatal exposure scenario given in the material.