A concentration of H2S above which ppm typically leads to rapid collapse?

• A. Greater than 1000 ppm
• B. 0-50 ppm
• C. 50-500 ppm
• D. 50-1000 ppm

Answer: (A)

H2S is extremely toxic and its danger rises with concentration. At about 1000 ppm and above, it rapidly inhibits cellular respiration by blocking cytochrome oxidase, so brain and other vital tissues are starved of oxygen almost immediately. This causes quick loss of consciousness and collapse, often within seconds to minutes. In addition, high levels blunt the sense of smell, so the warning odor can disappear just as danger is peaking, making detection even more dangerous. Lower ranges produce symptoms like irritation, dizziness, or headache, but the abrupt collapse seen at concentrations well above 1000 ppm is not typical at those levels.