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Competency Area 8: Protecting Humans from Pesticide Exposure

PO 47. Know the factors that affect particle drift and why they affect drift:

A. Droplet size
B. Wind speed
C. Nozzle distance from target
D. Temperature and humidity

The obvious intent of spraying a pesticide is to deposit droplets on the crop or soil surface. However, a lot can happen to a droplet in the distance between a nozzle and the crop or soil.

A  Droplet size. Smaller droplets fall more slowly, evaporate more quickly, and are more easily moved by wind. Therefore, they will be carried farther by wind before they are deposited.
B Wind speed. A stronger wind means droplets will be carried farther before they are deposited. This is true for any size droplet, though the effect is still greater on smaller droplets.
C Nozzle height. The higher a boom or nozzle is set above the target surface, the farther a droplet must travel before being deposited. This means the droplet will be in the air longer and subject to evaporation and wind longer.
D Temperature increases and humidity decreases. As the air gets warmer and/or drier, the water in the spray droplets evaporates faster. This makes droplets effectively smaller, making them more prone to drift.

Spray droplet size is the most important factor affecting the potential for particle drift. Wind speed is also very important. However, increasing droplet size can greatly reduce the wind’s effect. The active ingredient is not a significant factor in particle drift. The effects of formulation and physical properties of the spray mix are also much smaller than those of droplet size, wind speed, and nozzle height.