Noise, dust and manual handling in a small rice mill
A small rice mill near Barpeta employs six workers in a 12 m × 9 m × 4 m shed. The mill runs 10 hours a day in season. A safety survey finds noise of 98 dB(A) at the husker, 94 dB(A) at the polisher, and 88 dB(A) in the bagging area; respirable dust of 8 mg/m³ near the cleaner; and workers manually lifting 50 kg bags from floor to a 1.2 m high truck bed about 120 times per shift. Two workers complain of numbness in the fingers and one has lower-back pain. The mill owner has a budget of ₹1.5 lakh and wants to avoid lost workdays. The shed has open eaves but no exhaust fan. Workers do not use the earplugs provided because they say they cannot hear the mill.
1. Calculate the noise exposure of a worker who spends 4 h at the husker, 3 h at the polisher, and 3 h in bagging. Is it within the 90 dB(A) 8-hour limit? Show the dose calculation.
Read the worked answer
Use the standard 3 dB exchange rate. Permissible time at a level L is T = 8 / 2^((L−90)/3) hours.
- Husker, 98 dB(A): T = 8 / 2^(8/3) = 8 / 6.35 = 1.26 h.
- Polisher, 94 dB(A): T = 8 / 2^(4/3) = 8 / 2.52 = 3.17 h.
- Bagging, 88 dB(A): T = 8 / 2^(−2/3) = 8 / 0.63 = 12.7 h.
Dose = 4/1.26 + 3/3.17 + 3/12.7 = 3.17 + 0.95 + 0.24 = 4.36.
That is 436% of the permissible daily dose, far above 100%. The 8-hour time-weighted average is approximately 90 + 16.61 log10(4.36) = 90 + 10.6 = 100.6 dB(A). This is a serious hearing hazard: at this exposure, hearing damage is likely within a few years. The earplug programme has failed, so engineering controls are essential, not optional.
2. Recommend a control package within ₹1.5 lakh, prioritizing engineering controls over PPE, and explain why the earplugs failed.
Read the worked answer
Priority order: eliminate or enclose the noise source, then isolate, then administrative controls, then PPE.
A workable package:
- Acoustic enclosure for the husker and polisher with 50 mm mineral wool and perforated sheet, leaving maintenance doors: ₹45,000.
- Rubber anti-vibration mounts under the husker and motor, and flexible couplings: ₹12,000.
- Replace worn bearings and set clearances; a worn bearing often turns a 94 dB machine into a 98 dB one: ₹8,000.
- Exhaust fan and duct for the shed, plus a cyclone or bag filter at the cleaner to cut dust: ₹35,000.
- Local exhaust hood at the bagging spout and enclosed conveyors: ₹15,000.
- Low-cost lifting aid - a trolley and a 1.2 m loading platform, or two-person lifting: ₹25,000.
- Training, fit-testing and better earplugs as a last layer: ₹10,000.
Total approximately ₹1.5 lakh. The earplugs failed for three reasons: workers could not hear warnings and machine faults, the plugs were probably not fitted or worn correctly, and PPE depends on daily individual behaviour. Enclosure reduces the noise at source for everyone, including visitors. Keep earplugs for the remaining 85–90 dB areas and use communication headsets rather than removing plugs.
3. For the manual bag lifting, what ergonomic change would you recommend, and what should be done about the two workers with numbness in the fingers?
Read the worked answer
The 50 kg bag is too heavy for repeated lifting from floor to a 1.2 m truck bed, 120 times per shift. The load moment is high, the horizontal reach is awkward, and the floor-to-knuckle height lift is the worst zone. Recommended changes:
- Reduce bag size to 25 kg, or use two-person lifting for 50 kg bags.
- Provide a mobile scissor-lift table or a conveyor that rises to truck-bed height, so workers slide rather than lift. A simple roller conveyor and a 1.2 m platform can be built within the budget.
- Keep the load close to the body, avoid twisting, and rotate workers every 2 hours between bagging, cleaning and machine operation.
- Train workers to bend the knees, not the back, and to team-lift when a bag must be moved manually.
The two workers with numbness should be referred to a doctor for assessment of carpal tunnel syndrome or other nerve compression. Do not simply move them to another job without medical advice. Meanwhile, reduce their repetitive gripping and vibration exposure, provide anti-vibration gloves if tools are used, and record the cases. The lower-back pain case should be assessed for lifting restrictions. The mill should keep a simple health register and review the controls after one month.