Insecticide and squad planning for four IRS cycles
You are the malaria inspector for a division with 4,500 eligible houses for indoor residual spraying. The average plinth area is 55 m². The label dose is 2 g of 50% wettable powder per m². Four spray cycles are planned in the year. Six spray squads are available; each squad has two workers and can cover 22 houses per working day. The spray season allows 30 working days per cycle. The store has 1,200 kg of the wettable powder. Wastage during mixing and rinsing is estimated at 10%. Labour costs ₹550 per squad-day, insecticide ₹480 per kg, and supervision/transport ₹15,000 per cycle. One squad is likely to be diverted for dengue work for 5 days in cycle 2. The division must submit a revised plan to the chief health officer in two days.
1. Calculate the wettable-powder requirement for one spray cycle and for four cycles, including 10% wastage. State whether the 1,200 kg store stock is enough.
Read the worked answer
Total sprayable area per cycle = 4,500 houses × 55 m² = 247,500 m². At 2 g/m², product needed = 247,500 × 2 = 495,000 g = 495 kg per cycle. Adding 10% wastage: 495 × 1.10 = 544.5 kg per cycle. For four cycles: 544.5 × 4 = 2,178 kg. Store stock is 1,200 kg, so the shortage is 2,178 − 1,200 = 978 kg. The calculation uses the product label dose, not active ingredient; the 50% WP contains 1 g active ingredient per 2 g product, so the active-ingredient figure would be half. The practical conclusion is that the division cannot run four full cycles on current stock; it must procure 978 kg or reduce area/cycles with a documented risk-based decision.
2. Can six squads complete one cycle in 30 working days? What happens if one squad is diverted for five days, and how many squads should the revised plan request?
Read the worked answer
With six squads: 6 × 22 houses/day × 30 days = 3,960 houses. That is 540 short of 4,500, so six squads cannot complete the cycle. The requirement is 4,500/(22 × 30) = 6.82, i.e. 7 squads. With one squad diverted for 5 days, available squad-days = (6 × 30) − 5 = 175; coverage = 175 × 22 = 3,850 houses, a shortfall of 650. If 7 squads are deployed and one is diverted for 5 days, squad-days = (7 × 30) − 5 = 205; coverage = 205 × 22 = 4,510 houses, just enough. So the revised plan should request 7 squads, not 6, and keep a small buffer for reworking refused houses. The arithmetic matters because a nominal 6-squad plan looks adequate only if you ignore the diversion.
3. Estimate the direct cost of one cycle under the 7-squad plan and suggest two cost-reduction measures that do not reduce house coverage.
Read the worked answer
Insecticide: 544.5 kg × ₹480/kg = ₹261,360. Labour: 205 squad-days × ₹550 = ₹112,750. Supervision/transport: ₹15,000. Total for one cycle = ₹389,110. Four cycles would be about ₹1,556,440 at the same rates. Two measures that do not cut coverage: (1) reduce wastage from 10% to 5% by using calibrated measuring jars, reusing rinse water in the spray tank, and training mixers; at 5% the requirement falls to 519.75 kg, saving about ₹11,880 per cycle; (2) improve daily output per squad through route planning and pre-spray community notice, so the same houses are covered in fewer squad-days, cutting labour cost. Renegotiating the insecticide rate or buying in bulk may help, but that is procurement, not an operational saving. I would not reduce the dose or skip houses.