Which health effect is most associated with 500-1000 ppm H2S exposure?

• A. Respiratory irritation
• B. Rapid collapse
• C. Systematic poisoning
• D. Headache only

Answer: (C)

High-level exposure to H2S produces systemic poisoning rather than just local irritation. At 500–1000 ppm, the gas rapidly disrupts cellular respiration by inhibiting mitochondrial enzymes, which stops cells from using oxygen. That leads to widespread tissue hypoxia and dysfunction across the body, especially affecting the brain and other vital organs. Because the effects are not confined to the airways or a single symptom, this level of exposure is best described as systemic poisoning. Local irritation or headaches tend to appear at lower concentrations, but they don’t capture the severity and body-wide impact seen here. Rapid collapse can occur as part of this systemic toxicity, but the defining aspect is the whole-body poisoning that this concentration causes.