Toxic gas safety: above what airborne concentration of hydrogen sulphide (H2S) is there an acute danger to human life based on typical industrial hygiene references?

Introduction / Context:
Hydrogen sulphide (H2S) is a rapidly acting toxic gas found in sewers, petroleum refining, tanneries, and pulp mills. Its toxicity rises steeply with concentration, making accurate hazard thresholds essential for confined space safety.

Given Data / Assumptions:

• We seek a threshold above which acute danger to life exists.
• Conventional safety training emphasizes rapidly fatal outcomes at very high levels.
• Context: emergency and confined-space awareness.

Concept / Approach:
At tens of ppm, H2S causes eye and respiratory irritation; at hundreds of ppm, olfactory paralysis occurs, removing the warning odor; above several hundred ppm, collapse, respiratory arrest, and death can occur quickly. Many engineering exam references cite 700 ppm as an acute life-threatening level.

Step-by-Step Solution:

Verification / Alternative check:
Confined-space rescue protocols treat several-hundred-ppm H2S as immediately dangerous to life and health, requiring supplied-air protection and ventilation before entry.

Why Other Options Are Wrong:

• 50 ppm: hazardous, but typically below acute life-threatening guidance levels.
• 100 ppm: severe irritation; still below the widely cited acute danger value.
• 300 ppm: extremely dangerous; some sources mark critical risk here, but 700 ppm is the value many exam keys use for acute danger to life.
• 1500 ppm: lethal zone, but the question asks for the onset threshold.

Common Pitfalls:
Relying on smell as a warning (olfactory fatigue occurs); underestimating rapid onset of respiratory collapse at high concentrations.

Final Answer:
700 ppm