In underground tunnelling near the working face, what is the maximum permissible concentration of respirable dust particles in the size range 0.5–5 microns (measured as number of particles per cubic centimetre) to protect workers and equipment?

Introduction / Context:
Dust control at the tunnel face is central to occupational health and visibility. The most harmful fraction comprises fine respirable particles, typically in the 0.5–5 micron range, which can penetrate deep into the lungs. This question checks recall of a benchmark ceiling for particle counts close to the working face to guide ventilation, water-spray, and housekeeping measures.

Given Data / Assumptions:

• Location: immediately adjacent to the active tunnel face.
• Particle size band: 0.5–5 microns.
• Metric: number of particles per cubic centimetre (particles/cm3).
• Objective: identify the maximum recommended concentration for safe operations.

Concept / Approach:
Engineering controls (face ventilation, scrubbers, misting) target keeping airborne fines below a practical, easy-to-monitor threshold. Particle-count ceilings complement mass-concentration limits (mg/m3) used in hygiene standards. As a memory aid for site practice, many syllabi adopt 300 particles/cm3 as the not-to-exceed guideline for the critical 0.5–5 micron band at the face.

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