PANNA: Drift Occurs During and After Pesticide Applications

Pesticide drift is comprehensively defined as any airborne movement of pesticides off the target site. Classifying drift as to when and how it occurs makes it possible to predict which pesticides will be most problematic under different conditions. Spray drift occurs during and soon after a pesticide application, while post-application drift occurs after the application is complete.

Spray drift: During pesticide applications, winds or application equipment can blow spray droplets and vapors as those droplets evaporate (with liquid applications), or particles (with dust application) off site. Fine droplets generated by spray nozzles are the most problematic and can drift long distances before settling. Applicator error or misjudgment can be a significant source of drift. For example, when ground-rig operators fail to turn off spraying or blowing equipment when turning at the end of rows, blower fans or spray pressure may blow pesticides into roadways and neighboring properties. When aerial applicators misjudge the shut-off point when approaching the end of a field, pesticides may drift onto adjacent private property or cars and trucks on roadways instead of the crop. Applicators may also misjudge wind effects. Applications of gaseous fumigant pesticides always involve escape of the gases from the intended application site, generally through the normal (and presently legal) application process, but also through leaking equipment, containers, or tarps. Application-related drift is often visible as a cloud of mist (for liquids) or dust (for solids). Drift of gaseous pesticides like fumigants or volatilized liquids is invisible and often odorless, making it difficult to detect with the senses.

Post-application drift: Pesticide drift does not end when applications are complete. Post-application drift also may occur over many days and even weeks after a pesticide application. Post-application drift takes two forms.

* Volatilization drift: Because of their inherent physical properties, some pesticides readily volatilize from the leaf and soil surfaces on which they were initially deposited. They might be liquids or oils when applied, but evaporate in the heat of the day, drift for a distance, and re-condense when the temperature drops or when they contact a cool surface, just like water vapor condenses on a glass of iced tea on a humid day. This process is repeated many times as the pesticide is carried by prevailing winds. Fumigant pesticides used to treat homes, storage bins, and soil (before planting) are so volatile that they normally exist as gases or very volatile liquids or solids, rendering them extremely drift-prone. Because these pesticides are also applied in large quantities -- 100 to 400 pounds per acre -- they are the most problematic of all pesticides in terms of drift.

* Drift of pesticide-coated dust particles: High winds in agricultural areas create clouds of dust from pesticide-treated fields. This dust is eventually deposited in yards and parks, as well as in homes and cars, where it can be inhaled or ingested. Children are particularly at risk from this type of exposure because they play on the floor and often put their hands or other objects into their mouths. Both volatile and non-volatile pesticides may cling to dust particles and drift in this manner.