The Factories Act 1948, Section 87, grants the State Government power to declare a 'hazardous process'. Under Schedule relating to permissible levels, the Threshold Limit Value – Time Weighted Average (TLV-TWA) for carbon monoxide is 25 ppm (ACGIH 2023). If a worker is exposed to CO at 50 ppm for 6 hours and 10 ppm for 2 hours, the 8-hour TWA is:
- A 30 ppm — exceeds TLV-TWA
- B 40 ppm — exceeds TLV-TWA ✓
- C 25 ppm — exactly at TLV-TWA
- D 60 ppm — significantly exceeds ceiling value
Correct answer: B. 40 ppm — exceeds TLV-TWA
Explanation
TWA = [(C1 × T1) + (C2 × T2)] / total hours = [(50 × 6) + (10 × 2)] / 8 = [300 + 20] / 8 = 320 / 8 = 40 ppm. This exceeds the TLV-TWA of 25 ppm, indicating an unacceptable occupational exposure requiring engineering controls. The TWA formula weights each concentration by its duration over the standard 8-hour workday; it is not simply the arithmetic mean of the two concentrations.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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