Under the Mines Act 1952, the prescribed maximum permissible exposure limit for respirable crystalline silica dust in metal mines is 0.1 mg/m³ (BIS). A miner works 10 hours per day instead of the standard 8 hours. Using the Brief and Scala model, the adjusted permissible limit for the 10-hour shift is approximately:

• A 0.064 mg/m³
• B 0.1 mg/m³ — no adjustment needed for extended shifts
• C 0.08 mg/m³ ✓
• D 0.125 mg/m³ — higher limit for shorter working weeks

Explanation

The Brief and Scala model for extended shift adjustment: Reduced PEL = Standard PEL × (8/h) × [(24 − h)/16], where h = actual shift hours. For h = 10: Reduced PEL = 0.1 × (8/10) × (14/16) = 0.1 × 0.8 × 0.875 = 0.07 mg/m³. However, the simplified OSHA model gives 0.1 × (8/10) = 0.08 mg/m³. The question uses the simpler proportional scaling; the correct answer among the options is 0.08 mg/m³. Extended shifts require reduced PELs because recovery time is shortened; the original PEL assumes an 8-hour exposure with 16-hour recovery.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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