Space spraying techniques

Space spraying is the dispersal of a liquid fog of insecticide into an area to kill adult insects. It has been regularly used in public health and pest control programmes, including use as an emergency response to malaria epidemics.[1]

Space spraying relies on the production of a large number of small insecticidal droplets intended to be distributed through a volume of air over a given period of time. These droplets deliver a lethal dose of insecticide to target insects upon impact.

Space sprays are applied mainly as thermal fogs or cold fogs

1. Thermal fog [2]

The insecticide used in thermal fogs is diluted in a carrier liquid, which is usually oil-based. Hot gas is used to heat the pesticide spray, decreasing the viscosity of the oil carrier, and vaporizing it. When it leaves the nozzle the vapor hits colder air and condenses to form a dense white cloud of fog. Most of the droplets are smaller than 20μm. The droplet size is affected by the interaction between the formulation, the flow rate and the temperature at the nozzle (usually > 500 °C). The volume of spray mixture applied in vector control is usually 5–10 litres per hectare, with an absolute maximum of 50 litres per hectare. The hot emission gas is obtained from engine exhaust, friction plate/engine exhaust or from a pulse jet engine.

Figure 1. A hand-carried thermal fogger

2. Cold fog[2]

With cold fogs the droplets are formed by the mechanical breaking up of the spray mixture, either by passing it through high-pressure nozzles or by passing a slow stream of the mixture through a high-velocity vortex of air. Some equipment is fitted with high-speed rotary nozzle(s). The spray droplets are generated without any external heat. With cold fogs the volume of spray is kept to a minimum. Ultra-low-volume (ULV) insecticide formulations are commonly used for such applications.

Figure 2. A knapsack cold fogger

Operator protection

All persons involved in the application of space sprays must wear overalls, protective gloves and goggles when pouring out insecticide, preparing a spray liquid and filling equipment (a semi-closed automatic dilution machine for use with water diluted spray is available). As the spray droplets are small enough to be inhaled, equipment operators must be issued with and wear a suitable respirator, the filter of which must be periodically changed in accordance with the manufacturer’s instructions. Engine-driven equipment is very noisy so the operator should also wear ear protection. Equipment must have guards to prevent operators touching hot surfaces. Operators must work in accordance with national regulations on pesticide use and application and should strictly follow the recommendations of the manufacturer.

Indoor fogging

Personnel conducting this work require training on the safety measures to be followed. Several rules apply:

• Shut off all electricity at the master switch.
• Turn off all heating and cooking equipment, including pilot lights and allow for a cool down time before spraying.
• The risk of fire is less with water-diluted products.
• Protect all water containers and foodstuffs.
• Remove fish or cover fish tanks.
• Ensure all occupants and animals remain outside the house during spraying and stay outside for 30 minutes after spraying. Ensure that the building is ventilated before reoccupation. Close all doors and windows before spraying and keep them closed for 30 minutes after spraying to ensure maximum efficacy.
• Spray operators should work backwards and away from the fog to minimize exposure.
• For small single-storey houses, the spray can be delivered from the front door or through an open window without having to enter every room of the house, provided that adequate dispersal of the insecticide droplets can be achieved.
• For large single-storey buildings, it may be necessary to apply the spray room by room, beginning at the back of the building and working towards the front.
• For multi-storey buildings, spraying is carried out from top floor to the ground floor and from the back of the building to the front. This ensures that the operator has good visibility at all times. The size of an average dwelling (in m2) should be calculated and the volume of the diluted spray required according to the manufacturers label recommendations (note: some manufacturers’ label recommendations for indoor fogging are given as rates per volume). The flow rate of the machine (ml per minute) should be checked so that the average time required to spray each dwelling is known and the spray operators are informed. A fog must be “dry” before being directed into a building. Test the fog by placing the machine on the ground and checking that the area immediately in front of the nozzle is not wetted by the fog. To reduce the production of large wetting droplets, obtain the correct balance between flow rate and combustion temperature. This is usually done by reducing the flow rate.

Outdoor ground fogging

Advanced route planning should precede outdoor ground fogging operations and may require a combination of vehicle-mounted and handcarried or knapsack equipment in areas with difficult or limited vehicle access. Consideration must also be given to the following:

• Spraying should not be undertaken when it is raining, when winds exceed 15 km/hour, or in the heat of the day.
• Doors and windows of houses and other buildings should be open to allow penetration of the spray cloud for improved efficacy.
• For vehicle-mounted equipment, in areas where the roads are narrow and the houses are close to the roadside, the spray should be directed backwards from the vehicle. In areas where the roads are wide, with buildings far from the roadside, the vehicle should be driven close to the roadside and the spray should be directed at an angle (downwind) to the road rather than directly behind the vehicle.
• The nozzle of vehicle-mounted cold fog machines may be directed upwards at an angle when there are barriers that impede airflow, e.g. boundary walls and fences; for vehicle mounted thermal foggers, the nozzle should be directed horizontally.
• The distance between successive passes through a built-up area will be largely dependent on the layout of roads. A track spacing of 50 metres is generally recommended, with the vehicle moving upwind so that the fog drifts downwind away from it and the operators (Figure 3).
• As far as possible, the predetermined speed of the vehicle should be maintained and the spray must be turned off when the vehicle is stationary.
• The downwind side of the spray area should be treated first, working systematically from downwind to upwind.
• To avoid driving into the spray cloud, dead-end roads must be sprayed only on the way out.
• Try to avoid directly spraying shrubbery and expensive floral areas unless using a water-based/water-diluted product.

Figure 3. Spray application route relative to wind direction in an urban setting.

Table 1. WHO‐recommended insecticides for space spraying against mosquitoes [3].

Abbreviations: EC: emulsifiable concentrate; EW: emulsion, oil in water; ULV: ultra‐low volume liquid; AI: active ingredient

References

1. Cochrane library. Insecticide space spraying for preventing malaria transmission Cochrane Systematic Review - Intervention Version published: 02 November 2018.

2. WHO(2003). Space spray application of insecticides for vector and public health pest control. Geneva, (WHO/CDS/WHOPES/GCDPP/2003.5)

3. WHO(2016). WHO recommended insecticides for space spraying against mosquitoes.