During tunnel excavation safety monitoring, identify the acceptable on-site air quality thresholds at the heading: oxygen, carbon dioxide, hydrogen sulphide, and carbon monoxide should satisfy which combined limits?

Introduction / Context:
Ventilation in tunnels must maintain breathable air and prevent toxic exposure for workers. Regulatory and industry guidelines specify minimum oxygen and maximum contaminant limits to guide re-entry after blasting, continuous equipment operation, and routine work at the face. This question tests familiarity with commonly cited safe-threshold values for key gases.

Given Data / Assumptions:

• Parameters: O2, CO2, H2S, CO at the heading.
• Thresholds represent practical minima/maxima widely used on sites.
• Instrumentation (multi-gas meters) is used to verify compliance before entry.

Concept / Approach:

Safe practice requires O2 ≥ 19.5% to avoid hypoxia; CO2 ≤ 0.5% to limit asphyxiation risk; H2S ≤ 0.001% (≈10 ppm) due to its toxicity; and CO ≤ 0.01% (≈100 ppm) to prevent carboxyhemoglobin formation. Meeting all criteria simultaneously is necessary for worker safety and legal compliance.

Step-by-Step Solution:

Verification / Alternative check:

Post-blast ventilation plans typically target sustained airflow to clear NOx, CO, and SOx and verify with detectors that the above thresholds are met before resuming work.

Why Other Options Are Wrong:

• Any single parameter alone is insufficient; safe atmosphere requires all limits.
• Values higher in contaminants or lower in oxygen would be unsafe.

Common Pitfalls:

• Rushing re-entry without instrument confirmation.
• Ignoring localized pockets where heavier-than-air gases accumulate.

Final Answer:

All of the above.