Five drones were used for dust reduction at Lan Khon Muang, in front of City Hall, on Thursday. The number would be increased and the operation widened, the governor said, “Each drone could carry 5-10 litres of water and could be flown over a two-square-kilometre area for 15-20 minutes on one charge. Drones will spray only clean water into the air, with the Bangkok Fire and Rescue department supervising the operation.’’